View of Dissertation

DYNAMIC LARYNGO-TRACHEAL CONTROL FOR AIRWAY MANAGEMENT IN DYSPHAGIA

AARON JOHN HADLEY

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Dissertation Advisor: Dr. Dustin J. Tyler

Department of Biomedical Engineering

CASE WESTERN RESERVE UNIVERSITY

August 2013 CASE WESTERN RESERVE UNIVERSITY

SCHOOL OF GRADUATE STUDIES

We hereby approve the thesis/dissertation of:

Aaron John Hadley candidate for the Doctor of Philosophy degree*.

(signed) Dustin J Tyler, Ph.D

(chair of the committee)

Kenneth Gustafson

Michael Broniatowski

Patrizia Bonaventura

(date) 05/30/2013

*We also certify that written approval has been obtained for any proprietary material contained therein.

Table of Contents LIST OF FIGURES ...... VI LIST OF TABLES ...... VIII ACKNOWLEDGEMENTS ...... IX ABSTRACT ...... 1 CHAPTER 1 : INTRODUCTION ...... 3

DYSPHAGIA ...... 3 PHASES OF SWALLOWING ...... 5 LARYNGEAL ANATOMY...... 8 LARYNGEAL ELEVATION ...... 11 INNERVATION OF LARYNGEAL ELEVATION (HN, CN XII) ...... 16 RECURRENT LARYNGEAL NERVE ...... 18 FUNCTIONAL ELECTRICAL STIMULATION FOR LARYNGEAL ELEVATION ...... 20 Surface Stimulation for Laryngeal Elevation ...... 20 Intramuscular Stimulation for Laryngeal Elevation ...... 22 Nerve Stimulation for Laryngeal Elevation ...... 25 FUNCTIONAL ELECTRICAL STIMULATION FOR VOCAL FOLD CONTROL ...... 28 Surface Stimulation of the Vocal Folds ...... 29 Implanted Stimulation of the Recurrent Laryngeal Nerve ...... 31 Transtracheal/Transesophageal stimulation ...... 33 STIMULATION CONTROLLER ...... 35 PALATOMETRY ...... 37 Electric Sensors ...... 38 Positive and Negative Pressure Recording ...... 41 PROJECT SPECIFIC AIMS ...... 43 CHAPTER 2 - LARYNGEAL ELEVATION BY SELECTIVE STIMULATION OF THE HYPOGLOSSAL NERVE ...... 47

ABSTRACT ...... 47 INTRODUCTION ...... 49 METHODS ...... 52 Surgery ...... 52 Tetanic Stimulation and Video Recording ...... 54 Nerve Stimulation and EMG Twitch Response ...... 55 Video Analysis ...... 55 Selectivity Analysis ...... 57

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RESULTS ...... 58 Elevation ...... 58 Elevation Velocity ...... 61 Selectivity ...... 62 DISCUSSION ...... 63 CONCLUSION ...... 70 CHAPTER 3 - TARGETED TRANSTRACHEAL STIMULATION FOR VOCAL FOLD CLOSURE 73

ABSTRACT ...... 73 INTRODUCTION ...... 74 METHODS ...... 80 Modified Endotracheal Tube Design...... 80 Surgical Preparation ...... 81 Stimulation Trials ...... 83 Data Analysis ...... 84 RESULTS ...... 87 DISCUSSION ...... 93 CONCLUSION ...... 98 CHAPTER 4 - NEURAL NETWORK DETECTION OF SWALLOWING USING PALATOMETRY ...... 100

ABSTRACT ...... 100 INTRODUCTION ...... 101 Palatometry ...... 104 Time Delay Artificial Neural Networks...... 106 MATERIALS AND METHODS: ...... 108 Subjects and Palatometer Production ...... 108 Time Delay Artificial Neural Network Analysis ...... 113 RESULTS ...... 116 Palatometer Recordings ...... 116 Intra-Subject and Population-Trained Networks ...... 122 DISCUSSION ...... 126 Optimizing Thresholds for FES systems ...... 127 CONCLUSION ...... 131 CHAPTER 5 - DISCUSSION AND CONCLUSIONS ...... 133

OVERVIEW OF DISSERTATION ...... 133 CONTRIBUTION/RELATIONSHIP TO THE LITERATURE ...... 134 FUTURE STUDIES ...... 136

Stimulation ...... 136 Palatometry ...... 138 APPLICATION OF RESEARCH ...... 140 Rehabilitation ...... 140 Implantation ...... 141 OVERALL CONCLUSIONS ...... 142 BIBLIOGRAPHY ...... 144

List of Figures Figure 1-1: The phases of swallowing and bolus transport...... 6 Figure 1-2: Diagram of intrinsic laryngeal muscles ...... 10 Figure 1-3: Diagram of the muscles, nerves, and structures of laryngeal elevation...... 14 Figure 1-4: Nerve tracts of the Hypoglossal and First Cranial Nerve ...... 18 Figure 1-5: A change in peak elevation (mm) for laryngeal and hyoid bone vertical displacement...... 22 Figure 1-6: Diagram of the distal XII/C1 nerve complex ...... 27 Figure 1-7: Diagram of the pressure sensor locations ...... 39 Figure 1-8: Timing of onset, maximum, and offset pressure ...... 40 Figure 2-1: Lateral view of the significant anatomy in the canine neck...... 53 Figure 2-2 : A representative recording profile of laryngeal and hyoid movement over time...... 57 Figure 2-3: Net displacement of the hyoid bone and thyroid cartilage ...... 60 Figure 2-4: Example recruitment curves ...... 63 Figure 3-1 : Averaged positions and range of the RLN from the cross sections from Liebermann-Meffert ...... 79 Figure 3-2: Image of an electrified endotracheal tube...... 81 Figure 3-3: An isometric view of the significant anatomy ...... 82 Figure 3-4 : Example scaled glottal area recording during stimulation ...... 86 Figure 3-5 : Example images of the vocal folds ...... 86 Figure 3-6 : Threshold levels of the left LCA in one experiment...... 88 Figure 3-7 : Plot of the range of angles at which transtracheal stimulation was possible for each experiment...... 89 Figure 3-8 : Effect of frequency on vocal fold area under stimulation...... 91 Figure 3-9 : Effect of Isoflurane levels on glottal area...... 92 Figure 3-10 : Plot of glottal area under unilateral and bilateral stimulation...... 97 Figure 4-1 : Example negative and positive of dental mouthpiece production...... 109 Figure 4-2 : Oral mouthpiece with seven sensors in place...... 110 Figure 4-3 : Diagram of TDANN data preparation and analysis...... 113 Figure 4-4 : An example recording of five swallows ...... 117 Figure 4-5 : Examples from one subject of palatometry recordings for a variety of swallowing stimuli and non-swallowing activities...... 118 Figure 4-6 : Plots of the effect of varying the number of neurons and time delays on Accuracy, ROC area, and MSE...... 119 Figure 4-7 : Diagrams of selections of sensor patterns used for training...... 121 Figure 4-8 : Effect of varying the number or choice of sensors ...... 121 Figure 4-9 : ROC curves from all 40 sessions ...... 122

Figure 4-10 : Example of ROC curves from networks trained on one session ...... 123 Figure 4-11 : ROC curves of five networks applied to the same dataset...... 124 Figure 4-12 : ROC Curve of the best population-trained TDANNs ...... 125 Figure 4-13 : Barplot of the median maximum specificity ...... 129

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List of Tables Table 1-1: Timing of swallow events...... 8 Table 2-1: Average elevatory motion after stimulation of each electrode...... 59 Table 2-2 : Net laryngeal elevation induced by unilateral and bilateral stimulation ...... 61 Table 2-3 : Average peak velocity under stimulation ...... 62 Table 2-4 : Measurements of hyoid and thyroid kinematics ...... 68 Table 4-1: List of swallowing stimuli and non-swallow activities...... 111

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Acknowledgements This work was performed with the assistance of a multitude of people, and

would not have been possible otherwise. I am very thankful for Dr. Tyler, who has

provided exceptional assistance and guidance throughout the years. Special thanks to

Dr. Kate Krival, without whom the palatometry studies would not have been possible.

Additional thanks go to Ilya Kolb and Paul Thompson, who assisted with data analysis

and overnight experiments. Tina Emancipator taught me so much about the surgical

process and ensured everything was done to the best it could be. I would also like to

thank the members of my guidance committee, Drs. Gustafson, Broniatowski, and

Bonaventura, who provide me with new angles from their different viewpoints into my

research.

To my colleagues and fellow researchers, Elizabeth Hahn, Natalie Brill, Smruta

Koppaka, Erik Peterson, Daniel Tan, Nathan Makowski, Chris Heylman, Bradley

Plummer, and all the others with whom I discussed my work and whom shared in the experience, thank you for your feedback, assistance, humor, and tolerance throughout the years.

Outside of Wickenden, I have been blessed with wonderful family and friends

whom I can call my brothers and sisters. I could never thank my mother and father

enough for all the love and guidance through the years that have led me to this point. I

would especially like to thank my beautiful wife, Chanary, who has been my support and

my vent through all the good times and the bad, and I know will be there with me for all

the things yet to come. Thank you all, and stay awesome.

Dynamic Laryngo-Tracheal Control for Airway Management in Dysphagia

Abstract

AARON JOHN HADLEY

Control of the laryngo-tracheal opening is necessary to balance the body’s constant need of oxygen, phonate speech, and enable safe intake of food. A common result of traumatic brain injury and stroke is paralysis and paresis of the vocal folds, causing impaired breathing, hoarseness, and aspiration. Vocal fold adduction and laryngeal elevation serve as protective mechanisms to divert fluids and food away from the airway and into the esophagus during deglutition. The aims of the current study were to: 1) Examine selective hypoglossal nerve stimulation for laryngeal elevation, 2)

Optimize the stimulation angles and parameters of transtracheal stimulation, and 3)

Develop an automatic detection algorithm using natural signals from swallowing.

Hypoglossal nerve stimulation induced laryngeal elevation to a magnitude approximately equal to that of a natural swallow, and FINE electrodes were shown to be able to selectively activate the muscles of elevation. Transtracheal stimulation, when applied at the optimized angles, was able to induce complete vocal fold adduction. A time-delay artificial neural network was trained to sensitively and selectively detect swallowing using oral pressure signals. This research advances the creation of a closed- loop laryngeal stimulator for dysphagia protection by assessing novel stimulation paradigms, producing an automatic control signal, and combining laryngeal stimulation

1 measures for more complete protection. The results of this research strongly support advancing these techniques to pilot clinical trials.

Chapter 1 : Introduction This chapter introduces the goals of this research, followed by background on

dysphagia and the anatomy of swallowing. Existing treatment options are presented,

along with applications of functional electrical stimulation to the head and neck. Finally,

opportunities for improvement in therapy and surgical treatment are discussed,

developing into the specific aims of this project.

The purpose of this project was to develop components of a dual-action

electrical stimulation system with a natural command source for protection of the

airway during swallowing in dysphagics. Components of the system were tested using

surgical implantation and stimulation, along with recording in human subjects and

computational analysis. Our hypothesis was that transtracheal and hypoglossal nerve

stimulation would provide some improvement over existing dysphagia stimulation techniques and that a detection algorithm using palatometry could be developed for

detection of the oral swallow.

Dysphagia Dysphagia can be broadly defined as difficulty swallowing. Swallowing is a

reflexogenic action that must be performed appropriately for the safe transfer of oral

materials to the stomach. Malfunctions in the swallowing reflex often lead to aspiration,

which is leakage of the swallowed bolus into the larynx, trachea, and lungs. Aspiration

of contaminated materials leads to the development of infection within the lungs,

causing aspiration pneumonia. Aspiration pneumonia is the leading cause of death in

Parkinson’s disease (Troche et al. 2010) and stroke patients (Heuschmann et al. 2004).

Multiple motor disorders result in dysphagia, but stroke is the most common

cause of dysphagia (Howden 2004). Stroke can result in significant damage to regions of

the central nervous system, unilateral or bilaterally, depending on the site of the stroke.

There are many central regions important for the control of swallowing, including the

pre-motor cortex, NTS, NX, NXII, and nucleus ambiguus (Miller 1986). Damage to any portion of these pathways can result in paresis or paralysis of the swallowing musculature, resulting in reduced movement, timing errors, or complete inability to initiate the swallowing reflex. Dysphagia after stroke is associated with increased institutionalization and is a sign of poor long-term outlook (Smithard, Smeeton, and

Wolfe 2007). Additionally, patients with Parkinson’s disease are at high risk for dysphagia due to side effects of the motion disorder. The most common impairments were delay in triggering of the swallow reflex and slow pharyngeal transit (Ertekin et al.

2002). These impairments lead to increased opportunity for a bolus to aspirate, especially when tremor results in additional uncontrolled motion.

The clinical definition of dysphagia is fairly broad, so reported rates after stroke are variable, ranging from 19 to 81% (Martino et al. 2005). Three main diagnostic studies are performed to determine if a subject has dysphagia, and the diagnostic method affects the reported rates. First, an initial screening test is given to stroke patients (Martino, Pron, and Diamant 2000). This procedure is simple and can be applied with minimal clinical swallowing training. If any dysphagia is detected, then a more comprehensive test is applied by a specifically trained individual. The more comprehensive test includes cranial nerve testing and a variety of boluses to determine

4 the level of disability. Following this trial, if deemed necessary, a video fluoroscopic swallowing study (VFSS) is applied to determine the location and magnitude of any swallowing impairments. The barium-dosed boluses are followed in video to observe the location and timing of aspiration and determine why it is occurring. In a review of existing rates of dysphagia reporting, Martino et al. used VFSS as the gold standard, as only it truly allows measure of aspiration into the trachea.

Martino’s review of dysphagia rate reporting found that more detailed screening methods resulted in higher diagnosis rates of dysphagia in stroke patients. Basic clinical screening diagnosed dysphagia in 37%-45% of stroke patients, clinical testing diagnosed dysphagia in 51-55% of stroke patients, and instrumental testing (VFSS) found dysphagia in 64-78% of stroke patients. The authors propose that the reason for the variability between studies is each clinician uses a different definition of dysphagia. To combine the results of these different studies, aspiration was defined as a part of dysphagia, so that all aspirators are dysphagic, but not all dysphagics are aspirators. Under this filtering, it was found that 22-50% of tested patients had aspiration, while 64-78% of patients were diagnosed with dysphagia (Martino et al. 2005). The delineation between aspiration and difficulty swallowing is key for the proper application of treatment: difficulty swallowing by itself is a sign of potential problems, but aspiration leads to higher incidence of pneumonia and death.

Phases of Swallowing Swallowing, or deglutition, can be defined as “the semiautomatic motor action of the muscles of the respiratory and gastrointestinal tracts to propel food from the oral

cavity to the stomach" (Miller 1986). Deglutition is widely accepted to be composed of

three phases: oral, pharyngeal, and esophageal (Lang 2009). The oral phase consists of

tongue contractions that move the bolus from the oral cavity to the pharynx. The

pharyngeal phase includes vestibular closure, relaxation of the upper esophageal

sphincter (UES), reflection of the epiglottis, closure of the vocal folds, and pharyngeal peristalsis, all of which lead to the propulsion of the bolus into the esophagus. The esophageal phase consists of the esophageal peristalsis propelling the bolus to and through the lower esophageal sphincter. A display of positions of bolus location during the phases of swallowing can be seen in Figure 1-1. The different phases are coordinated with a series of voluntary action, reflexes, and swallowing pattern generators in the brainstem. Stroke, Parkinson’s disease, or other dysphagia causing infirmities can affect any of the phases, depending on which regions of the nervous system are affected.

Figure 1-1: The phases of swallowing and bolus transport. (Smith Hammond and Goldstein 2006)

The oral phase of swallowing can be further broken into two periods. First is the

Oral Prepatory phase, in which the tongue and mouth process an introduced material

into a bolus (Lang 2009). This phase includes mastication (chewing) and saliva

production to create a malleable bolus that can be propelled through the mouth. This

phase ends with the collection of materials in a bolus in the anterior portion of the

mouth. The Oral Transport phase, the second portion of the oral phase, includes

propulsion of the bolus through the mouth and into the pharynx. Oral transport requires

patterned motion of the tongue and jaw, as the bolus must be kept complete for fully

effective swallowing. Both portions of the oral phase of swallowing are capable of being performed consciously or unconsciously, as the reflexogenic activities patterns can be replicated voluntarily or can be secondarily controlled by proprioception and sensation.

The pharyngeal phase of deglutition is a series of reflexes that begin when the bolus enters the pharynx. Motions during this phase include hyoid elevation, laryngeal elevation, epiglottal dislocation, vocal fold closure, nasal seal, etc. Vocal fold closure seals the trachea, and the body reflexively halts inspiration, a brief cessation dubbed swallowing apnea. The levator veli palatini raises the soft palate in conjunction with halting breathing and diaphragm activity (Miller 1986). Using videofluoroscopic swallowing studies, the timing of these events and the position of the bolus have been measured (Mendell and Logemann 2007). The sequential order of events varied from subject to subject, and the duration of the oral and pharyngeal phase was related to age, with older patients requiring a longer time period from onset of bolus transit to esophageal opening. Timing relationships between oral activities are shown in Table 1-1.

Table 1-1: Timing of swallow events. Average timing of a series of swallowing activities in relation to the onset of UES opening. Of primary note is the increase in time from onset to conclusion of swallow for the increase in age. (Mendell and Logemann 2007) The esophageal phase of swallowing begins once the bolus has passed through

the upper esophageal sphincter and enters the esophagus. The esophagus is a muscular

tube that propels the bolus to the stomach using peristaltic motion. These reflexive

motions are initiated by mechanical and chemical sensors in the esophagus that return to the autonomic nervous system (Richards and Sugarbaker 1995). Esophageal dysphagia gives the patient the sensation of food sticking in their chest, as opposed to the coughing or inability to initiate swallowing that is common in oropharyngeal dysphagia. Esophageal dysphagia is usually either caused by muscle dysfunction or a

mechanical obstruction like a lesion on the esophageal wall (Castell and Donner 1987).

Once the bolus has entered the esophagus it is no longer a threat to the airway, so the

protective mechanisms activated as part of the pharyngeal swallow relax and return to

resting states.

Laryngeal Anatomy

The larynx, which contains the vocal folds and serves as the doorway to the

lungs, is a collection of cartilaginous structures. The thyroid cartilage is the largest

cartilage and is visible to the outside as the “Adam’s apple” or “voice box”. Within the

thyroid cartilage are the vocal folds, which are mucous membranes stretched between

the anterior inside of the thyroid cartilage to the arytenoids cartilages, which rest in the

posterior of the thyroid cartilage. The two arytenoid cartilages are pyramidal structures

that rotate and translate to change the tension and position of the vocal folds. The cricoid cartilage lies inferior to the thyroid cartilage and appears like the first tracheal ring. Motion of the cricoid cartilage alters the position of the arytenoids, adjusting the tension of the vocal folds.

Five intrinsic laryngeal muscles control the relative position of the vocal folds

(Ludlow 2005). The thyroarytenoid muscle (TA) is a primary component of the vocal folds, and its tension affects the tension of the vocal folds, adjusting pitch and tone. The interarytenoid muscles (IA) connect the two arytenoids cartilages and contract them together, resulting in vocal fold adduction. The lateral cricoarytenoid muscles (LCA) causes rotation of the arytenoids cartilages such that the vocal folds adduct, whereas the posterior cricoarytenoid muscle (PCA) causes the arytenoids to rotate and abduct the vocal folds. These four muscles are innervated by the recurrent laryngeal nerve

(RLN), a branch of the vagus nerve. The fifth intrinsic laryngeal muscle, the cricothyroid muscle (CT), causes lengthening of the vocal folds, which in turn assists vocal fold adduction. Vocal fold adduction seals the airway from the pharynx, preventing inspiration, expiration, and aspiration during swallowing. The cricothyroid muscle is

9 usually innervated by the superior laryngeal nerve, but is periodically innervated by the recurrent laryngeal nerve (Mu and Sanders 2009). A diagram of each of the laryngeal muscles and their induced motions can be seen in Figure 1-2.

Figure 1-2: Diagram of intrinsic laryngeal muscles and their effect on the motion of the cartilages.(Ludlow 2005)

Laryngeal Elevation Laryngeal elevation is one of the most important protective mechanisms used by the body during swallowing. In fact, according to Burnett, “pathologically reduced or delayed laryngeal elevation is the most common cause of aspiration in persons with dysphagia, either as the primary swallowing dysfunction or as a part of a composite of kinematic and temporal deficiencies” (Burnett et al. 2003). Laryngeal elevation can be broadly described as the superior and anterior motion of the larynx during swallowing, but this motion incorporates contraction of multiple muscle groups and motion of bones and cartilages. The hyoid bone and larynx are not articulated to the skeleton, connected only by muscles, ligaments and other soft tissues. Any time a muscle attached to one of these hyolaryngeal components contracts, the cartilage or bone will pull toward the skeletal connection (Pearson et al. 2012).

Superior and anterior motion of the larynx provides benefits to swallowing in a few different ways. The motion pulls the larynx into a position inferior to the base of the tongue, shifting the airway out of the bolus path. Additionally, laryngeal elevation aids epiglottal deflection, in which the epiglottis flips over and covers the airway, creating a

“slide” for the bolus to travel to the esophagus. The anterior motion of the thyroid cartilage applies tension to the esophagus, aiding increased upper esophageal sphincter opening. The increasingly open esophagus makes it easier for the bolus to reach its proper target. Laryngeal elevation is a reflexive part of the pharyngeal phase of swallowing, and while parts can be induced consciously, it cannot be consciously prevented.

The hyoid bone is a free-floating bone in the neck positioned superiorly to the thyroid cartilage. The hyoid serves as a tethering point for the thyroid cartilage, as there are no muscles attached to the thyroid cartilage that independently have the capability to pull the larynx anteriorly. Instead, the hyoid is drawn forward by contraction of muscles under the tongue, and muscles attaching the thyroid to the hyoid pull the larynx in an angular combination of superior and anterior movement. The Geniohyoid and Mylohyoid muscles are the two primary muscles that induce the anterior motion of the hyoid bone. The mylohyoid muscle is shaped like a sheet, forming the floor of the jaw and connecting the hyoid to the mandible. The geniohyoid muscle inserts into the most anterior portion of the mandible, causing anterior approximation of the hyoid during contraction. Studies have shown that decreased motion of the hyoid correlates strongly with aspiration (Kim, Kang, and Kim 2009). A review of videofluoroscopic studies showed that patients observed to have reduced hyoid elevation are 3.7 times more likely to aspirate than patients with normal hyoid excursion (Perlman, Booth, and

Grayhack 1994).

Laryngeal elevation is often measured using videofluoroscopic swallowing studies (VFSS). These studies record x-rays at a high frequency so the series of images can be viewed as a video. To measure the motion of a bolus, a supply of water or food is dyed with barium, a non-toxic material that blocks x-rays, allowing the location of the bolus to be seen in each frame. The hyoid bone and larynx and their relative motion to each other or a stable point such as one of the cervical vertebra is measured. Because

VFSS requires exposing the patient to radiation, testing is limited to 3-10 swallows. This

limitation allows for high variability between individual trials, and impairment could be

missed completely. Beyond the health of the subject, things that may adjust the

measurement of hyoid and laryngeal elevation and increase the variability in the

literature include number of recordings per bolus size, methodology of hyoid tracking,

choice of initial positions, plane of reference, density of barium mixture, gender,

mandibular plane angle, patient height, and definition of "healthy" subjects (Molfenter

and Steele 2010).

The thyrohyoid is the only muscle in the human directly linking the thyroid cartilage and hyoid bone together. When contracted, the thyrohyoid muscle causes approximation of the hyoid bone and thyroid cartilage. Depending on the tension in

other muscles, this approximation can result in laryngeal elevation or depression of the

hyoid. A diagram illustrating the anatomical position of the thyrohyoid is shown in

Figure 1-3. Because the larynx is attached to the trachea and has a larger mass of other

structures attached to it, an imbalance of suprahyoid muscle forces will cause the

thyrohyoid contraction to pull the hyoid bone inferiorly. Superior hyoid elevator muscles include the digastric and stylohyoid muscles.

Figure 1-3: Diagram of the muscles, nerves, and structures of laryngeal elevation in the human. The thyrohyoid (TH) and geniohyoid (GH) are the primary muscles that move the larynx and hyoid in a superior and anterior motion.

Pearson et al. performed anatomical measurements in cadavers to determine the structural capabilities of the muscles in the head and neck that cause hyolaryngeal elevation (Pearson et al. 2012). They proposed a model of the larynx in which a few collective groups of muscles functioned as "slings" to move the hyolaryngeal complex.

The anterior sling included the submental muscles, and the proposed posterior sling included the long pharyngeal muscles, such as the stylopharyngeus, salpingopharyngeus, and palatopharyngeus. The thyrohyoid works alone in this model

to approximate the thyroid and hyoid. Measurements of muscle length, angle, diameter,

density, fascicle lengths, and composition were recorded to determine the capability of

each muscle to move the hyolaryngeal complex superiorly. Summed force vectors

showed no statistically significant difference between the submental muscles and the

long pharyngeal muscles for causing superior motion, though both were stronger than

the thyrohyoid. Even though the suprahyoid muscles were stronger, the group determined that the thyrohyoid alone could cause 2/3 of the average swallowing elevation, assuming maximal contraction of the muscle. This 12.1 mm of thyro-hyoid

approximation is a significant portion of the total movement when viewed in a single axis (Leonard et al. 2000). The paper, however, had no analysis of the anterior motion which is an important part of hyolaryngeal displacement, and only measured the effect of force along the vertical axis. Superior motion is only a portion of the swallowing reflex, causing the desired vestibular closure but not opening the esophagus.

Additionally, this analysis does not factor in the potential hyoid depression that would occur if the suprahyoid muscles were slack and stretched.

Using video recordings, Burnett measured an average of 17.56 ± 4.17 mm of laryngeal elevation during natural human swallows (Burnett et al. 2003). Leonard et al. used fluoroscopy to separate the recorded elevation into individual components and found an average of 16.9 mm of hyoid motion and 12.1 mm of thyroid-hyoid approximation (Leonard et al. 2000). Shaw et al. measured the length of the geniohyoid during swallowing and found that the final swallowing length of the geniohyoid was

75±2% of the initial length (Shaw et al. 1995). This decrease transfers to approximately

12.5 mm of hyoid movement toward the symphysis of the mandible.

The dysphagic patient population often has comorbidities that result in the need for a tracheostomy and placement of an endotracheal tube. Terk et al. performed videofluoroscopic swallowing studies to determine if tracheostomy tubes would hinder laryngeal elevation by applying additional forces to the system. No significant differences were found, either for hyoid bone displacement or larynx-to-hyoid bone approximation, for any changes in tracheostomy tube presence, cuff status, or capping status (Terk, Leder, and Burrell 2007). Therefore, the presence of a tracheostomy tube is not expected to have a negative impact on patients’ swallowing health, which is good due to the complexity of treatment overlap necessary for many comorbidities.

Innervation of Laryngeal Elevation (HN, CN XII) The muscles of laryngeal elevation are innervated by both cervical and cranial nerves. The mylohyoid muscle and the digastric muscle are innervated by the mandibular branch of the trigeminal nerve (V3). The thyrohyoid and geniohyoid muscles are innervated by branches of the first cervical nerve (C1) that travels temporarily with the hypoglossal nerve (HN, Cranial Nerve XII). This XII/C1 nerve complex innervates these two muscles of elevation and also the muscles of the tongue: the genioglossus, hyoglossus, and styloglossus muscles. Unilateral paralysis of the mouth from damage to the hypoglossal nerve has been shown to cause swallowing difficulty, but not as

significantly as bilateral paralysis. Unilateral control of the laryngeal elevators is

sufficient for inducing the protective effects of superior and anterior motion of the

larynx (Salame et al. 2006).

Salame et al. performed dissections in 23 cadavers to quantify the location and branching of the hypoglossal nerve (Salame et al. 2006). The hypoglossal nerve (HN, CN

XII) exits the skull through the hypoglossal canal and travels downward medial to the internal jugular vein, internal carotid artery, and cranial nerves IX, X, and XI. There are no branches of the HN to the tongue prior to the joining of the ansa hypoglossi. Cranial nerve 1 joins with the hypoglossal nerve soon after exiting the skull and then the descending hypoglossal (consisting of axons from C1) branches when the remaining

XII/C1 nerve bundle turns horizontal and travels anteriorly. The nerve branch to the thyrohyoid is an offshoot of the horizontal XII/C1 nerve complex in 17/46 examples seen by Salame, and was a branch off of the descending hypoglossal in 29/46 specimens. The branch to the geniohyoid was found on the hypoglossal nerve following separation of the ansa hypoglossi 36/46 times and at an average of 10 mm after the ansa hypoglossi split apart (Salame et al. 2006). The remaining 10/46 times the branch was an offshoot of the descending hypoglossal. All other infrahyoid muscles, including the omohyoid, sternohyoid, and sternothyroid muscles, were innervated by offshoots of the descending hypoglossal or descending cervical nerve. An image of the most common nerve branching pattern and the elevator muscles is displayed in Figure 1-4.

Figure 1-4: Nerve tracts of the Hypoglossal and First Cranial Nerve as they merge and branch to innervate the submental and infrahyoid muscles.

Recurrent Laryngeal Nerve The Intrinsic laryngeal muscles (ILMS) that control the position of the vocal folds are innervated by the Recurrent Laryngeal Nerve (RLN). The RLN is a bilateral branch of the vagus nerve (Cranial nerve X) that “recurs” from the thorax, creating a strange anatomical quirk that is a byproduct of evolution and embryologic development

(Perlman and Schulze-Delrieu 1997). The vagus nerve exits the skull and then travels inferiorly to innervate the heart, lungs, stomach, and other internal structures. The RLN does not form until the Vagus has bypassed the larynx and entered the chest, adding unnecessary length to the RLN. The branch from the left vagus nerve wraps around the aorta and travels superiorly adjacent to the trachea and esophagus in the trachea- esophageal groove. On the right side the branch wraps around the right subclavian artery and then “recurs” to the larynx. Because the right subclavian artery is superior to the aorta, the right RLN has a shorter length (7cm vs. 13 cm) (Liebermann-Meffert et al.

1999). The RLN at its branching point is at its furthest distance from the trachea and 18

esophagus, moving closer as it nears its insertion point at the larynx. As stated by

Liebermann-Meffert, “[The RLNs] never lay in a straight line in the groove between the trachea and the esophagus, until just before entering the pharynx.” In addition to the positional changes, the RLN changes in diameter as it travels superiorly, reducing from

3.4 to 1.8 mm from branching point to laryngeal entry on the left as smaller branches to the esophagus and trachea branch off, and reducing from 3.0 to 1.2 mm on the right side (Liebermann-Meffert et al. 1999).

Because the RLN innervates three of the muscles that induce vocal fold adduction and only one muscle that induces vocal fold abduction, gross stimulation of the RLN induces a net vocal fold adduction. Stimulation trials in canines demonstrated that bilateral RLN stimulation caused a thickening of the vocal fold and an 11.27% decrease in vocal fold length (Titze, Jiang, and Lin 1997), primarily due to contraction of the thyroarytenoid.

The recurrent laryngeal nerve tracts can be tracked from the RLN branching point back up through the vagus as a discrete group. The RLN itself is a monofascicular nerve until immediately before branching into the larynx. Tracking of the nerves to the larynx through the RLN show a few specific patterns: Most of the fibers are motor, almost all the fibers leave the brainstem in the most rostral rootlet of the vagus nerve, and these motor fibers form a discrete group in the trunk of the vagus prior to branching into the RLN (R R Gacek, Malmgren, and Lyon 1977). Using horseradish peroxidase studies, it was found that there are about twice as many adductor nerve

fibers than there are abductor nerve fibers in the RLN, which is logical because there are

more muscles that are considered to be adductors. While the authors noted that a

majority of each muscle's fiber appeared on one side of the RLN in their samples, the

fibers are interspersed throughout the nerve and not grouped functionally. Electrical

stimulation would likely activate a broad swath of functional groups simultaneously.

Toward this end, “any attempt at providing selective adduction of abduction function in

a specific muscle must take place at the level of the muscle and its neuromuscular junction” (Richard R. Gacek 2001).

Functional Electrical Stimulation for Laryngeal Elevation Functional Electrical Stimulation (FES) is the application of electrical current to

the nerves or muscles of the body to induce functional effects such as movement or

sensation. FES has been used for diverse applications such as enabling motion after

paralysis, sensation of amputated limbs, pain relief, and bladder control (Peckham and

Knutson 2005). When the muscles of the neck have weakened or lost their nervous

system control due to disease or incident, FES has the opportunity to strengthen or re-

create motions that are not functioning effectively. Electrical stimulation systems have

been applied to the neck to reenable two of the primary protective mechanisms of

swallowing: laryngeal elevation and vocal fold adduction.

Surface Stimulation for Laryngeal Elevation Electrical stimulation of the laryngeal elevators, primarily the geniohyoid,

mylohyoid, and thyrohyoid muscles, has been tried with two methods in humans: using

either surface stimulation or implanted intramuscular electrodes. Due to the invasiveness of intramuscular stimulation, only surface stimulation is used clinically. The most commonly marketed surface stimulation device is VitalStim, produced by DJO

Global (Vista, CA, USA). VitalStim uses surface electrode patches placed over the neck, adhered over the thyrohyoid or submental muscles. Current is applied using a battery powered, hand-held stimulator. The device is approved for use during eating or as a rehabilitation exercise therapy, strengthening the muscles by repeated use (Kiger,

Brown, and Watkins 2006).

Tests of surface stimulation for laryngeal elevation have shown varying results.

The most useful are a series of studies by Humbert et al. in which electrical stimulation was applied to ten different positions on the neck while neck movement was analyzed

(Humbert et al. 2006). Videofluoroscopic video tracked the hyoid bone and subglottic air column during stimulation. Stimulation induced significant depression of the larynx and hyoid during rest and swallows. Their recordings showed that during normal swallowing, the larynx elevates approximately 20 mm and the hyoid bone moves about

5 mm anteriorly. Surface stimulation, however, caused approximately 10 mm of descent when electrodes are placed over the larynx. This counter-swallowing motion would likely impede swallowing, and using the NIH-SSS, the stimulated swallows were judged to be significantly less safe than non-stimulated swallows. Figure 1-5 below shows that stimulation caused a net movement of 8 mm of hyoid depression and 5mm of laryngeal depression during swallowing. This ratio means that the thyrohyoid and larynx were drawn together about 3 mm.

Figure 1-5: A change in peak elevation (mm) for laryngeal and hyoid bone vertical displacement. Data above zero indicate that the peak laryngeal or hyoid bone elevation was higher in stimulated swallows than in non-stimulated swallows. (Humbert et al. 2006)

In a review article, Logemann discussed the clinical acceptance of VitalStim and the lack of clinical research that had been performed prior to release. She posits that the depression is a result of electrical stimulation activating the sternohyoid muscle, a muscle that connects the hyoid to the scapula and causes depression of the hyoid. This muscle is in close proximity to the thyrohyoid and, when stimulated, overpowers any elevation induced by thyrohyoid contraction (Logemann 2007). Baijens et al. found that stimulation and different electrode positions caused no significant change in the magnitude of hyoid motion (Baijens et al. 2012). Surface stimulation, even when applied by a trained professional, does not reliably add to laryngeal elevation.

Intramuscular Stimulation for Laryngeal Elevation

Surface stimulation is a minimally invasive method of applying stimulation, but it

does not provide targeted access to specific muscles at an amplitude that produces

laryngeal elevation. The next level of invasiveness in electrical stimulation is temporary

implantation of electrodes into the muscles, known as intramuscular electrodes. Wire

electrodes are implanted through the skin using a needle, while the electrode leads

remain outside the body for application of stimulation and to allow retraction of the

electrodes at the end of a study. No surgery is required, but a mild anesthetic is usually

applied to the insertion site.

Burnett et al. performed a stimulation study in healthy subjects and recorded

the resulting larynx motion (Burnett et al. 2003). The mylohyoid, geniohyoid, and thyrohyoid muscles were chosen for implantation because of their expected resulting motion and their accessibility for surgical implantation. The stylohyoid was excluded because of location, the anterior belly of the digastric was excluded because it is not activated in 25% of swallowers and contraction can lower the jaw.

After a small application of lidocaine to the insertion site, needle electrodes were inserted into the muscles to implant hook-wire electrodes which were then

stimulated using Nicolet Viking IV system (Madison, WI). Currents were applied at a

level below the subjects’ pain threshold (3-6 mA, 200us, 30 Hz) while video motion was

recorded, allowing for video capture of the thyroid prominence. The video was analyzed

by manually marking the location of the thyroid prominence over time on a computer

screen, and the resulting data was lowpass filtered at 3 Hz. This lowpass frequency is

lower than one would expect as appropriate as a significant amount of the motion

occurs at a higher frequency.

The magnitude of elevation was calculated by measuring the difference in mean position between the resting state and mean of maximum difference, and velocity was calculated by using the peak of the first derivative of the filtered data. Because the natural swallow has a peak motion that is maintained for a very short duration, the peak location is measured from three data points. Because stimulated tetanic motion can be sustained for an extended time, the maximum motion was measured by obtaining the mean of 500 milliseconds or 30 data points. The averaging duration of half of a second is long, but allows for accurate measure of sustained closure, especially when the motion is truly sustained. The manual measurements of thyroid location and change in distance were performed independently by a pair of examiners to allow for intraexaminer reliability calculations to obtain accurate measurements.

Burnett measured that unilateral stimulation of a single muscle created an average of 5.08 ± 3.81 mm of laryngeal elevation, and paired muscle stimulation created an average of 8.90 ± 5.50 mm of laryngeal elevation, both significantly less than the

17.56 ± 4.17 mm of motion induced during a natural swallow. Paired muscle stimulation caused significantly more elevation than individual muscle elevation, but neither was able to replicate the natural swallow. Velocity was also significantly faster with paired muscle stimulation than single muscle stimulation, but did not match natural swallowing.

Paired muscle stimulation was able to replicate ~50% of natural swallowing, and

the discussion states that this should be “adequate” for assisting laryngeal elevation in

patients with dysphagia (Burnett et al. 2003). Whether this is sufficient to assist

swallowing is not actually studied in this paper, but should be in the future. There was

variety in elevation amounts between participants and the muscle stimulated. The

authors cite the example of the thyrohyoid, which caused the most elevation in one

subject but the least in another. This is explained as a result of electrode placement or

normal variation in anatomy. The final conclusion of this data is that if one wants to replicate the natural swallow, single or paired intramuscular stimulation is not sufficient to cause the full amount of motion. The magnitude of elevation caused by paired stimulation is only equivalent to about half of the natural swallow, so a new method of stimulation needs to be used to cause double the effect of paired intramuscular stimulation.

Nerve Stimulation for Laryngeal Elevation If electrical stimulation through intramuscular electrodes is insufficient, then a more invasive method would be likely to add muscular contraction. Electrodes placed on a nerve have the capability of activating the muscles that are innervated by that specific nerve, and at a lower current than required for intramuscular or surface stimulation (Peckham and Knutson 2005). Nerve electrodes vary in invasiveness from extraneural to intraneural to intrafascicular to sieve electrodes, with a generalization that the more invasive an electrode is, the more selective of specific axons it will be. The

25 closer the electrode is to the axon, the lower current will be required to activate that specific axon.

The Flat Interface Nerve Electrode, or FINE, is an extraneural electrode that reshapes a nerve into a configuration that increases the surface area of a nerve, separating the fascicles and bringing them to the surface (D.J. Tyler and Durand 2002).

FINEs are used for obtaining selectivity of desired muscle groups in one single nerve.

Each of the contacts on a FINE will activate the axons closest to it first as current is increased, meaning multiple contacts will each cause different responses. Combinations of electrodes can be stimulated to generate combined activations, resulting in functional effects from activation of specific muscle groups at specified amplitudes, pulse widths, and frequencies.

Yoo et al. researched electrode stimulation of the hypoglossal nerve to selectively stimulate the muscles of tongue motion, specifically the genioglossus, styloglossus, and hyoglossus muscles. The genioglossus causes tongue stiffening and protrusion, while the styloglossus and hyoglossus result in tongue retraction. Activation of these muscles will decrease upper airway resistance for improved breathing to prevent obstructive sleep apnea (Paul B. Yoo and Durand 2005). Yoo et al. placed a FINE electrode on the XII/C1 nerve proximal to the geniohyoid branching point. Additional electrodes were placed on each of the distal branches: geniohyoid, genioglossus, and combination styloglossus and hyoglossus. Yoo et al. stimulated each of the branches and the main trunk of the nerve individually while recording electromyography from the

individual muscles and electroneurography from the FINEs on the nerve branches , seen in Figure 1-6 (P. B Yoo and Durand 2005).

Figure 1-6: Diagram of the distal XII/C1 nerve complex as studied by Yoo et al. Figure 6 from Yoo et al 2004.(Paul B. Yoo, Sahin, and Durand 2004) By applying current to specific electrodes, the resulting activity of each of the muscles or nerve branches can be recorded, compared, and used to calculate the selectivity of that specific contact (P. B Yoo and Durand 2005). Selectivity was calculated as a ratio of normalized target muscle response and the summated normalized response of all of the recorded muscles, targeted and non-targeted. Yoo et al. also set thresholds for electrode significance: the targeted muscle had to activate at least 70% of the muscle’s maximum activation, and the non-targeted muscles required activation level less than 30% of their normalized maximum. Each of the twelve contacts were stimulated and the resulting output was rectified, integrated, and normalized to generate recruitment curves, which was then used to calculate selectivity for given stimulus levels. The FINE electrode was able to selectively stimulate each of the

27 branches of the FINE and the muscles, allowing selectivity levels greater than 0.87 for each branch (Paul B. Yoo, Sahin, and Durand 2004). Yoo’s hypothesis that the genioglossus could be selectively stimulation was proved, and additionally it was shown that a FINE at this location would be able to selectively stimulate the Geniohyoid muscle, enabling laryngeal elevation. Applying stimulation of each of these branches showed that airway resistance is reduced a significant amount under stimulation of genioglossus, styloglossus, and hyoglossus, or the whole hypoglossal nerve, which activates these muscles and the geniohyoid (Paul B. Yoo and Durand 2005).

Functional Electrical Stimulation for Vocal Fold Control The intrinsic laryngeal muscles, which are innervated by the recurrent laryngeal nerve, control the area of the glottis. Gross activation of these muscles results in net vocal fold closure, sealing the airway and preventing inspiration, expiration, and aspiration. The electromyography (EMG) activity of the intrinsic laryngeal muscles has been shown to be linearly related to change in subglottic pressure, signifying a relationship between EMG and vocal fold area (Nasri et al. 1995). Electrical stimulation has been applied experimentally and clinically to the laryngotracheal complex to affect the positioning of the vocal folds for use in inspiration, vocalization, and deglutition.

Stimulation of the intrinsic laryngeal muscles is performed through surface stimulation and nerve cuff stimulation; Intramuscular stimulation is not an acceptable choice due to the small size and positioning of the muscles.

Surface Stimulation of the Vocal Folds In addition to inducing laryngeal elevation, the creators of VitalStim claimed in the device's manual that transcutaneous stimulation with their device can assist with true vocal fold closure during swallowing. The intrinsic laryngeal muscles, however, are very deep, and Humbert recently showed that surface stimulation can cause hyoid depression, suggesting the sternohyoid is being activated (Humbert et al. 2008). The purpose of this study was to determine if vocal fold closure was induced under surface stimulation. Stimulation with the VitalStim device was performed at 80Hz with pulse width of 700us, pulse amplitude ranged from 0 to 25 mA. Bipolar surface electrodes were placed on the skin in one of ten different placement patterns under the neck. A laryngoscope was used to visualize the vocal folds, and the average angle of vocal folds was measured.

Only one location caused a significant adduction of the vocal folds, and that was only 2.8 degrees of adduction while the mean resting angle was 59.2 degrees, but this

4.7% reduction in area is likely not clinically meaningful (Humbert et al. 2006). Another location induced a non-significant 2.3 degrees of abduction. Stimulation up to 8.0 mA was used, and while higher amplitudes may cause more contraction, patient tolerance became an issue. The authors concluded that surface electrical stimulation is not a

viable method of controlling vocal fold motion due to the minimal change and the

potential for hyoid and laryngeal depression under stimulation.

Though surface stimulation over the intrinsic laryngeal muscles has been shown to be ineffective for controlling the vocal folds, other targeted methods of surface stimulation may prove to be more effective. Sanders et al. applied a stimulation probe to the surface of an anesthetized canine neck close to the tracheoesophageal groove, the expected location of the recurrent laryngeal nerve (Ira Sanders et al. 1987). Pressure was applied to move the probe as close as possible to the RLN to potentially reduce the required current to induce motion. Laryngoscopy was used to view and record the vocal folds. Frequencies from 10-40 Hz caused vocal fold abduction of approximately 3 mm, while frequencies 50Hz and higher caused vocal adduction. Stridor occurred at frequencies above 60 Hz, signifying that the glottis was effectively obstructed. Direct stimulation of the vocal folds performed subsequently gave identical results. The frequency results indicate that the laryngeal muscles have innate differences in muscle type resulting in variable activation. Sanders et al. hypothesize that the muscles of laryngeal abduction are fast-twitch muscles that will reach tetany at lower frequencies, whereas the muscles of adduction are slower-twitch muscles that reach tetany at a higher frequency and then overpower the vocal fold abductors (I Sanders, Aviv, and

Biller 1986). The frequency modulation capabilities of the recurrent laryngeal nerve provide opportunity for a single electrode to selectively control vocal fold motion for both adduction and abduction. These results were repeated in the monkey neck:

Stimulation from 10 to 30 Hz caused abduction, while frequencies above 30 Hz caused adduction, with maximal adduction occurring at 100 Hz (I Sanders et al. 1987). Unilateral and bilateral stimulation was applied in this study, with the result of inducing total

airway occlusion with bilateral stimulation that could not obtained with unilateral

stimulation. The placement of the targeted surface electrodes in these studies is critical,

as improper positioning will result in activation of laryngeal strap muscles, causing

laryngeal elevation or depression. Motion of the larynx and trachea will result in motion

of the targeted nerve, altering the effectiveness of the electrodes and current

stimulation parameters. Furthermore, surface stimulation requires placement by a

trained physician and constant monitoring for accuracy.

Implanted Stimulation of the Recurrent Laryngeal Nerve Because surface stimulation is insufficient to regularly activate the RLN,

implanting electrodes near to or onto the RLN is necessary for chronic application.

Studies of RLN stimulation using direct stimulation with probes or nerve cuffs have been

performed for altering glottal area for speech, inspiration, and swallowing. Direct nerve

stimulation with an implanted cuff alters the fundamental frequency of a given note

voice by tensing the thyroarytenoid muscle, thickening the vocal folds (Broniatowski et al. 2008).

In vocal fold paralysis, the paramedial positioning of the vocal folds causes increased resistance in the airway, preventing free flow of oxygen to the lungs.

Abduction of the vocal folds is primarily caused by contraction of the posterior cricoarytenoid, which causes rotation of the arytenoids to bring the vocal folds together.

Zealear et al. applied stimulation to the bilateral canine posterior cricoarytenoid using deep brain stimulation electrodes implanted into the muscle near to the RLN insertion point. Stimulation was shown to increase vocal fold area on anastamosed nerves

(Zealear et al. 2009). There was a gradual decrease in unstimulated glottal area,

signifying that the glottis was becoming more medialized over time as the muscles

adjusted in tone following transection and anastamosis. Adductors had increased tone

after reinnervation. During breathing, stimulation caused and maintained increased vocal fold area, which decreased resistance of the airway and allowed increased flow.

Bilateral stimulation caused immediate improvement of exercise tolerance, while unilateral stimulation did not allow for improved exercise tolerance until the nerve had

reinnervated the muscle. These results were confirmed in chronic implants of paralyzed

canine larynges (Nomura et al. 2010), increasing the glottal area and allowing the

canines to perform exercise for significantly longer amounts of time.

Implanted nerve cuffs on the vagus nerve or the recurrent laryngeal nerve

branch of the vagus induce vocal fold motion. Vanschandevijl et al. performed unilateral

implantation of a helical electrode upon the recurrent laryngeal nerve in horses.

Laryngoscopic analysis was performed by measuring the angle between the midline and

the ipsilateral epiglottal cartilage. Stimulation currents were in the 1 mA and 250

microsecond pulse width range, much lower than the 8mA required for surface

stimulation in the smaller canines (Humbert et al. 2006). Stimulations performed at 25

Hz induced sustained abduction of the vocal folds (Vanschandevijl et al. 2011).

A chronic study of implanted nerve electrodes in humans has been performed to measure the effect of RLN stimulation on aspiration and dysphagia. Stimulation was applied in pulses at 1.2 mA and pulse widths from 188-560 microseconds at a frequency

of 42 Hz. Laryngoscopic video of the vocal folds was recorded, and subjects were

provided with barium-dosed foods and liquids while recording videofluoroscopy.

Analysis of the laryngoscopy showed significant vocal fold adduction for the subjects

(Broniatowski et al. 2010). One notable result of the study is that in the four subjects

that obtained complete glottal closure due to stimulation, there was no recurrence of

pneumonia for the duration of the study.

Transtracheal/Transesophageal stimulation Surface stimulation for vocal fold closure has been shown not to be clinically

effective, mainly due to poor selectivity and specificity. Because surface stimulation of

the RLN is not a reliable option for humans, it is theorized that stimulation could be

applied to the recurrent laryngeal nerve by "surface" stimulation of the tracheal or

esophageal wall. Properly placed electrodes could use transtracheal or transesophageal

stimulation to activate nerves that lie on these structures. The RLN lies in the

tracheoesophageal groove, a position abutting both the trachea and esophagus lateral

to the place they touch. In 10 anesthetized canines, Sanders et al. inserted a blunt probe

with an electrode into a tracheostomy while current was applied. Video of the vocal

folds was measured to determine the activation levels of the intrinsic laryngeal muscles.

Stimulation was repeated in the esophagus by orally placing the probe to a position that

activated the RLN through the esophageal mucosa. The lowest threshold for

transtracheal stimulation was found across the posterior-lateral wall of the tracheal 1

cm inferior to the cricoid cartilage. Current amplitudes as low as 1.0 mA were necessary for transtracheal stimulation, while 2.0 mA was required for transesophageal

stimulation (Ira Sanders et al. 1987). These current levels are expected to be clinically tolerated as prior studies have shown that stimulation of vaginal mucosa has been tolerated up to 30mA and “caused no mucosal damage” (Kraus et al. 1987). Berke et al.

performed both transtracheal and direct nerve stimulation and determined that 10

times as much voltage was required to induce vocal fold motion using transtracheal

stimulation than direct nerve stimulation (Berke et al. 1988). Vocal fold position was

dependent on the frequency applied, similar to responses given using transcutaneous

stimulation or direct nerve stimulation.

The authors discuss the clinical applications of this research, deciding that

transesophageal stimulation has some true capabilities for laryngoscopic analysis,

including during anesthesia. The researchers developed an indwelling device for

transesophageal stimulation of the RLN. Their device applies stimulation in a superior

location such that the only nerve being stimulated is the branch of the RLN to the

posterior cricoarytenoid. The tested Laryngeal Electrode Platforms were able to control

the posterior cricoarytenoid with levels as low as 3 mA and were able to stimulate the

main trunk of the RLN with inferior electrodes on the device. The device had to be

specifically made to fit each individual subject, which required anaesthesia just to

produce the mold.

The researchers discuss the use of transtracheal stimulation in patients who

have undergone tracheostomy to aid determination of the source of vocal fold paralysis.

Transtracheal stimulation would allow for determination if the RLN is still viable or

whether the problem is in the mechanics of the vocal fold, such as cricoarytenoid

arthritis (Ira Sanders et al. 1987).

Stimulation Controller Current stimulation devices for dysphagia are controlled by a hand-held device.

Stimulation is applied after a button is pressed by the user or their caretaker, and is

applied either for a preset amount of time or until the button is pressed again. Laryngeal

elevation devices like VitalStim require the user to attach the electrodes and ensure the

leads to the electrodes are not pulled during stimulation. The RLN stimulation system

used in Broniatowski’s studies requires proper placement of a transmitter over the

chest, which is help or taped into place. If a user desires to eat, they have to insert the

food into their mouth, apply stimulation, swallow, turn off stimulation, and repeat.

While manual control of the device requires conscious thought and can help the

dysphagic focus on their swallowing, it complicates the process and adds potential for

error.

Burnett et al. performed combined stimulation and control experiments to determine whether healthy adults could trigger intramuscular stimulation in synchrony with volitional swallows (Burnett et al. 2005). Electrodes were inserted into the thyrohyoid and mylohyoid muscles bilaterally. Stimulation parameters were 200us pw,

0.5-6.0 mA, 30 Hz. External video recordings, EMG, button pressing, and movement of the thyroid were measured. Stimulation duration was either 1 or 2 seconds. Natural mylohyoid activity onset preceded laryngeal elevation by 345 ms, and the thyrohyoid

35 activity onset preceded laryngeal elevation by 52 milliseconds. When the subjects controlled their own stimulation, the application of stimulation occurred 256 ms after mylohyoid activity onset. The authors describe this pattern as appropriate because stimulation of the thyrohyoid would occur prematurely if the stimulus was applied prior to mylohyoid activation. They determine that allowing personal decision to determine the best onset time of stimulation will be the greatest benefit, assuming the user has adequate oral sensor, limb motor, and cognitive function to make the decision and execute the swallow.

FES devices for other parts of the body such as the arm or leg use some sort of natural recording paradigm to automatically determine when and how much they should activate. To control motion of the arm, it is inefficient for the system to require a functional arm to push buttons to activate the non-functional arm. A closed-loop, self- contained system is optimal because it requires minimal conscious input to activate the system and thus is able to be used “naturally”, or as close as possible to replicating the original function. To improve FES devices for swallowing, a natural controller needs to be developed that could automatically recognize swallowing prior to the bolus reaching the inferior pharynx, the point when aspiration is likely to happen. To design a controller, a command signal needs to be found from the body that can be recognized in the early stages of swallowing, restricting the signal choice to something from either the preparatory or transport phases of oral swallowing.

Considered signals for further experimentation included Eustachian tube pressure, submental or levator veli palatini muscle EMG, and palatometry. Swallowing generates pressure in the ear during swallowing and yawning, something that has been recorded using an earpiece in the past (Kumazawa, Honjo, and Honda 1977). The reliability of that signal and accuracy is expected to be low in a non-clinical setting, as motion of the sensor and other activities are likely to adjust the pressure as well. The levator veli palatini is a muscle of the soft palate that seals the nasal passage during swallowing (Kuehn and Moon 1994). In conjunction with EMG recordings of other muscles active during swallowing like the genioglossus and masseter, EMG of the veli palatini could serve as a final trigger for activation (Wheeler, Chiara, and Sapienza 2007).

Recording EMG from the veli palatini would require implantation of an electrode into the muscle. This procedure would leave an electrode lead exiting the body likely through the nose or mouth, and would not be aesthetically pleasing to the user and add complications.

Palatometry Palatometry is the recording of pressure by the tongue on the hard palate of the mouth. Palatometry serves as a measure of oral activity by measuring tongue pressure against specific points on the roof of the mouth. Sensors can be adhered to the surface of the mouth or part of a removable mouthpiece, enabling recordings to be made for clinical testing, exercise, or research (Ono et al. 2009). Existing clinical devices such as those produced by Iowa Oral Pressure Instruments generally function using air pressure bulbs (Pouderoux and Kahrilas 1995). The bulbs are placed into the mouth and held or

glued into position and then stimuli are provided to the user. A tube from the bulb exits

the mouth and connects to a barometer that records the pressure induced by the

tongue (Nicosia et al. 2000). Similar pressure sensors can be placed into the cheeks to

measure labial pressure (Engelke, Jung, and Knösel 2011). Studies using pressure bulbs

have shown the relationship between increased pressure and increased bolus size, user

emphasis, and submental EMG (Yeates, Steele, and Pelletier 2010).

Youmans et al. performed studies with air bulb palatometry to determine the pressure generating capability of the tongue. Subjects were instructed to apply maximal isometric pressure against the bulb with their tongue or provided with a bolus and told to swallow. Isometric pressure application generated an average of 59.97 kPa (standard deviation 13.62, range 32 to 94 kPa) (Youmans and Stierwalt 2006). It was found that a swallow generated an average of 30.48 ± 13.41 kPa, range 5.67 to 67.33 kPa. A reliable palatometry system would need to be able to measure pressures in this range to effectively report the activity generated in the mouth. Air pressure bulbs are capable of recording these pressures, but they are large and require tubing to exit the mouth, affecting the natural swallow, preventing natural mastication, and resulting in recording of something artificial.

Electric Sensors Electric pressure sensors have much smaller profile than air pressure bulbs, enabling them to fit more naturally in the mouth. Wires extending from electrical pressure sensors are thinner and more flexible than tubing, enabling easier exit from the oral cavity around the posterior molars. This allows the user to fully close their mouth

without stopping data transmission. Additionally, with the smaller size, more can fit into

the mouth. Ono and Hori developed a mouthpiece with seven pressure sensors spaced

in the oral cavity. The pressure sensors enabled recording across the surface of the hard

palate, enabling much fuller understanding of tongue placement during swallowing

activity. A diagram of the sensor locations is shown in Figure 1-7.

Figure 1-7: Diagram of the pressure sensor locations.(Ono, Hori, and Nokubi 2004) Ten healthy subjects were provided with 15mL boluses of water and pressure

data was recorded. Ono and Hori extracted specific time points and pressure values

including onset, time of max pressure, time off, and maximum magnitude. What was

found was a general anterior to posterior pressure onset wave, characterized by

activation first of Sensor 1, then Sensor 2, then Sensor 3. This pattern was replicated in

the timing of maximum magnitude, as the peaks followed the anterior to posterior

pattern, seen in Figure 1-8. Sensor 1 recorded a significantly higher magnitude than every other sensor, all of which were about the same (non significant differences). The pressure on the anterior sensor lasted for 0.90 seconds, sensor three (which began 0.25

39 seconds later) endured pressure for only 0.62 seconds, with pressure offset occurring before sensor 1 was released.

Figure 1-8: Timing of onset, maximum, and offset pressure at each of the seven sensors.(Ono, Hori, and Nokubi 2004) In a later study, the same experimental palatal plate with seven sensors was used to record tongue pressure against the roof of the mouth during chewing and resulting swallowing (K. Hori, Ono, and Nokubi 2006). On average there were 25 chews, the amplitude of each slowly increasing over time such that the later strokes created the highest magnitude of palatometry pressure. The data was analyzed by focusing on a few important points in the chewing cycle. Pressure began in the anterior portion of the mouth during the occlusal phase and then extended into the opening phase of the mouth. The magnitude of tongue pressure was significantly highest at channels 1 and 5.

In every subject the tongue pressure was 2.8-6.9 times larger in the later phases of chewing than in the earlier phases of swallowing.

Ono and Hori also recorded pressure in post-stroke patients, instead using a 5- sensor sheet. In stroke subjects during dry swallows, the pressure on the non-paralyzed side was significantly stronger than any other location. The pressure applied on the non- paralyzed side was significantly higher than any of the midline locations also. In the healthy subjects there were no significant differences between any pressure sites

(Kazuhiro Hori et al. 2005). The researchers used a “simpler sensor sheet” that does not have to be individually made for each subject. Their prior required a dental fitting and embedding of sensors into a mouthpiece that fit only that subject. This five sensor device was designed for temporary placement by a trained professional using denture adhesive, which works well for experimental testing. The device is found to be equally effective for recording pressure patterns, but the pressures recorded were lower than those found using the hard mouthpiece, either due to the sensors themselves, the reduction in size, or the existence/absence of a mouthpiece (Kazuhiro Hori et al. 2009).

Positive and Negative Pressure Recording Swallowing is expected to generate negative pressure by the action of the tongue pulling away from other portions of the mouth. This creation of closed compartments is something the air bulb pressure recordings and other pressure sensors are incapable of accurately measuring. Engelke et al placed tubes into the mouth to record oral pressure during the closed mouth swallow. They found that between -50 and -150 mbar of pressure were generated by closed compartments and swallowing

(Engelke, Jung, and Knösel 2011). To measure negative and positive pressure at different

locations simultaneously around the mouth, Keiser and Kennedy developed an

electronic mouthpiece with sensors for recording negative pressure (Kieser et al. 2007).

Kennedy hypothesized that swallowing recordings would show: 1) defined

individual patterns, 2) significant negative pressures, and 3) differences in pressures on

anterior-posterior axis are important for swallowing (Kennedy et al. 2009). Using a metal

device with 8 sensors, three of which are on the hard palate, Kennedy recorded bolus

propulsion during swallowing at three points along the midline of the palate. All three

sites had an initial brief sharp drop in pressure during swallowing. The recordings

showed that the anterior sensor generated mostly negative pressure during swallowing,

never showing any positive pressure in some subjects. This is contrary to the research by

Ono and Hori, which showed that there was significant positive pressure in the anterior mouth, to higher amplitude than any other sensor.

Analysis was done by measuring the difference between adjacent sensors and

also by taking the derivative of the patterns to determine changes in pressure. The delay

in pressure onset was measured as well. Pressure wave onset began anteriorly, as

expected, but timing of each sensor’s onset was subject and situation dependant.

Integrated root mean square area of the pressure trace was used to measure the

"effort" required to generate the pressure at the location. Pressure profiles within an

individual showed a high degree of reproducibility between visits (intra-subject

variability was low), while inter-subject variability was higher. The group concludes that

"It is thus questionable that there will ever be "norms" of sequential activity, a

conclusion that would have a profound influence on clinical rehabilitative thinking"

(Kennedy et al. 2009).

Project Specific Aims The main objective of this research is to develop a combined laryngeal elevation

and vocal fold closure stimulation system, and to develop a control source for a natural,

closed-loop controller to activate an FES device for dysphagia. This research is composed of two specific aims.

Assess the capabilities of transtracheal stimulation and hypoglossal stimulation for replicating the protective reflex motions of the vocal folds and larynx.

Our first aim is to determine whether two novel mechanisms of stimulation for airway protection are as effective as existing stimulation paradigms and what impact paired stimulation has upon the results of either method alone. Our hypothesis is that transtracheal stimulation will allow targeted vocal fold closure, hypoglossal nerve stimulation will produce more laryngeal elevation than intramuscular stimulation, and that vocal fold closure and laryngeal elevation can be performed synchronously.

In Chapter Two we evaluate hypoglossal nerve stimulation as an alternative

method of artificially generating laryngeal elevation. We hypothesize that a single cuff

implanted upon the hypoglossal nerve will at least replicate the laryngeal elevation that

is produced by intramuscular stimulation. The common hypoglossal, its branches, and

the elevatory muscles in the canine model are stimulated while movement of the larynx

and EMG of the submental muscles are recorded.

Chapter Three discusses our evaluation of transtracheal stimulation as an innovative, minimally invasive method of initiating vocal fold closure. Electrodes were

placed on the wall of the canine trachea to provide minimally invasive stimulation to the

RLN. The optimal location and capabilities of stimulation of this electrode was studied by recording electromyography (EMG) of the intrinsic laryngeal muscle activity and observing vocal fold motion via laryngoscopy. Additionally, we simultaneously

stimulated for both elevation and vocal fold adduction using hypoglossal nerve stimulation as discussed in Chapter Two and transtracheal stimulation discussed in

Chapter Three.

Develop a system that reliably detects swallowing for use as a control source in an FES

system.

To develop a closed-loop controller, a sensitive and specific control source is

needed to automatically detect the initiation of a swallow. We hypothesize that the

force of the tongue against the hard palate during the oral transition phase of

swallowing creates a detectable pattern that we can record that is a specific and

sensitive for a swallow event and could be implemented to activate an FES device.

Chapter four discusses this research, in which we developed a palatometer for recording a series of positive and negative pressures in the mouth. Recent studies have shown that significantly higher positive and negative pressures are produced in the mouth during deglutition than other oral activities. Recordings of pressures on the palate, the EMG of the submental muscles, and breathing patterns were made from

healthy and dysphagic subjects while performing oral activities, including swallowing

various consistencies of food, speech, chewing, and coughing to determine the specific

patterns that occur only in swallowing.

Using the recordings, we then optimized an algorithm that specifically and

sensitively recognizes the oral transport phase of swallowing. The pressure patterns

were analyzed to create a time delay artificial neural network that was trained for identification of deglutition and could be used as a control source for laryngeal stimulation for dysphagia.

Chapter 2 - Laryngeal Elevation by Selective Stimulation of the Hypoglossal Nerve

The material in this chapter has been submitted to the Journal of Neural Engineering.

Abstract Objective: Laryngeal elevation protects the airway and assists opening of the esophagus during swallowing. The geniohyoid, thyrohyoid, and mylohyoid muscles provide a majority of this elevatory motion. This study applied functional electrical stimulation to the XII/C1 nerve complex using a nerve cuff electrode to determine the capabilities of neural stimulation to induce laryngeal elevation.

Approach: Multi-contact FINE electrodes were implanted onto the XII/C1 nerve complex at locations proximal and distal to the thyrohyoid branching point in 5 anesthetized canines. Motion of the thyroid cartilage and the hyoid bone was recorded during stimulation of nerve cuffs and intramuscular electrodes.

Main Results: Nerve stimulation induced 260% more laryngeal elevation than intramuscular stimulation (18.8 mm vs. 5.2 mm, p<<0.01), and 228% higher velocity

(143.8 vs. 43.9 mm/s, p<<0.01). While stimulation at all cuff and electrode locations elevated the larynx, only the proximal XII/C1 nerve cuff significantly elicited both thyroid-hyoid approximation and hyoid elevation. In all proximal XII/C1 nerve cuffs (n =

7), stimulation was able to obtain selectivity of greater than 75% of at least one elevatory muscle.

Significance: These results support the hypothesis that an implanted neural interface system can produce increased laryngeal elevation, a significant protective mechanism of deglutition.

Introduction Dysphagia, defined as difficulty swallowing, affects 10 million people a year in the United States and is diagnosed in 37 to 78% of stroke victims (Martino et al. 2005).

During healthy swallowing, many protective reflexes are evoked to guide the bolus into the esophagus. These mechanisms include, but are not limited to, oral collection, velopharyngeal closure, epiglottal deflection, swallowing apnea, esophageal relaxation, superior and anterior movement of the larynx, and vocal fold adduction (Medda et al.

2003). Errors in the swallowing reflex lead to aspiration: the ingestion of foreign material past the vocal folds and into the lungs. Aspiration pneumonia, infection of the lungs caused by aspirated bacteria, is the leading cause of death following the initial injury for stroke patients (Heuschmann et al. 2004) and Parkinson’s disease patients

(Troche et al. 2010). To prevent aspiration, at-risk patients are first prescribed a blend of rehabilitation and specialized diet. If these changes are insufficient, invasive surgical interventions like tracheostomy and PEG tube implantation are applied, resulting in a reduction in quality of life (Blitzer, Brin, and Ramig 2009). The inability to engage in the social aspects of eating and drinking results in many dysphagic patients ‘cheating’ on their restrictions. There is a need for an advanced treatment that is less severe than surgical alterations if the simple rehabilitative therapy is insufficient.

Functional electrical stimulation (FES) is the application of electrical pulses for activating healthy nerves and muscles that have partial or complete loss of central nervous system control. FES is used in the neurologically impaired to restore walking, grasping, bladder control, and many other functions (Peckham and Knutson 2005).

Electrical stimulation can be applied to the skin surface, to the muscle, or directly to the

nerve. Stimulation is more selective and requires lower current when the electrodes are

closer in proximity to the individual axons. Dysphagia due to paralysis of the head and

neck can be treated by application of FES. In the literature, FES has been used for

reanimating and strengthening two of the protective mechanisms that occur during the

swallowing reflex: vocal fold adduction and laryngeal elevation.

Vocal fold adduction creates a glottal seal, preventing bolus flow to the trachea

during swallowing. The vocal folds are situated in the larynx, connecting the thyroid and

arytenoid cartilages. The glottic seal is caused by bilateral contraction of the lateral

cricoarytenoid, thyroarytenoid, interarytenoids, and cricothyroid muscles, four of the

intrinsic laryngeal muscles (D J Tyler 2007). The recurrent laryngeal nerve (RLN) innervates the intrinsic laryngeal muscles, and stimulation of the RLN causes net vocal fold adduction, sealing the glottis (Broniatowski et al. 2001). Fluoroscopic swallowing

studies verified the clinical application of stimulated vocal fold adduction by preventing

the aspiration of swallowed boluses (Broniatowski et al. 2010).

Another important protective mechanism during the swallowing reflex is

laryngeal elevation. Contraction of the geniohyoid, mylohyoid, and thyrohyoid muscles

shifts the larynx into an anterior and superior position shielded by the reflected

epiglottis (Shaw et al. 1995). The thyrohyoid muscle, which connects the thyroid

cartilage to the hyoid bone, approximates these structures together, resulting in

elevation of the larynx (Hong et al. 1997). Tension of the geniohyoid and mylohyoid

muscles pulls the hyoid bone anteriorly, drawing the larynx superiorly when the

thyrohyoid maintains isometric length or contracts. The anterior motion, combined with

relaxation of the esophageal sphincter, leads to opening of the esophagus and

expansion of the pharynx. Burnett et al (Burnett et al. 2003) determined that

stimulation with intramuscular (IM) electrodes of either the suprahyoid or infrahyoid

muscles replicates about 30% of the elevation that occurs during a natural swallow, and paired intramuscular stimulation improved net laryngeal elevation to 50% of a natural swallow. Kagaya demonstrated with intramuscular electrodes that sequential activation of the geniohyoid followed by the thyrohyoid, as occurs naturally, resulted in increased laryngeal elevation compared to simultaneous stimulation (Kagaya et al. 2011).

While intramuscular stimulation can induce laryngeal elevation, nerve stimulation is known to generate increased contraction of targeted muscles (Horch and

Dhillon 2004). The muscles of elevation are innervated by a pair of nerves: the mylohyoid muscle is innervated by cranial nerve branch V3, while the geniohyoid and

the thyrohyoid muscles are innervated by branches of the first cervical spinal nerve. The

first cervical spinal nerve (C1) merges with the hypoglossal nerve (cranial nerve XII) en

route to the geniohyoid and thyrohyoid muscles, creating the XII/C1 nerve complex

(Figure 1) (Kahrilas 1993; Salame et al. 2006). The hypoglossal nerve innervates the

genioglossus, hyoglossus, and styloglossus muscles, which control the protrusion of the

tongue (Paul B. Yoo, Sahin, and Durand 2004). Stimulation of the XII/C1 nerve complex has been studied for the treatment of obstructive sleep apnea, which could be improved by inducing tongue protrusion (Paul B. Yoo and Durand 2005). Yoo et al demonstrated

that the geniohyoid can be selectively stimulated using a Flat Interface Nerve Electrode

(FINE) (D. J Tyler and Durand 1997) on the XII/C1. Because at proximal locations the

XII/C1 includes axons to both the geniohyoid and thyrohyoid, stimulation should induce laryngeal elevation that could be used to improve swallowing in dysphagia. Activation of the axons to the tongue will unnaturally alter bolus flow during swallowing, so selective stimulation will be necessary to avoid activation of the tongue protruders.

Because both the geniohyoid and thyrohyoid are innervated by the XII/C1 nerve

complex, we hypothesize that functional electrical stimulation applied to the XII/C1

nerve complex will elicit laryngeal elevation. We hypothesize that nerve stimulation will

generate elevation that at least replicates the elevation from intramuscular stimulation.

We also hypothesize that a FINE placed on the XII/C1 will be able to selectively stimulate

the geniohyoid and thyrohyoid muscle without activation of the genioglossus muscle.

Methods

Surgery Five adult canines (11.8 – 17.5 kg) were anesthetized with an initial IV injection of

Pentothal and maintained with 0.5-4.0% ventilated isoflurane through an endotracheal

tube. EKG, blood pressure, blood oxygenation, and breathing rate were continuously

monitored. Normal body temperature was maintained with a circulating water heating

pad while fluids were provided intravenously. All animal care and experimental

protocols were in accordance with NIH guidelines and approved by the Institutional

Animal Care and Use Committee of Case Western Reserve University. 52

The canine was laid in a supine position and a surgical incision was made along

the midline of the neck from the suprasternal notch to the mylohyoid. The incision was

spread laterally to access the XII/C1 nerve complex deep to the digastric muscle

approximately 5 cm lateral to the hyoid bone. Custom-made FINEs were bilaterally

implanted in three locations on the XII/C1 nerve complex near the branching point of

the thyrohyoid muscle (Figure 1). The first electrode was placed proximally to the

branching point (ProxXII). The second electrode (DistXII) was placed distally to the

branching point on the continuation of the XII/C1. The third electrode was implanted on

the branch of the XII/C1 to the thyrohyoid (THXII). The positioning of these electrodes

can be seen in Figure 2-1. A tracheostomy was performed at the fourth tracheal ring and an endotracheal tube was inserted to allow for anesthesia and air flow without restricting larynx and upper airway movement.

Figure 2-1: Lateral view of the significant anatomy in the canine neck. The visual markers are shown at their locations, along with the measured motions typical to laryngeal elevation.

Bipolar pairs of hook electrodes were made from perfluoroalkoxy (PFA)-coated

stainless steel leads (Fort Wayne Metal, Fort Wayne, IN) and bilaterally implanted into

the thyrohyoid (TH), geniohyoid (GH), genioglossus (GG), and mylohyoid (MH) muscles

using a 22-gauge needle at the motor point. The placement of the electrodes into the motor point was confirmed using stimulation and replaced if muscle contraction was not observed. The placement of these electrodes allowed for their use in both intramuscular (IM) stimulation and electromyography (EMG) recording.

Tetanic Stimulation and Video Recording Colored markers were solidly anchored with suture to the surface of the thyroid

cartilage, the hyoid bone, and the arch of the mandible. A video camera (Panasonic

GS300, Osaka, Japan) was placed laterally such that the markers were visible in the

frame with a ruler placed next to the neck for calibration. A digital video recorder

(Panasonic DMR-EH55) captured the laryngeal motion seen through the video camera at

30 frames per second. The video recordings were exported to a computer for movement

analysis in MATLAB (version 2011a). A Crishtronics (Cleveland, Ohio) stimulator (0-5 mA,

0-500 us, 0-50 Hz) or a Tektronix (Beaverton, OR) AFG-3022B pulse generator (0-10 V, 0-

500 us, 0-100 Hz) were used to evoke tetany by stimulating each electrode with biphasic

pulses at frequencies greater than 30 Hz. The necessary stimulation parameters to

induce tetany were found prior to video recording. Tetanic intramuscular stimulation

amplitudes were generally at or below 5 V or 5 mA, while tetanic nerve stimulation

parameters were at or below 1 V or 1 mA. To maintain mouth closure during

54 stimulation, the jaw and snout were wrapped with gauze and anchored during stimulation. Stimulation was performed unilaterally and bilaterally. Each video recording included a period of rest greater than one second, the stimulation period, and return to rest.

Nerve Stimulation and EMG Twitch Response A custom MATLAB (MathWorks, version 7.1) program controlled the Crishtronics stimulator to create pulse amplitude and pulse width modulation recruitment curves.

Biphasic stimulation pulses were applied at 4 Hz and EMG was recorded from the four ipsilateral and four contralateral muscles. An AC –coupled differential amplifier

(Cambridge Electronic Design, Cambridge, UK, CED-1902) filtered EMG signals with a gain of 90-990, a 10 Hz -1000 Hz bandpass, and a 60 Hz notch filter. Data were recorded using a National Instruments (Austin, Texas) DAQCard-6036E data acquisition device at a sampling rate of 2400 Hz. The program recorded, rectified, integrated, and then averaged five EMG twitch responses to each stimulus parameter. The integrated voltage results of each muscle were normalized to the maximum integrated EMG twitch response and the noise floor of that muscle from the entire experiment. Recruitment curves were generated for each muscle and electrode contact.

Video Analysis The recorded videos of tetanic stimulation (frequency >30 Hz) were imported into a custom automated MATLAB program that scanned the video frame-by-frame and extracted x-y locations of the colored markers prior to stimulation, during stimulation, and during the return to resting position. The locations of the markers were converted

55 from pixels to millimeters using the calibration ruler in the video frame, which was used to calculate thyroid-hyoid distance, hyoid-jaw distance, and thyroid-jaw distance, and example of which is displayed in Figure 2-2. A 500 ms window rolling average was used similar to as performed by Burnett (Burnett et al. 2003). Displacement was measured as the maximum change in millimeters of the distance between two markers, whether contraction or expansion. Velocity was measured by finding the peak of the first derivative of the signal position data of the thyroid markers following lowpass filtering at 10 Hz, a frequency chosen to retain the step-like response induced by stimulation.

Additionally, the time required to reach 80% of the maximum change in position of each recording following stimulation application was measured.

Figure 2-2 : A representative recording profile of laryngeal and hyoid movement over time. The initial position of the thyroid cartilage is designated as the origin. Stimulation was applied to the implanted nerve cuff on the XII/C1 at 50 Hz using bipolar 5 mA pulses with a 400 microsecond pulse width.

Selectivity Analysis Selectivity levels were calculated for each muscle to determine whether the nerve cuffs were able to activate the elevatory muscles without causing undesired genioglossus activity (Paul B. Yoo, Sahin, and Durand 2004). An electrode was defined as selective for elevation if the normalized activity of the target muscle, Vn(target), was greater than 0.5 while the normalized genioglossus activation, Vn(GG), was less than

0.2. These activation levels were chosen to ensure that the target muscle was significantly active and the genioglossus muscle was inactive. Selectivity level, S(i), was calculated as the ratio of the target muscle’s normalized activation to the sum of all the ipsilateral muscles’ normalized activation (Paul B. Yoo, Sahin, and Durand 2004).

Selectivity was calculated for each elevatory muscle as the target. Additionally, because 57 a single location could simultaneously activate both elevatory muscles but not the genioglossus, selectivity was calculated for the combined thyrohyoid and geniohyoid muscle pairing when the two elevatory muscles reached threshold at the same stimulation parameters.

Vn(target muscle) If Vn(target)>0.5 and Vn(GG)<0.2), Si()= ∑Vn(target and non-target muscles)

Results

Elevation A total of 96 video recordings were made at supra-maximal stimulation levels that produced full contraction. 34 other recordings were made at sub-maximal levels to generate recruitment curves for the stimulation. Only the recordings at supra-maximal levels were used to measure the magnitude of maximum possible motion.

Net laryngeal elevation is the combination of thyroid-hyoid approximation and hyoid elevation. The largest net laryngeal elevation was produced by stimulation of the

DistXII nerve electrode (18.8 ± 5.9 mm) or the ProxXII nerve electrode (17.4 ± 5.6 mm).

Stimulation of either of these electrodes induced significantly more elevation than any of the other electrode locations, including the intramuscular electrodes (p < 0.01) (table

1). Stimulation of the THXII nerve electrode resulted in a non-significant amount of laryngeal elevation (0.1 ± 4.6 mm, p = 0.95), but did cause contraction of the thyrohyoid, depressing the hyoid toward the thyroid. These results are displayed in Table 2-1

Average Motion Net Laryngeal Hyoid Thyroid-Hyoid [mm (±std)] Elevation Elevation Approximation ProxXII 17.4* (5.6) 9.7*(9.1) 7.7*(6.0) DistXII 18.8*(5.9) 17.3*(6.1) 1.4(6.2) THXII 0.1 (4.6) -4.8† (3.8) 4.8* (2.5) TH IM 3.8* (3.4) -3.1†(3.2) 6.9*(3.5) GH IM 5.2*(2.8) 8.5*(3.4) -3.3†(3.2) MH IM 8.7*(3.1) 12.5*(3.2) -3.8†(0.5) Table 2-1: Average elevatory motion after stimulation of each electrode. Elevation (hyoid and laryngeal) and thyroid-hyoid approximation, motions that occur during the natural swallow, are given positive values. Depression of the thyroid or hyoid and spreading. Depression of the thyroid or hyoid and spreading of the thyroid- hyoid distance, motions that do not occur during swallowing, are negative. * Significant motions replicating swallowing (p < 0.01) † Significant motions counter to swallowing (p < 0.01)

Thyroid-hyoid approximation and hyoid elevation were induced in different

amounts depending on the stimulating electrode and position chosen. Significant

thyroid-hyoid approximation was seen when the ProxXII, THXII, or TH IM was stimulated

(p<<0.01). Significant hyoid elevation was measured when the ProxXII, DistXII, GH IM,

and MH IM were stimulated (p<<0.01). The only stimulation site that caused both

significant thyroid-hyoid approximation and hyoid elevation is the ProxXII (p<0.01). At

the ProxXII location the XII/C1 nerve contains axons to both the supra- and infrahyoid

muscles, unlike any other tested electrode location. Net motion of the hyoid bone and

thyroid cartilage during stimulation can be seen in Figure 2-3.

Motion of the Thyroid and Hyoid under Stimulation 50

30 Hyoid

Thyroid Thyroid and Hyoid Cartilage Position (mm) Position Cartilage and Hyoid Thyroid

-10 Rest ProxXII DistXII THXII TH IM GH IM MH IM

Figure 2-3: Net displacement of the hyoid bone and thyroid cartilage following stimulation of each of the nerve cuffs and intramuscular electrodes. Stimulation of the ProxXII illustrates hyoid elevation (HE) and TH approximation, DistXII only exhibits hyoid elevation, thyrohyoid IM stimulation causes hyoid depression and TH approximation, and geniohyoid IM stimulation shows hyoid elevation and thyroid- hyoid expansion.

We stimulated each nerve cuff and intramuscular electrode pair unilaterally and bilaterally while recording motion. There was no significant difference between unilateral and bilateral stimulation (p>0.05) for any stimulation site, although bilateral stimulation did elicit a non-significant increase in elevation over unilateral stimulation for every electrode placement, shown in Table 2-2.

LE (mm ± std) Unilateral Bilateral ProxXII 17.0 ± 5.6 18.2 ± 5.7 DistXII 18.4 ± 6.1 20.4 ± 5.3 THXII -0.3 ± 4.9 1.1 ± 4.6 TH IM 3.7 ± 4.1 4.1 ± 1.4

GH IM 4.9 ± 3.2 6.1 ± 0.7 MH IM 7.4 ± 3.0 11.4 ± 0.6

Table 2-2 : Net laryngeal elevation induced by unilateral and bilateral stimulation at each electrode location. Though bilateral stimulation induced more laryngeal elevation at each nerve cuff, the increase was non-significant (p>0.05) for each pairing.

Elevation Velocity The electrodes that induced the most laryngeal elevation also induced the

highest velocity, displayed in Table 2-3. The larynx moved at an average peak velocity of

116.5 ± 43.8 mm/s under stimulation of the ProxXII and 143.8 ± 54.1 mm/s under

stimulation of the DistXII, significantly faster than other stimulation locations (p < 0.02).

Average peak velocity across all intramuscular stimulations was 37.1 ± 20.9 mm/s. The

increase in velocity was primarily due to the increase in distance travelled during the

roughly same time period: the markers reached 80% of their final position in 0.25 ± 0.13

seconds during nerve stimulation and 0.29 ± 0.17 seconds during intramuscular

stimulation, a non-significant difference. An average of 0.27 ± 0.14 seconds were

required to reach 80% of each trial’s laryngeal elevation.

Velocity mm/s ± std 10Hz Lowpass LE 3Hz Lowpass LE ProxXII 116.5 ± 43.8 67.2 ± 24.7 DistXII 143.8 ± 54.1 77.1 ± 27.2 THXII 46.0 ± 31.6 20.8 ± 11.4 TH IM 27.8 ± 11.9 16.0 ± 11.2 GH IM 43.9 ± 25.8 19.4 ± 11.5 MH IM 50.0 ± 17.6 31.8 ± 10.8

Table 2-3 : Average peak velocity under stimulation at each of the electrode locations. 3 Hz lowpass filtered signals are significantly slower than 10 Hz filtered velocity measurements (p<0.05).

Selectivity The goal of selectivity analysis is to determine if each cuff and its contacts have

stimulation parameters that allow target of elevatory muscles without motion of the

tongue. All seven of the seven implanted ProxXII cuffs were capable of selectively

stimulating at least one of the elevatory muscles above 50% of its maximum activity

while maintaining the genioglossus at a level <20% of its maximum normalized

activation. Of the seven ProxXII cuffs, six had a contact which could selectively stimulate

the thyrohyoid muscle (S(ProxXII)= 0.95 ± 0.07), and three could selectively stimulate the

geniohyoid muscle (S(ProxXII)=0.90 ± 0.11). Three of the seven DistXII cuffs could

selectively stimulate the geniohyoid muscle (S(DistXII)=0.83 ± 0.11). Six of the seven THXII cuffs could selectively stimulate the thyrohyoid (S(THXII)=0.98 ± 0.02). Example

recruitment curves from a single subject are shown in Figure 2-4.

Figure 2-4: Example recruitment curves from contacts at different locations on the same XII/C1 nerve. (a) ProxXII contact 2: The geniohyoid muscle is activated at the lowest current, followed by the thyrohyoid muscle. (b) ProxXII contact 4: The thyrohyoid and geniohyoid muscles are activated with the same current levels. (c) ProxXII contact 6: The thyrohyoid muscle is activated at the lowest current, while the geniohyoid is only partially activated. (d) DistXII: Geniohyoid and genioglossus activity occurs, but no thyrohyoid muscle activity is recorded.

Discussion Stimulation of the ProxXII and the DistXII nerve electrodes caused significantly

more laryngeal elevation (17.4 mm and 18.8 mm) than any other method (p<0.01). Each

of these electrode sites did so through different ratios of hyoid elevation and thyrohyoid

contraction (table 1): stimulation of the ProxXII caused roughly even amounts of hyoid

elevation and thyroid-hyoid approximation (9.7 and 7.7 mm), whereas stimulating the

DistXII caused significant hyoid elevation but little thyroid-hyoid approximation (17.3

and 1.4 mm). The XII/C1 nerve at the ProxXII location contains axons to the thyrohyoid

muscle while the DistXII does not, so the recorded thyrohyoid activation and

approximation is anticipated. Leonard et al performed videofluoroscopic swallowing studies in humans and measured hyoid and thyroid excursion during the natural swallow (Leonard et al. 2000). They found an average of 16.9 mm of hyoid elevation and

12.1 mm of thyrohyoid approximation, a relatively even split of motion. The balanced stimulated motion approach obtained by ProxXII is a closer match to natural swallowing.

In the human, geniohyoid contraction will move the hyoid anteriorly, while thyrohyoid contraction will cause perpendicular, superior movement of the thyroid. Due to the anatomy and supine positioning of our animal model, the markers moved in one axis. Our experiment showed the largest net laryngeal elevation occurred when the

Distal XII branch was stimulated, despite this nerve branch not innervating the thyrohyoid muscle and causing non-significant thyrohyoid contraction. Human

stimulation of this location would likely cause significant anterior movement but

minimal superior motion because the flaccid thyrohyoid would not draw the infrahyoid

structures toward the hyoid as happens during natural swallowing.

It is not known how the varying ratios of hyoid elevation and thyroid-hyoid

approximation elicited by each nerve cuff location will affect dysphagia, specifically

whether maximal laryngeal elevation is the main goal or if combined hyoid elevation

and thyroid-hyoid approximation is necessary. Future studies of the effect of XII/C1

stimulation on swallowing in human subjects should analyze if balanced co-contraction

is necessary for aspiration prevention or whether maximal laryngeal elevation is a better

indicator of dysphagia protection. This could be performed by selectively stimulating the

XII/C1 during videofluoroscopic swallowing studies and recording motion of both the hyoid and thyroid and their effect on aspiration prevention.

Yokoba et al measured the reduction in geniohyoid length in canines during natural swallowing to be 9.31±1.20% of the resting length (Yokoba, Hawes, and Easton

2003). We measured that the hyoid-jaw resting length in our experiment was ~100 mm, from which we can estimate that about 9.3 mm of hyoid elevation should occur during a natural swallow in our models. Stimulation of the ProxXII (9.7 mm) or DistXII (17.3 mm) nerve cuffs obtains this amount of hyoid elevation, while intramuscular stimulation induces an average of only 8.5 mm (table 1).

Hyoid elevation, however, is only one part of laryngeal elevation. While the contracting geniohyoid and mylohyoid draw the hyoid anteriorly, the relaxed thyrohyoid muscle elongates and does not draw the thyroid an equal distance, causing elongation of the thyroid-hyoid space. The increase in thyroid-hyoid distance during suprahyoid stimulation occurs as a result of the hyoid elevating more than the larynx. In contrast, stimulation of the THXII or THIM causes significant depression of the hyoid. Stimulation of the THXII reduced the thyroid-hyoid distance (4.8 mm, p<0.01), but because the stimulation also significantly depressed the hyoid bone (-4.8 mm) (p<0.01), there was a

net result of non-significant laryngeal elevation (0.1 mm). Stimulation of only infrahyoid muscles draws the hyoid bone and thyroid cartilage together.

There are no data in the canine literature measuring the net laryngeal elevation or thyroid-hyoid contraction during a natural swallow. Hong et al recorded an average movement of 5.8 mm of net laryngeal elevation when the anesthetized canine thyrohyoid muscle was directly stimulated (Hong et al. 1997). We recorded an average

of 3.8 mm of laryngeal elevation when intramuscularly stimulating the thyrohyoid (table

1). No average resting length was provided by Hong et al, so this difference in

magnitude may be attributed to the larger canines used in their study (25-30 kg vs. 12-

18 kg), but it is not clear.

To contrast nerve and intramuscular stimulation’s effect on laryngeal elevation,

the DistXII nerve cuff and the IM GH provide the most direct anatomical comparison.

The DistXII innervates the geniohyoid, genioglossus, styloglossus, and hyoglossus, and of

these, only the GH elevates the larynx. The stimulation of this nerve branch will cause

full contraction of the ipsilateral geniohyoid muscle. Unilateral intramuscular

stimulation of the geniohyoid elicited 4.9 mm of laryngeal elevation, only 27% of the

18.4 mm of laryngeal elevation induced by nerve stimulation. Burnett et al reported that

unilateral intramuscular stimulation of the geniohyoid muscle in humans was able to

recreate ~30% of the laryngeal elevation of natural swallowing (Burnett et al. 2003).

Though different models, canine neural stimulation and natural human deglutition

cause approximately the same increase in laryngeal elevation. Toward this end, the

66 increase in elevation elicited by nerve stimulation may be seen as equivalent to natural swallowing elevation (18.8 mm from canine DistXII vs. 17.6 mm natural human swallow)(Burnett et al. 2003).

Natural swallowing includes contraction on both sides of the head and neck, but unilateral stimulation would be preferred due to the simpler surgery and reduced hardware required. This study demonstrated the application of stimulation to the contralateral muscle or nerve had no significant additive effect to laryngeal elevation

(table 2). The addition of bilateral stimulation to the ProxXII nerve cuff increased laryngeal elevation by only 7%. This means that if the magnitude of motion is the only important factor for aspiration prevention, an implanted system would only require unilateral implantation. The benefits of bilateral implantation include increased force, stiffness, opportunity for selectivity, and balance of sensation for the user, and if any of these measures prove to be critical for protection, then bilateral stimulation is the proper choice.

For comparison to literature, the motion signals were lowpass filtered at 3 Hz, and then the velocity was calculated (Burnett et al. 2003). Full velocity results for each stimulation source and motion can be seen in Table 2-3, along with the measured velocity after the signals were lowpass filtered at 3 Hz as by Burnett (Burnett et al.

2003). In Burnett’s analysis of laryngeal motion, the movement signals were low-pass filtered at 3Hz to remove recording variation from frame to frame. The 3Hz low-pass filter reduces velocity by ~50% compared to the 10Hz lowpass filter that maintains the

step response of stimulation application (table 3). These 3Hz filtered canine IM

velocities approximately match the filtered human IM stimulation velocities recorded by

Burnett: IM mylohyoid stimulation elicited 31.8 mm/s of canine laryngeal elevation,

while Burnett’s unilateral muscle stimulation was able to elicit 31.25 mm/s of human

laryngeal elevation, shown in Table 2-4. Even when filtered, nerve cuff stimulation

remains faster than intramuscular stimulation.

Literature Geniohyoid Laryngeal Elevation Larynx Velocity Contraction (mm ± std) (mm/s ± std) (mm ± std) (3Hz Lowpass) Human Natural Swallow 13.8a 17.56 ± 4.17b 72.67 ± 29.98 b 16.9c 16.9 HE and 12.1 THc Human Unilateral IM 5.08 ± 3.81b 31.25 ± 15.53b Human Bilateral IM 15.6 (GH)d 7.6 ± 1.5 (TH IM)d Canine Natural Swallow ~9.3e Canine IM Stim 5.8 (TH IM)f Canine ProxXII 9.7 ± 9.1 17.4 ± 5.6 67.2 ± 24.7 Canine DistXII 17.3 ± 6.1 18.8 ± 5.9 77.1 ± 27.2 Canine IM Stim 8.5 ± 3.4 5.2 ± 3.5 (IM mean) 19.1 ± 11.8 (GH) 3.8 ± 3.4 (TH IM)

Table 2-4 : Measurements of hyoid and thyroid kinematics during natural swallows and stimulation in the human and canine literature compared to the nerve and intramuscular stimulation results of this study. The darkened boxes contain the results from this study. a (Shaw et al. 1995) b (Burnett et al. 2003) c (Leonard et al. 2000) d (Kagaya et al. 2011) e (Yokoba, Hawes, and Easton 2003) f (Hong et al. 1997)

Non-selective stimulation of the XII/C1 results in GG, SG, and HG activity, causing tongue motion. Because uncontrolled motion of the tongue will complicate deglutition, selective stimulation is necessary. All seven of the Proximal XII cuffs obtained selectivity

(S(n)>0.94 ± 0.07) of at least one elevatory muscle. This means the canine XII/C1 is sufficiently organized at this location to selectively stimulate elevation with nerve cuff electrodes. This is similar to the results of Yoo et al, who showed that a selectivity level of 0.97 for the Geniohyoid can be obtained with proper electrode design and positioning of contacts upon the XII/C1 nerve complex (Paul B. Yoo and Durand 2005).

The implanted flat interface nerve electrode (FINE) reshapes the nerve without causing chronic damage, allowing for increased selectivity (Dustin J. Tyler and Durand

2003). Fascicular rearrangement does not induce mechanical nerve trauma, as long as the applied forces remain below a safe level, which can be ensured by proper sizing of the device. Computer modeling of human XII/C1 fascicular organization from histology will allow for optimal FINE design to obtain selectivity and prevent mechanical trauma. If the fascicular organization at the DistXII or ProxXII locations is not optimal for selective stimulation with an extraneural cuff, then individual nerve cuffs could be implanted at the distal extremes of the nerve branches exclusively to the geniohyoid and the thyrohyoid muscles. At these locations, each cuff would activate only a single elevatory muscle, removing the need for selective electrodes to prevent activation of non-target muscles. While this would require more invasive surgery in a complex and highly vascularized region, the stimulator would not need to be more complex, as multiple contacts will be necessary even in a single cuff system.

In Figure 4, the recruitment curves of selected electrodes in one subject are displayed. The four recruitment curves illustrate the selective capabilities of the XII/C1 nerve at varying locations. If only geniohyoid stimulation is desired, ProxXII contact 1 could be stimulated at 0.2 mA. If only thyrohyoid activation is desired, either ProxXII contact 5 or THXII could be stimulated. In order to replicate swallowing’s motion, in which the geniohyoid contracts and then the thyrohyoid contracts, a stimulator could apply current to a pair of contacts to cause full contraction of both muscles with desired timing. Kagaya et al demonstrated that subsequent stimulation of the geniohyoid and then the thyrohyoid was able to cause increased laryngeal elevation over simultaneous stimulation (Kagaya et al. 2011).

Conclusion Direct stimulation of the XII/C1 nerve complex causes significantly more laryngeal elevation than intramuscular stimulation. A single FINE electrode, placed unilaterally upon the XII/C1 nerve complex, is able to elicit laryngeal elevation at a magnitude and velocity equivalent to that of a natural swallow, whereas intramuscular stimulation elicits only a fraction of the motion. Nerve cuff electrodes can obtain selective stimulation of the geniohyoid and thyrohyoid, avoiding undesired tongue activity, and are a viable option for functional restoration of swallowing.

Laryngeal elevation is one of two major protective mechanisms used for diverting food away from the airway. The other, vocal fold adduction, has been successfully demonstrated in humans to reduce aspiration using stimulation of the recurrent laryngeal nerve. A combined system that incorporates FES for both laryngeal

70 elevation and vocal fold adduction may further improve airway protection in dysphagics to reduce incidence of aspiration pneumonia and improve patient quality of life. Based on the selectivity of stimulation of XII/C1 for elevation without tongue motion, we are confident that a system with one electrode on the recurrent laryngeal nerve for vocal fold closure and a second on the XII/C1 could restore most of the natural protections during swallowing.

Chapter 3 - Targeted Transtracheal Stimulation for Vocal Fold Closure

Abstract Paralysis of the structures in the head and neck due to stroke or other

neurological disorder often causes dysphagia, leading to aspiration pneumonia and

death. The recurrent laryngeal nerve (RLN), which innervates the intrinsic laryngeal

muscles that control the vocal folds, travels superiorly in parallel to the trachea in the

tracheoesophageal groove. We hypothesize that functional electrical stimulation

applied with transtracheal electrodes can obtain controlled vocal fold adduction that

could be used to prevent aspiration. Bipolar electrodes were placed at intervals of 15

degrees around the interior mucosal surface of the canine trachea, and current was

applied to the tissue while electromyography (EMG) activity from the intrinsic laryngeal

muscles and laryngoscopy of vocal fold movement was recorded. The lowest EMG

thresholds were found at an average location of 100 degrees to the left of the ventral

midsagittal line and 128 degrees to the right. A rotatable pair of bipolar electrodes

spaced 230 degrees apart would be able to bilaterally stimulate both RLNs in every

subject. Laryngoscopy showed complete glottal closure under transtracheal stimulation

in six of the eight subjects, and importantly, this closure was maintained under

simultaneous FES-induced laryngeal elevation. Transtracheal stimulation is an effective

tool for non-surgical application of FES that could be used to assist in airway protection in dysphagia patients.

Introduction Dysphagia, defined as difficulty swallowing, is a condition that affects 10 million people a year in the United States and is diagnosed in 37% to 78% of stroke victims

(Martino et al. 2005). During healthy swallowing, protective mechanisms and reflexes prevent bolus flow into the trachea and direct its passage into the esophagus. Some of these mechanisms include oral collection, velopharyngeal closure, epiglottal deflection, swallowing apnea, upper esophageal sphincter (UES) relaxation, vocal fold adduction, and superior and anterior movement of the larynx (Medda et al. 2003). Aspiration, the inhalation of foreign material, often occurs when these protective mechanisms are impaired or uncoordinated. Aspirated material can result in pneumonia, which is the most common cause of death in stroke survivors (Heuschmann et al. 2004) and

Parkinson’s disease patients (Troche et al. 2010). Patients at risk for aspiration are initially prescribed a blend of rehabilitation exercises and specialized or restricted diet. If these measures fail to adequately protect the airway, further restriction of oral consumption and more aggressive surgical interventions, like tracheostomy and PEG tube implantation, are often required. These procedures lead to a reduction in quality of life (Blitzer, Brin, and Ramig 2009) as the patient is unable to engage in the social aspects of eating and drinking. Despite the risks, patients at risk of pneumonia often cheat on their restrictions. Additionally, approximately a liter of saliva and secretions are swallowed every day. Because the standard surgical and behavioral changes result in an unsatisfactory patient state, an improved treatment is needed.

Functional electrical stimulation (FES), the application of electrical pulses to

activate healthy nerves and muscles that have lost central nervous system control, is

used in the disabled to restore walking, grasping, bladder control, and many other

functions (Peckham and Knutson 2005). Electrical stimulation can be applied to the skin

surface, to the muscle, or directly to nervous tissue to induce a desired effect. Increased

proximity to the individual axons allows electrical stimulation to be more selective and

requires lower current. FES has already been used for reanimating and strengthening

two protective mechanisms that occur during the swallowing reflex: laryngeal elevation

and vocal fold adduction.

Laryngeal elevation, the superior and anterior motion of the hyoid and larynx,

serves as an important protective mechanism during the swallowing reflex. The larynx is

composed of the thyroid and cricoid cartilage and contains the vocal folds, which guard

the opening to the trachea and lungs. Contraction of the geniohyoid (GH), mylohyoid

(MH), and thyrohyoid (TH) muscles pulls the hyoid bone and larynx forward into a

protected position and aids the reflection of the epiglottis over the airway (Shaw et al.

1995). Burnett et al applied paired intramuscular FES to these laryngeal elevator muscles in aware humans and were able to recreate 50% of the elevation occurring

during natural swallowing (Burnett et al. 2003). The geniohyoid and thyrohyoid muscles are innervated by a merger of the first cervical nerve (C1) and the hypoglossal nerve

(XII). Nerve cuff stimulation is able to induce significantly more laryngeal elevation than intramuscular stimulation, as discussed in Chapter Two.

In addition to laryngeal elevation, FES has also been applied to induce vocal fold adduction for dysphagic aspiration protection. Natural vocal fold adduction occurs through bilateral contraction of the lateral cricoarytenoid (LCA), thyroarytenoid (TA), interarytenoids (IA), and cricothyroid (CT) muscles, four of the intrinsic laryngeal muscles (ILM). The recurrent laryngeal nerve (RLN), a branch of the vagus nerve, innervates the intrinsic laryngeal muscles. Tetanic stimulation of the RLN has been shown to cause net vocal fold adduction (I Sanders, Aviv, and Biller 1986). Complete vocal fold adduction creates a glottal seal, halting airflow and preventing bolus leakage into the trachea. Broniatowski et al chronically implanted a nerve cuff electrode in five dysphagic patients and demonstrated that RLN stimulation caused functionally significant vocal fold adduction (Broniatowski et al. 2010). In four of the five subjects, neural stimulation prevented the occurrence of aspiration pneumonia. This effect was experimentally verified by modified barium swallowing studies which exhibited that stimulated vocal fold adduction prevented the aspiration of swallowed boluses, demonstrating the effect is clinically relevant. The RLN branches from the vagus nerve

(Cranial nerve X) under the aorta on the left and under the subclavian artery on the right and travels superiorly to the larynx in the tracheoesophageal groove, abutting the trachea, the esophagus, and thyroid gland.

Unilateral resection of the RLN results in paralysis of the ipsilateral vocal fold, causing hoarseness of voice, increased resistance to breathing, and incomplete glottal closure. During thyroidectomy operations, surgeons use stimulation probes to determine the location of the RLN so they can prevent damage during the delicate

removal of the thyroid gland. Transtracheal and transesophageal stimulation, the

stimulation of nerves inside the neck using “surface” electrodes on the epithelium of the

trachea or esophagus, has been tested for determining RLN viability and controlling

vocal fold position. Sanders et al inserted a single monopolar probe through either the

oral cavity or a tracheostomy to apply gross stimulation to the tracheal and esophageal

surface (Ira Sanders et al. 1987). This stimulation induced vocal fold motion, which was

observed by laryngoscopy. Kraus et al developed an indwelling electrode platform which

could be temporarily placed superior to the upper esophageal sphincter (Kraus et al.

1987). Electrodes on the platform were positioned to activate only the branches of the

RLN to the posterior cricoarytenoid (PCA) muscle, inducing vocal fold abduction meant to improve respiration.

Sercarz et al induced motion and altered canine voice with transtracheal

electrodes and artificial expiration, showing effects similar to what Broniatowski

performed with a nerve cuff (Sercarz et al. 1991). Sercarz et al adhered two large

electrodes to contralateral sides of the endotracheal tube at positions 120 degrees from

the ventral midsagittal line; this modified endotracheal tube was inserted through a

tracheostomy and used to stimulate the RLN, tensing the vocal folds and changing vocal

tone. Sercarz’s design required stimulation pulses with amplitudes of 10-20 mA to

obtain vocal fold adduction, an amplitude that the author states would not be tolerated

by unanaesthetized subjects. Applied stimulation generated an electric field through a

large volume of the neck from the cathode to the anode on the contralateral side,

oriented primarily in the same axial plane of the body. To reduce the threshold current,

the transtracheal electrodes and the generated electrical field should run in parallel and

in close proximity to the recurrent laryngeal nerve (Rattay 1998).

To optimally place transtracheal electrodes, the angular location of the RLN in relation to the trachea must be found. Liebermann-Meffert dissected a series of human cadavers to measure the relative distance of the RLN from the trachea (Liebermann-

Meffert et al. 1999). Cross sections of the neck at levels one and four centimeters inferior to the cricoid cartilage were taken from 10 cadavers, and the position of the RLN in relation to the trachea and esophagus was noted. Using ImageJ (NIH) and

Liebermann-Meffert’s figures, we recorded the angular location of each nerve using a centroid of the trachea as the origin and the intersection of the ventral aspect and the y- axis as the midsagittal line. Our review of the article’s figures showed that the left recurrent laryngeal nerve was on average positioned at an angle of 98.3 ± 29.8 degrees from the ventral midsagittal line, and the right RLN was positioned 117.3 ± 25.6 degrees to the right of the ventral midsagittal line of the trachea, as shown in Figure 3-1. There is high variability seen in the cross sections, especially at the inferior 4 cm level, meaning a single electrode position may not be effective for every subject. There is a smaller distribution of nerve locations at the superior cross sections closer to the larynx.

Additionally, the more inferior positions of the nerve are further away from the trachea.

Liebermann-Meffert reported that the left recurrent laryngeal nerve is closer to the trachea, remaining within 10 mm of the tracheal wall until seven centimeters below the cricoid. The right RLN is reliably closer than 10 mm away from the wall for only the most superior 2 cm of the RLN. The distance of the RLN from the tracheal electrodes will

78 directly impact the magnitude of current that is required to stimulate the nerve, therefore it is expected that more superior locations will allow lower current thresholds, especially on the right.

Figure 3-1 : Averaged positions and range of the RLN from the cross sections from Liebermann-Meffert (Liebermann-Meffert et al. 1999). 1cm inferior to the cricoid cartilage, the left RLN is positioned an average of 118.5 ± 17.9 degrees from the ventral surface, and the right RLN is positioned 136.4 ± 13.24 degrees from the ventral surface. Four centimeters inferior to the cricoid cartilage, the left RLN is positioned 78.0 ± 25.0 degrees from the ventral surface, and the right RLN is positioned 98.0 ± 19.7 degrees from the ventral surface. Stars illustrate the average location of the RLNs.

Both laryngeal elevation and vocal fold adduction occur in the natural swallow, and both are important protective mechanisms that can be induced by FES. Toward this end, in conjunction with a study of transtracheal stimulation for vocal fold closure, we

will also induce elevation of the larynx by stimulating the XII/C1 nerve complex. We hypothesize that an optimal angular location can be found in the trachea for transtracheal stimulation of each recurrent laryngeal nerve and that stimulation at this location will cause vocal fold adduction and glottal closure. No study has yet determined whether these two stimulation methods can be applied simultaneously: stimulated activation of elevation may negatively impact the positioning of transtracheal or nerve cuff electrodes that induce vocal fold closure. We hypothesize elevation of the laryngeal anatomy will not negatively affect transtracheal stimulation for vocal fold adduction.

Methods

Modified Endotracheal Tube Design Pairs of electrodes and wire were attached longitudinally around the surface of a

7.5 mm endotracheal tube (Mallinkrodt, St. Louis, MO) at a spacing of 45 degrees. An example transtracheal balloon is shown in Figure 3-2. The electrodes were custom-made

using rectangular pieces of foil tape (3M, Minneapolis, MN) with an exposed surface

area of approximately 25mm2. Perfluoroalkoxy (PFA)-coated stainless steel leads (Fort

Wayne Metal, Fort Wayne, IN) were deinsulated at their tips and connected to each

contact. The wire was led longitudinally along the length of the tube for connection to

stimulators. Colored lines from each electrode were drawn up the length of the tube to

display the angular location of the electrodes for observation while the inflated balloon

and electrodes were inside the trachea without deflation, removal, and reinsertion. The

deflated endotracheal balloon was sheathed within a cylindrical piece of heat shrink to

prevent the electrodes from being dislodged during insertion. Bipolar pairs of hook

electrodes were constructed with stainless steel wire for use in intramuscular (IM)

stimulation and electromyography (EMG) recording. 80

Figure 3-2: Image of an electrified endotracheal tube. Contacts are arranged longitudinally every 45 degrees around the circumference of the balloon.

Surgical Preparation Seven adult canines (11.8 – 17.5 kg) were anesthetized with an initial IV injection of Pentothal and maintained with ventilated isoflurane (0.5-4.0%) through a non-modified endotracheal tube. Normal body temperature was maintained with a circulating-water heating pad while IV fluids were provided through a catheter in the forelimb. EKG, blood pressure, blood oxygenation, and breathing rate were continuously monitored. All animal care and experimental protocols were in accordance with NIH guidelines and approved by the Institutional Animal Care and Use Committee (IACUC) of

Case Western Reserve University.

An incision was made along the midline of the neck from the suprasternal notch to the mylohyoid in an anaesthetized canine in a supine position. The trachea and recurrent laryngeal nerve deep to the sternohyoid and sternothyroid were exposed. 81

Custom-made Flat Interface Nerve Electrodes (FINE) (D. J Tyler and Durand 1997) were placed bilaterally on the RLN. Additional cuffs were placed bilaterally upon the XII/C1 nerves proximal and distal to the branching point to the thyrohyoid muscle, as seen in

Figure 3-3.

Figure 3-3: An isometric view of the significant anatomy for the experiment, including the XII/C1 nerve complex and the recurrent laryngeal nerve.

Bipolar pairs of hook electrodes were implanted using a 22-gauge needle into four intrinsic laryngeal muscles (PCA, LCA, TA, and CT) and the laryngeal elevators (TH,

GH, and MH). A tracheostomy was performed between the third and fourth cartilage ring and modified endotracheal tube was inserted into the trachea. The sheath was retracted following implantation and then the balloon was inflated to seal the airway for anaesthesia provision and also to press the electrodes to the tracheal wall.

Stimulation Trials Stimulation and EMG recording was performed using a Crishtronics (Cleveland,

OH) stimulator controlled by a custom MATLAB program. EMG of the intrinsic laryngeal

muscles was amplified (gain: 30-330) and filtered (bandpass: 10 Hz -1000 Hz; notch 60

Hz) with an AC –coupled differential amplifier (Cambridge Electronic Design, CED-1902,

Cambridge, UK) and recorded at a sampling rate of 2400 Hz using a DAQCard-6036E

data acquisition device (National Instruments, Austin, TX). The program generated

recruitment curves using pulse amplitude and pulse width modulation of charge-

balanced, square stimulus pulses. Five pulses of each stimulus value were applied (4Hz,

0-5mA, 0-500 us) and the integrated rectified EMG response of each muscle was averaged for the five pulses.

Electrical stimulation was applied to longitudinally-oriented bipolar electrode

pairs pressed against the tracheal wall at 24 annular locations by rotating the

endotracheal tube in 15o increments between trials. In one experiment, transtracheal stimulation was performed at spacing of 22.5 degrees, resulting in 16 stimulation locations instead of 24. At each location, pulse amplitude modulation was performed to determine the threshold of electrical stimulation required to activate the ipsilateral laryngeal muscles. Similar stimulation trials were performed using the FINE nerve cuff placed on the RLN. A laryngoscope (Karl Storz, Tuttlingen, Germany) was inserted into mouth and laid in position to allow steady viewing of the vocal folds. Tetanic transtracheal stimulation (25-70 Hz) was applied at supra-threshold amplitudes to the

83 bipolar transtracheal electrode pairs and the FINE nerve cuffs while video of the vocal folds were viewed and recorded for post-analysis using a digital video recorder.

A Tektronix (Beaverton, OR) AFG-3022B pulse generator (0-10 V, 0-500 us, 0-

100Hz) was used to stimulate the hypoglossal nerve to produce laryngeal elevation by inducing contraction of the geniohyoid and thyrohyoid as described in Chapter Two.

Transtracheal stimulation was applied prior to and during elevation to determine whether the addition of laryngeal elevation would alter the vocal fold motion induced by transtracheal stimulation. Vocal fold motion and glottal area was recorded using laryngoscopy as described above, while a video camera (Panasonic GS-300, Osaka,

Japan) recorded the extent of laryngeal elevation. The two video sources were combined and time-synced using a Vitec MX-4 mixer (Chatillon, France) and recorded to the digital video recorder for measurement. Transtracheal stimulation for laryngoscopy recording was applied at a known effective location based on earlier EMG recordings.

Data Analysis EMG data was rectified, integrated, and normalized to measure the activation of the intrinsic laryngeal muscles for each transtracheal stimulation location. The threshold for activation of a muscle at a specific location was defined as 50% of the muscle’s maximum activation across all locations. The current amplitude required for threshold activation at a pulse width of 500us was extracted for each angular location. If threshold could not be reached within an amplitude of 5 mA, that angular location was recorded as having a threshold of >5mA, which was the limit of the stimulator.

Laryngoscopic video was imported into a custom designed MATLAB program

which measured the glottal area in pixels. The temporal plot of motion was recorded

and measured to obtain the relative change from the resting glottal area. During the

stimulated period, the minimum glottal area was calculated by recording the minimum

number of pixels using a sliding 500 ms averaging window (15 frames). If stimulation

caused vocal fold opening, then the maximum glottal area was measured by taking the

maximum number of pixels using the sliding 500 millisecond averaging window. The

glottal area in pixels of each video session was scaled such that the glottal area when stimulation was not applied was defined as 1. Complete closure of the glottis (zero pixels of glottal area) was defined as 0. The scaled maximum closure was recorded along with notes of stimulus parameters, isoflurane levels, and whether hypoglossal stimulation for elevation was performed. An example stimulation trial is shown in Figure

3-4, and examples of the vocal folds seen via laryngoscopy can be seen in Figure 3-5.

Example Stimulation

0.8

0.6 Glottal Area 0.4 Stimulation

Standardized Glottal Area 0.2

0 2 4 6 8 10 Time (seconds)

Figure 3-4 : Example scaled glottal area recording during stimulation. On application of stimulation the vocal folds quickly close, and the vocal folds reopen when stimulation is halted.

(a) (b)

Figure 3-5 : Example images of the vocal folds, (a) relaxed and (b) stimulated, resulting in complete glottal closure.

Results The close proximity of the laryngeal muscles and the implanted bipolar fine wire

electrodes occasionally caused muscle activity from one side of the larynx to be

recorded by the contralateral EMG electrodes, despite stimulation being applied

unilaterally. Electrodes that recorded equal levels of EMG during contralateral stimulation and ipsilateral stimulation were removed from the data set. In these experiments the EMG electrodes were likely damaged or dislodged due to muscle motion.

For every experiment we were able to obtain low-noise EMG recordings from at least one muscle on each side of the larynx. Of a possible 14 recordings of each unilateral muscle, we recorded 13 TA, 10 PCA, and 11 LCA muscles. Cricothyroid activation during RLN stimulation occurred in only one of the seven canines tested and it

occurred bilaterally in the one canine. Transtracheal stimulation of the RLN bilaterally

activated at least one unilateral ILM in all specimens with stimulation amplitude under 5

mA. The maximum transtracheal stimulation amplitude required to meet threshold of

the intrinsic muscles was 4.1 mA, and currents as low as 0.62 mA activated ILMs via

transtracheal stimulation in one canine. The nerve cuff electrode on the RLN required

as little as 20 μA (0.02 mA) to obtain threshold activation of the intrinsic laryngeal muscles, much less than the transtracheal electrodes. In every canine more than one

direction of the transtracheal electrodes relative to the ventral midline was able to

activate each RLN. An example of a single muscle’s range of thresholds can be seen in

Figure 3-6. The optimal locations, based on averaging the minimum threshold locations,

were 100 ± 22 degrees on the left and 128 ± 37 degrees on the right, displayed in Figure

3-7. The average angular range of possible stimulation locations for each electrode was

61 ± 26 degrees.

Figure 3-6 : Threshold levels of the left LCA in one experiment. The three regions mark threshold required to activate the EMG to the specified percentage of the maximum level of that muscle during the experiment. The left LCA is activated by electrodes on the left side of the trachea, near the left RLN. The lowest threshold was found at 75 degrees, marked by the arrow.

Figure 3-7 : Plot of the range of angles at which transtracheal stimulation was possible for each experiment. The diamond represents the location with the lowest EMG threshold. The averaged optimal locations were calculated to be 100 degrees on the left and 128 degrees on the right, as illustrated by the vertical lines.

The angular difference between the right and left optimal locations for each canine was calculated. The average angular spacing between optimal electrode locations was 230 degrees ± 27 degrees. For every specimen, a spacing of 230 degrees would be effective at placing electrodes in bilateral stimulation locations.

There was an average of 13125 pixels in the open glottal area across the 678 laryngoscopic recorded stimulations. Transtracheal stimulation induced vocal fold

89 motion when the electrodes were placed within the proper angular areas of the trachea.

Video analysis showed a change in vocal fold area depending on the stimulation pulse and frequency. When the frequency was below 30 Hz, net vocal fold opening was observed. If the frequency was higher there was net closure, displayed in Figure 3-8.

Isoflurane level affected the stimulation results on the vocal folds. Stimulation under high isoflurane levels (>2%) resulted in stimulation abducting the vocal folds, counter to the expected adduction Figure 3-9. Across all experiments, with a suprathrehsold stimulation current, stimulation frequency >40Hz, and isoflurane levels

<= 2%, transtracheal stimulation caused complete closure in six of eight canines, with an average glottal closure to 8.8 ± 20.5% and a median closure to 1.6% under stimulation.

Stimulation of the nerve cuffs upon the RLNs caused complete closure in seven of eight canines, causing an average of 1 ± 2% of the glottal area, significantly greater closure than transtracheal stimulation (p=0.03).

Frequency modulation effect on vocal fold area

1.5

0.5

Standardized vocal fold area fold vocal Standardized 20 25 30 35 40 45 50 Frequency (Hz)

Figure 3-8 : Effect of frequency on vocal fold area under stimulation. Values greater than one indicate vocal fold abduction, while values less than one indicate vocal fold adduction

Effect of isofluorane on vocal fold movement for experiment 4 1.6

1.4

1.2

0.8

0.6

0.4

0.2

Standardized vocal fold after area stimulation <2% 2% >2% Isofluorane level

Figure 3-9 : Effect of Isoflurane levels on glottal area. When isoflurane levels were higher than 2%, stimulation induced vocal fold abduction.

There was no significant difference between unilateral and bilateral transtracheal stimulation on reducing the glottal area (p=0.13). Unilateral transtracheal stimulation induced complete closure in 6 of 8 canines, while bilateral stimulation was able to induce complete closure in only one of the five canines in which bilateral stimulation was applied, though it did obtain >95% closure in three of those five. In our very first canine, transtracheal stimulation was not effective at inducing vocal fold closure, though nerve cuff stimulation induced complete closure. In the fifth canine we were unable to induce more than 10% closure, which we later determined was due to the isoflurane levels. In this canine the anaesthesia levels were maintained at >2%,

causing an abduction response to stimulation of the RLN. Further study of isoflurane level adjustment demonstrated that under high levels of isoflurane electrical stimulation induced vocal fold abduction and under lower anaesthesia levels those same stimulation parameters induced vocal fold adduction.

When current was applied to the XII/C1 nerve using the implanted nerve cuffs, stimulation caused contraction of the geniohyoid and thyrohyoid muscles which induced elevation of the larynx and hyoid. Laryngoscopy of the vocal folds during elevation showed that transtracheal stimulation of the RLN still accomplished vocal fold closure.

Transtracheal stimulation during laryngeal elevation was able to induce complete closure in 2 of the 3 canines in which measurable video was recorded. In the other two canines in which dual transtracheal stimulation and laryngeal elevation was performed, laryngoscopic video was unable to record an acceptable view of the vocal folds during the maintained elevation. In the three canines that elevated closure was recorded, complete closure could be induced by unilateral transtracheal stimulation during non- elevated positions. In the trials in which elevation was performed, non-elevated stimulation caused an average closure to 9.5 ± 9.4% of the vocal fold area, while elevated vocal fold closure caused closure to 3.6 ± 6.1%. Transtracheal stimulation during laryngeal elevation induced significantly more (p=0.029) vocal fold closure across all of these recordings.

Discussion In the canine neck, the optimal angles for transtracheal stimulation were found

to be 100 degrees to the left and 128 degrees to the right of the midsagittal line. Our

review of Liebermann-Meffert’s figures on human anatomy shows similar angles, 98.3

degrees on the left and 117.3 degrees on the right. These are two different models, but

we expect these similar results due to the anatomical similarities in the path of the RLN.

During supine, non-preserved dissection of our specimens, we observed that the nerve

was positioned against or near the tracheoesophageal groove, but the exact angular

location could not be accurately determined due to gravity shifting the nerve during

dissection.

Six of the seven left RLNs were activated by electrodes placed at either 90° or

105° to the left, and five of the seven right RLNs were activated by electrodes placed at

135° to the right, with another activated at 120°. The remaining inactivated RLN on each side came from the same experimental canine (Exp. 3). The shift of both curves appears to be of a similar rotational distance, about 45 degrees “clockwise” on each side (Figure

3-7). In observing the pattern of the stimulation locations for this experiment, the rotational measurements were either axially inaccurate (rotated 45 degrees axially) or the RLNs were in a strangely distinct position from the hypothesized anatomical position. To adjust for this rotational variation across the studies, the angular distance between the optimal position for left and right stimulation was recorded. An average difference of 230 ± 27 degrees was found, signifying that rotatable pairs of bipolar electrodes 230 degrees apart would be able to stimulate both RLNs if properly positioned. On application of this observation to the results, it was found that any angle between 210 and 248 degrees would be able to stimulate bilaterally in every canine.

As we expected, the design and proximity of the cuffs on the nerve allowed for a

much lower threshold than prior studies, reducing power requirements for a stimulator.

The largest minimum pulse amplitude that was required in any canine to activate the intrinsic muscles was 4.1 mA, and currents as low as 0.62 mA were able to activate the

RLN in one canine. Sercarz’s device required 10-20 mA of current, most likely because of the size and positioning of single electrodes 120 degrees on each side. Stimulation at these levels could induce sensations that could range from tingling to discomfort to pain, and could potentially trigger coughing reflexes. This would be a significant problem if occurring while stimulating for vocal fold adduction. However, sensation may instead subconsciously assist the swallowing reflex by increasing conscious effort. Electrical stimulation on the tracheal mucosa in aware, unanaesthetized humans has not been performed, so electrified tracheostomy tubes will need to be tested in the human model prior to clinical use. The experimental devices could be temporarily placed in canines who could inform the researchers the type and magnitude of sensation being felt. If pain or other problematic sensations are noted at lower currents than functional effects, then transtracheal stimulation may not be a viable solution for improving vocal fold adduction in dysphagics, in which case nerve cuff stimulation remains an effective method.

It is unknown how much glottal closure is necessary to functionally protect the lungs from aspiration. However, because we were able to induce complete closure in 6 of the 8 canines, transtracheal stimulation would be sufficient. In our very first canine, transtracheal stimulation was not effective, though nerve cuff stimulation induced

complete closure. In the fifth canine we were unable to induce more than 10% closure,

which we later determined was due to the isoflurane levels. To apply these results to

dysphagic patients, this stimulation method would have to be measured and tested in

an alert deglutition model that included laryngoscopy and modified barium swallow

studies.

This study found that unilateral stimulation induced no significant difference in

glottal closure from bilateral stimulation. Sanders found that unilateral stimulation

causes over 95% closure, depending on the stimulation parameters (I Sanders, Aviv, and

Biller 1986). The tetanic contraction of the unilateral muscles from FES results in the

ipsilateral vocal fold adducting past the midline of the glottis, crossing to the

contralateral side. The additional glottal closure from bilateral stimulation is minimal,

and in some canines it was found that the addition of bilateral stimulation reduced

glottal closure. In Figure 3-10, a recording of glottal area is shown in which both

unilateral and bilateral transtracheal stimulation are applied in succession. Bilateral

stimulation induced less glottal closure than either unilateral stimulation. This could

occur because the stimulation of the RLN causes activation of the posterior

cricoarytenoid, which induces abduction of the vocal folds by displacement of the

arytenoids. The combined contraction of all of the abductors and adductors causes a

non-natural displacement of the arytenoids, resulting in incomplete closure of the vocal

folds under bilateral stimulation, but not during unilateral contraction. To this end, it appears that unilateral stimulation should be sufficient to protect the airway, and would also require a simpler stimulator design.

Left-Bilateral-Right Stimulation 1.4 Glottal Area Left Stimulation 1.2 Right Stimulation

0.8

Left Right Bilateral L B R 0.6

0.4 StandardizedGlottal Area 0.2

0 0 5 10 15 20 25 30 35 40 45 50 Time (seconds)

Figure 3-10 : Plot of glottal area under unilateral and bilateral stimulation.

Laryngeal elevation did not affect the ability of transtracheal stimulation to induce vocal fold closure. The motion of the larynx during elevation stimulation required movement of the laryngoscope during video recording, but because vocal fold adduction is measured as a percentage of pre-stimulation glottal area, the percentage of closure is comparable. Elevation was able to be sustained for periods greater than a minute, allowing for repositioning of the laryngoscope and multiple stimulation parameters during a single elevatory session. A single device with multiple channels for stimulation could be used for inducing laryngeal elevation and vocal fold adduction, as these two systems are independent and not counter-active to each other.

Conclusion Transtracheal stimulation can be used as a tool for further study of the recurrent laryngeal nerve and vocal fold control. A tracheostomy provides easy access for an indwelling device to be placed and removed. Many patients who have dysphagia are required to undergo a tracheostomy, so a large population exists that would not require further surgery to be fitted with a device. Patients who have already undergone tracheostomy and have to place and remove tracheostomy tubes on a daily basis would have little problem inserting an electrified tracheostomy tube.

Transtracheal stimulation can be applied to induce vocal fold adduction. The optimal angles on each side of the trachea were found to match expected angular position of the RLN from prior anatomic studies. A modified tracheostomy tube with electrodes could be used in dysphagic patients such that stimulation would allow vocal fold protection during swallowing. Laryngeal elevation, another protective mechanism, can be applied simultaneously to replicate the natural swallowing motion without harming the elevatory or adductory effect of the stimulation methods. The currents required for transtracheal stimulation are below the expected pain threshold, but this would need to be determined in an aware human subject. Following a study of sensory thresholds and optimal human placement of transtracheal stimulation, a permanent device should be developed that also incorporates laryngeal elevation and a natural controller for automatic protection during deglutition.

Chapter 4 - Neural Network Detection of Swallowing Using Palatometry

Abstract Measurement of the oral transport phase of swallowing is a complicated process

requiring either placement of obstructive sensors or sitting within a larger device for

recording. Further, detection algorithms that can distinguish oral events, such as

swallow, from time-dependent patterns of lingual-palatal pressure are imprecise,

resulting in much false detection. An oral swallowing detection device that can be

custom fit to the user and trained to automatically recognize the oral phase of

deglutition is described here. Seven pressure sensors were placed on a molded mouthpiece fitting over the upper teeth and hard palate. Pressure recordings were made during a variety of swallow and non-swallow activities. The signals and swallow times were then input into a time-delay artificial neural network to create a fit to the objective function, after which a threshold analysis was used to grade the accuracy of the network. The neural networks obtained a median swallow detection accuracy of

94% on intra-session recordings, while a network trained across a selection of subjects was able to obtain an accuracy of 92% for both healthy and dysphagic subjects. The lingual-palatal pressure signals are sufficient to selectively and specifically recognize the act of swallowing.

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Introduction Deglutition, or swallowing, is a complex process consisting broadly of three phases: Oral, pharyngeal, and esophageal. The oral phase corresponds to the receipt, preparation, collection, and propulsion of the material from the anterior portion of the mouth to the pharynx. The collection and propulsion of the bolus are critical for complete swallowing, as weakness or inefficiency of either action can result in residual materials in the mouth. The pharyngeal phase of swallowing is composed of reflexogenic actions that seal the airway, prevent aspiration, and move the bolus completely to and through the upper esophageal sphincter. The esophageal phase is a fully non-volitional, peristaltic motion that pushes the bolus to the stomach.

Dysphagia, or difficulty swallowing, is a disability that affects a significant proportion of geriatric patients. The inability to safely engage the protective mechanisms of the pharyngeal swallow leads to aspiration, defined as the inhalation of oral materials. Aspiration is a significant predictor of pneumonia due to the likelihood of an aspired bolus containing bacteria that infect the lungs. In fact, dysphagia is the leading cause of death for people following stroke (Heuschmann et al. 2004) and those with Parkinson’s disease (Troche et al. 2010). Initially, patients are prescribed rehabilitation and adjustment of diet to thickened materials (Nicosia and Robbins 2001).

If this is insufficient, more aggressive clinical interventions are required including percutaneous endogastronomy tube implantation and nil per os (nothing by mouth)

(Blitzer, Brin, and Ramig 2009).

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If the natural protective reflexes of deglutition have been weakened or lost due

to central nervous system injury, functional electrical stimulation can be used to activate

the necessary muscle groups. Functional electrical stimulation (FES) is the application of

electrical pulses to activate healthy nerves and muscles that have lost central nervous

system control (Peckham and Knutson 2005). Currently, FES devices to aid protection of the airway include implanted and surface electrical stimulation devices which are used to induce laryngeal elevation and vocal fold closure by electrically activating the nerves that control the muscles of action (Burnett et al. 2005; Broniatowski et al. 2010). These

devices are controlled by some form of external activator, usually a hand-held button

device that contains the battery and pulse generator. The controller is used to apply stimulation manually because continuous stimulation would cause fatigue of the stimulated muscles and apnea from sustained vocal fold closure. Self-applied stimulation has been studied in the past and can be useful for improving concentration during swallowing (Burnett et al. 2005). The research devices were activated by the user in time for activation of the thyrohyoid muscle, approximately 0.25 seconds after the mylohyoid naturally activates. Because the user is swallowing throughout the day, an average of 580-2000 times, continuously using a hand controller would apply considerable mental and physical strain to an already-disabled patient (Garliner 1979).

An ideal FES device would have an automated swallowing recognition system using a natural signal for use as a controller. This would remove the need for manual control, simplifying use of the device. Prior studies have attempted to detect swallowing using recording and analysis of chewing sounds, arm motion, vibration, laryngeal

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movement, and infrahyoid EMG, to varying degrees of success (Das, Reddy, and

Narayanan 2001). Amft and Troster obtained a recall rate of 0.84 with a combination

sound/infrahyoid EMG system, meaning that 84% of swallows were detected (Amft and

Troster 2006). The system, however, recorded a high number of false positives, resulting

in a system that was very imprecise and would not be efficient for properly detecting

swallowing. Prior studies of swallow detection were performed for recording activity

throughout the day for research and clinical analysis. Beyond this, there is a need for an automated swallowing recognition system for use as a control system. Because stimulation would need to be applied during the pharyngeal phase of swallowing, the automatic controller would need to activate based on signals prior to the pharyngeal phase, during the oral phase of swallowing. Natural signals occurring during the oral phase include electromyography from the submental or jaw muscles and pressure of the tongue against the roof of the mouth.

Submental electromyography (sEMG) is a common recording taken during swallowing studies. Surface electrode patches are placed on the skin and electrical activity is measured, the amplitude of which is correlated to the firing rate of the underlying musculature. High levels of submental muscle activity can be measured during swallowing, which is why it has been used for detection in prior studies. The difficulty with sEMG is that significant activity is measured during other activities, including speech, chewing, and pressure generation (Wheeler, Chiara, and Sapienza

2007). The submental region is composed of many different muscles, including those of the tongue. The signals recorded by surface EMG are predominantly proximal muscles,

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which will be the geniohyoid and mylohyoid. The muscles of the tongue are oriented in

all three dimensions and are constantly changing in distance and position, complicating

the possibility of selectively recording the activity of any specific one (Hiiemae and

Palmer 2003). This constant movement and variety of shape also reduces the potential

for permanent implant of recording electrodes into the desired muscles

Palatometry Palatometry is a recording method used clinically for diagnosis of tongue

strength, as a rehabilitation platform, and as an exercise device to strengthen the

tongue. Studies of tongue pressure generally use air-pressure bulbs that are held

temporarily in the mouth or adhered to the hard palate. Systems like the Iowa Oral

Pressure Instruments (IOPI) record pressure from air-filled bulbs (Pouderoux and

Kahrilas 1995). Air bulb systems are connected to tubes that exit the mouth to transmit

pressure signals to a recording device (Nicosia et al. 2000). These systems have been

used to record the magnitude of induced pressure and the pattern of oral pressure that occurs during swallowing and isometric pressure application. Isometric pressure application, where the patient is told to press as hard as possible on their palate with their tongue, is a common test used to determine the strength of the tongue. Low maximum pressure values are often indicative of dysphagia.

While air-bulb measurements are useful for clinical study of tongue strength, the size and obstructiveness impedes recording of natural swallowing patterns. To counter the obstruction of air-bulbs, groups have transitioned to smaller electronic sensors. Ono and Hori developed an artificial sensor plate using resistive sensors for recording oral

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pressures (Ono, Hori, and Nokubi 2004). Electronic pressure sensors were included into a custom-fit mouthpiece while a series of stimuli and instructions were provided. The recorded pressure patterns matched expected biomechanical models of swallowing,

consistent with anterior, circumferential, and then posterior pressure application. Ono

presented results from a series of ten subjects that showed a repeated pattern of oral

pressure application. The full swallow pressure pattern was found to last an average of

0.9 seconds from onset to offset, with a delay of 0.25 seconds between onset of

anterior and posterior pressure (Ono, Hori, and Nokubi 2004).

In addition to positive pressure, the mouth has the ability to induce suction by

pulling the tongue away from the palate or inhaling air from the oral cavity into the

lungs. Existing resistive sensors in air-bulb palatometers were not calibrated or designed

to record negative pressure, so Keiser et al produced a mouthpiece with sensors that

could record both positive and negative intra-oral pressure (Kieser et al. 2007). Eight

sensors were built into a mouthpiece designed individually to fit each subject: Two

palatal sensors corresponded to the central and posterior midline sensors used in Ono’s

design, and an additional six sensors were placed on the labial and lingual sides of the

central incisor, canine, and first molars for measuring cheek and lateral tongue pressure

activity. Farland consistently found negative pressure induced in the posterior mouth

prior to oral bolus propulsion, signifying suction was a primary bolus propellant (Farland

2011). Analysis of the timing of onset and offset pressure on the midline sensors

showed “considerable intra-individual variability”, meaning every swallow was different,

even when swallowing similar materials (Kieser et al. 2007).

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The pattern of significant positive and negative pressure on selected swallows has variability, but components may be similar enough to be recognized. A detector using palatometry signals would likely emphasize amplitude and activity levels. Other activities that may have similar levels of activity include direct tongue pressure on the hard palate and chewing, events that occur under direct specific targeted control of the mouth. A system to differentiate between swallows, non-swallows, and high-activity non-swallows would need to weigh the patterns of swallowing along with pressure amplitudes to sensitively recognize the desired activities.

Time Delay Artificial Neural Networks Artificial neural networks (ANNs) are computational algorithms that are able to solve complicated problems using artificial learning to fit a desired output based on a given input. ANNs are composed of artificial neurons, which are smaller computational units. Each neuron takes input data, multiplies each input by a weight, sums the resulting values, and then provides an output to the mass network. The network combines the values and provides a cumulative output based on the values from each individual artificial neuron. The network’s output is improved upon by the process of backpropagation, in which the weights within each neuron are adjusted until the output is optimally fit to a desired function (McClelland and Rumelhart 1986).

In a time delay artificial neural network (TDANN), the inputs provided to each neuron are time-series data. The previous time points of each signal input are used by the network to generate the output function, by weighing the change of a signal over time instead of only the instantaneous value of the sensors. The number of delays and

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number of neurons involved affects the training process and output of the data. Too few neurons will likely not learn the optimal mapping system, while too many neurons may fit the output perfectly for a given set of data but not be general enough to fit a new set of input data, a process known as memorization (Pulliam 2013). A well designed neural

network will be applicable to both the original data set and similar data sets, creating a

fitting function that can be used across sessions and subjects.

Hypotheses

We hypothesized that the oral pressure pattern generated by swallowing would be

distinctive from non-swallowing activities and can be sensitively and specifically

detected by a computer algorithm. The time-delay neural network will be able to

recognize the temporal pattern that is present in the natural swallow, generating the

algorithm that will be effective across a variety of subjects, including both healthy and

dysphagic subjects.

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Materials and Methods:

Subjects and Palatometer Production Prior to enrollment all subjects underwent a comprehensive general health and

swallowing disorder pre-screening. Primary inclusion criteria for Parkinson’s disease

subjects included: clinical diagnosis of idiopathic PD, age between 30-79 years, Hoehn

and Yahr stage I-III when off anti-parkinsonian medication, and the ability to safely consume an oral diet meeting modified levels 6 or 7 on the Functional Oral Intake Scale

(Crary, Mann, and Groher 2005) . Primary exclusion criteria for the healthy and dysphagic subjects included: existing stroke, presence of dementia, any medical or musculoskeletal contraindications to participation, or dental concerns warranting refusal to wear the palatal mouthpiece.

Participants who meet initial screening qualifications were seen for an initial visit, and then for up to three subsequent visits. The first visit involved the completion of a brief oral mechanism examination, interview, and clinical swallowing screening. Following the consent process, primary impression of the upper teeth was made. A proper sized impression tray was selected for the subject, then a mixture of Jeltrate Alginate Fast-Set

(Dentsply Caulk, Delaware, USA) or Delikit putty (HappiDen, Seoul, South Korea) was placed in the tray and inserted into the mouth. Following the necessary hardening, the impression was removed and rinsed. Subsequently, the impression was cast in

ResinRock (Whip-Mix Corporation, Louisville, KY, USA) to create a mold of the subject’s mouth. An example impression and resin cast of the mouth is displayed in Figure 4-1.

The subject was then released until their return visit. Following hardening of the resin, a

108 temporary splint PVC sheet (Patterson Dental, St. Paul, MN, USA) was molded using an

Econo-Vac (Buffal Dental Mfg Co, Syosset, NY, USA) onto the resin mouthpiece, which was then cut away to create a plastic mouthpiece designed to fit that subject individually.

Figure 4-1 : Example negative and positive of dental mouthpiece production.

Seven miniature stainless-steel diaphragm pressure transducers (type 105S,

Precision Measurement Company, Michigan, USA), capable of recording positive and negative pressure, were lightly encased in MED-4011 (NuSil, Carpenteria, CA, USA). For the first four subjects, the seven sensors were encased together into a seven-sensor pattern, displayed in Figure 4-2. To accommodate the curvature of the palatal ridge, for the remaining subjects each sensor was encased individually. The seven sensor

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placements replicated the small, medium, and large patterns of seven sensors

presented by Hori and Ono (Hori 2009). The PDMS-encased sensors were secured into

position using Pink Denture Carving Wax (Master Dental Laboratory Waxes, Seattle, WA,

USA). Dental wax sufficiently held the sensors in place and filled in gaps between the

PDMS and the mouthpiece, preventing accumulation of materials. The wires from the

seven sensors were interwoven and curved behind the posterior molar, adhered with

wax to the buccal aspect of the mouthpiece, and led to exit the mouth through the right

labial commisure. A custom-built wheatstone bridge amplifier was designed to filter and amplify the signals from the seven sensors en route to data acquisition by the computer.

Figure 4-2 : Oral mouthpiece with seven sensors in place.

When the subjects returned, they were fitted with their custom-mouthpiece to determine if it was properly sized and to ensure it did not significantly affect their natural oral activity. Following a brief acclimation period, subjects were provided with stimuli of swallowing materials or instructed to perform a non-swallowing behavior.

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Swallowing stimuli included 5mL or 10 mL water, thickened liquid, pudding, 5 and 10 mL

of fruit, crackers, and saliva (dry swallow). Non-swallowing activities included rest,

isometric pressure, counting to 20 (speech), singing “Happy Birthday”, and coughing.

Full listings of the activities are provided in Table 4-1.

Swallowing Non-Swallowing Saliva/Dry Swallow Singing (Happy Birthday) Sequential Sipping Chewing 5 mL Water Speech (Counting 1-20) 10 mL water Coughing Carbonated Water Rest Thickened Juice Isometric Pressure Fruit Cocktail Inter-Swallow Activity Pudding Crackers

Table 4-1: List of swallowing stimuli and non-swallow activities.

To confirm oral swallowing activity, EMG recording of suprahyoid and infrahyoid

muscles was performed during the trials. Significant muscle activity is expected during swallowing as the submental muscles control the motion of the tongue and laryngeal elevation. Electromygraphy recording electrodes were applied to the subject over the submental and infrahyoid muscles. To detect swallowing apnea and confirm swallowing, two breathing belts (Pneumotrace, ADInstruments, Colorado Springs, CO, USA) were attached to the subject to record respiration. One belt was placed around the upper torso, and another around the abdomen. Wires from the EMG electrodes, breathing belts, and palatometer were connected to the custom-built amplifier and filter system.

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The output from the amplifier of the seven pressure sensors, two EMGs, and two breathing belts was visualized and recorded using a BNC-2110 DAQ (National

Instruments, Texas, USA) and a custom built program in Labview (v 8.5, National

Instruments) and sampled at 1000Hz. Data of each swallowing stimuli and instruction period was recorded for analysis in Matlab (v2012a).

A time notation was made whenever swallowing was observed, whether a stimuli was provided or not. The computerized data for each study was annotated to mark the oral transport phase as specified by a speech and language pathologist with reference to the recorded timing (ton, toff). The time period of maximum oral activity within each swallowing period was further annotated for training a neural network (txon, txoff). This emphasis period was an approximately 0.5 second region within the specified time range that included the onset or first significant change in pressure values. The purpose of this further annotation was to ensure the network trained to recognize the onset region of each swallow, and not try to obtain the same result on the quieter portions of the time region. Periods of isometric pressure were marked for network analysis (Isoon, Isooff). Isometric pressure application caused the largest magnitude pressures, so they were given a negative value within the objective function to increase likelihood the network will not detect isometric pressure as swallowing.

Following the study the data was smoothed using a lowess filter and then divided into 100 ms sections. The 100 ms sections of each sensor were averaged and standard deviations of each of those data sections were recorded for each sensor.

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Time Delay Artificial Neural Network Analysis

After filtering and annotation into swallow and non-swallow events, the data was setup to input a selection of the seven sensors and their standard deviations into a time delay neural network. The input the network was trained to fit was an objective function defined by the timing of the peak activity region and isometric pressure application periods. The objective function ascribed a value of 1 to the peak activity regions and a value of -1 to isometric pressure regions. The neural network uses current and prior data equal to the number of inputs multiplied by the number of time delays to calculate the output of the network, a single value for each time point. A flowchart of the setup of the network preparation and analysis process is shown in Figure 4-3.

Figure 4-3 : Diagram of TDANN data preparation and analysis.

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For each recording session, 40% of the swallows, isometric events, and non-

swallow events were designated as validation data. The neural network assigned was

trained on the remaining 60% of the swallows. The swallows were randomly divided

between training and validation, with a balance of each of the different oral activities

included for both the training and validation datasets of the neural network. For each

network, a specific number of artificial neurons and number of input time delays were programmed into the network for training. Training was terminated when the mean square error (MSE) of the network’s fit to the validation data was minimized and not improved upon for 50 consecutive iterations of training. Mean square error is a measure of the closeness of fit of the output to the objective function. The termination parameters were chosen to ensure that the network had sufficent opportunity to improve.

The fitted output of the neural network provides a confidence measure for every

timepoint of its likelihood of being a swallow. The output of the neural network for the

validation data was broken into individual swallow/non-swallow sections, and the

maximum output confidence measure of each region was recorded. A rolling threshold was applied to the maximum values of each region such that if a value was over threshold the region was considered detected. Adjustments to the threshold level resulted in varying true positive and false positive rates, creating a receiver operating characteristic (ROC) curve, and allowing for measurement of the accuracy of the network. The networks were run to completion, and the optimal network, as determined by the sum of the maximum accuracy (Accmax) and ROC area, were saved.

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These network weights and biases then could be applied to other data sets to measure the applicability of that trained network on other recording sessions of that subject and other subjects. The equations for the true positive rate, false positive rate, and accuracy at each threshold level are displayed below.

= = = + 푇푃 푇푃 푇푟푢푒 푃표푠푖푡푖푣푒 푅푎푡푒 푆푒푛푠푖푡푖푣푖푡푦 푃 푇푃 퐹푁 = (1 ) = = + 퐹푃 퐹푃 퐹푎푙푠푒 푃표푠푖푡푖푣푒 푅푎푡푒 − 푆푝푒푐푖푓푖푐푖푡푦 푁 푇푁 퐹푃 TP + TN = = + TP + TN + FN + FP 퐶표푟푟푒푐푡 퐴푐푐푢푟푎푐푦 퐶표푟푟푒푐푡 퐼푛푐표푟푟푒푐푡 The optimal number of hidden layer neurons and delays was determined by training a series of networks using different input parameters and analyzing the resulting outputs. A representative sample set of six sessions from two subjects was selected to minimize the output error and maximize the efficiency of training. The data was placed into time delay neural networks programmed with a variety of neurons (10-

80) and time delays (2-12). The networks were trained and output measures were recorded to determine the optimal number of delays and artificial neurons, then these architecture parameter values were used for analyzing the remainder of the data.

Once the optimal number of neurons and delays were found, TDANN training was performed individually for each session, for each subject’s collective sessions, and for a combined selection of sessions from a group of different subjects. Once the networks are optimized for their targeted training and validation data, the networks can

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be applied to other sessions and other subjects to measure the overall effectiveness of a

network. Because the pattern of swallowing is expected to be similar across subjects, a

trained TDANN should obtain a similar degree of accuracy when applied to other

subjects. Varieties of sensor combinations, including all seven, the midline three, and

the T-shape used in Ono and Hori’s latest design, were chosen for each run of the

network to determine the effectiveness of a selection of sensors compared to the full

set.

Results

Palatometer Recordings A total of 40 sessions were recorded: 22 sessions were recorded from a set of twelve healthy subjects and 18 sessions were from seven dysphagic subjects. The healthy subjects included seven females and five males, average age 42.1 ± 16.2, while

the dysphagic subjects included two females and five males, average age 62.1±8.7.

Pressure sensor data offset was removed using data prior to stimuli application for

calibration. Data from sensors that became disabled in the middle of a session were

zeroed.

Each session was annotated for timing of swallows. There were a total of 2058

swallows: 1114 from healthy sessions and 944 from dysphagic sessions. An example

recording of five swallows can be seen in Figure 4-4. Example swallows of eight different

stimuli and four non-swallowing activities are displayed in Figure 4-5.

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Figure 4-4 : An example recording of five swallows of 10mL of thickened liquid (Nectar consistency). The seven palatal sensors are plotted along with the recorded EMG and breathing belt (BB) activity.

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Figure 4-5 : Examples from one subject of palatometry recordings for a variety of swallowing stimuli and non-swallowing activities.

Once the data was annotated, a subset of six sessions from two subjects were

compiled and analyzed with the TDANN, specifically with the goal of optimizing the

parameters for swallowing detection. The architecture parameters of number of hidden

layer neurons and the number of input time delays were varied for the combined

dataset and the network effectiveness as measured by MSE, Accmax, and ROC area were

recorded. Each pairing of neuron volume (10, 20, 30, 40, 60, and 80) and delays (2, 3, 4,

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5, 7, 10, 12) was trained, validated, and graded ten times. Figure 4-6 shows the effects

of varying these parameters on effectiveness.

Figure 4-6 : Plots of the effect of varying the number of neurons and time delays on Accuracy, ROC area, and MSE. The darkened lines signify the optimal values of 7 delays and 40 neurons.

Increasing the number of delays decreased MSE significantly up until the number

of delays reached 5, then it plateaued, showing no significant improvement when more

delays were added. Accmax and ROC curve area peaked when either five or seven delays were used; fewer or more delays reduced the accuracy, though only a non-significant

amount except for in relation to 12 delays. Seven delays showed a non-significant

increase in accuracy and ROC area over five delays. Similar results were obtained for progressively increasing the number of neurons, as optimal values in MSE, ROC area,

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and Accmax were found in the range of 20-40 neurons. Generally, increasing the number of neurons improved the ability of the network to fit the training data, but only up to a point did the increase in neurons improve the output accuracy and ROC area of the network. Fewer or greater numbers of hidden layer neurons reduced the accuracy and

increased the MSE. There was no significant difference in scoring between twenty and

forty neurons. Given these results, further training was performed with networks using

these parameters: 5-7 delays and 40 neurons.

In addition to altering the number of neurons and time delays, the number of

sensor inputs provided to the network was adjusted to determine the effectiveness of

varying numbers and orientations of sensor patterns. Using the same data set of six

sessions, a hidden layer of 40 neurons, and five time delays, a network was trained using

either all seven sensors, a T-shaped set of five sensors, the three midline sensors, the

anterior and posterior midline sensors, and only the posterior midline sensor. Diagrams

of these placements can be seen in Figure 4-7, and the output measures of these

arrangements can be seen in Figure 4-8. As expected, increasing the number of sensors

improved the outcome measures. There was no significant difference between five and

seven sensors (p>0.05) for any of the grading measures, meaning that though seven

sensors was the most accurate, five sensors may be sufficient for a swallow detection

system.

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Figure 4-7 : Diagrams of selections of sensor patterns used for training.

Figure 4-8 : Effect of varying the number or choice of sensors on Accmax, ROC area, and

MSE. The full set of seven sensors allowed for the highest Accmax and ROC area, as well as minimum MSE. There was no significant difference between the seven sensors and the T-shaped five sensor pattern on these output measures (p>0.05).

Training was performed for each individual session ten times using 40 hidden

layer neurons, seven delays, and seven input sensors. The average Accmax was found to be 0.960 ± 0.028, and average ROC area was 0.982 ± 0.021. In four of the recording sessions, an optimal network was generated that obtained a perfect accuracy for the training and validation data, displayed in Figure 4-9. The lowest Accmax measured for an individual subject was 0.869, and the minimum ROC area was 0.902. The healthy networks obtained a mean Accmax of 0.964 ± 0.026, while the dysphagic networks

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obtained a mean Accmax of 0.955 ± 0.032. There was no significant difference between

the two, meaning that the network design is equally effective for healthy subjects and

the dysphagic subjects that would most likely use an oral palatometer.

Figure 4-9 : ROC curves from all 40 sessions with their best self-trained network individually plotted. Networks that obtained a perfect ROC area are marked with *, while curves with ROC area greater than 0.99 are marked with a †.

Intra-Subject and Population-Trained Networks Eight subjects returned for the full three visits. Each subjects’ three sessions

were compiled and a network was trained on the compiled data. The three single-

session-trained networks and the multi-session intra-subject network were each applied to the three datasets to measure intra-subject efficacy of the TDANN training process.

Intra-subject variability in the recording sessions led to the networks not remaining as

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effective when applied to a new dataset from the same subject. On average there was a

4.7% decrease in Accmax between sessions, the median Accmax decreasing from 0.9661 to

0.912. The intra-subject network, when reapplied to each of their three datasets, had a median Accmax of 0.9660, the same as the individually trained datasets. The addition of data from the same subject did not negatively impact or improve the network’s accuracy. Examples of these single-session and intra-subject networks applied to

individual sessions can be seen in Figure 4-10 and Figure 4-11.

Figure 4-10 : Example of ROC curves from networks trained on one session then applied to another session from the same subject.

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Figure 4-11 : ROC curves of five networks applied to the same dataset. To compare patterns between subjects, the pressure amplitudes of each session was scaled. The pressure signals were adjusted in amplitude such that the median peak pressure of each sensor during 10mL water swallowing was equal, increasing the amplitude of weaker signals and decreasing the amplitude of stronger signals. This standardization was performed to remove variation due to adjustments in the equipment, scaling the amplitudes to allow the training algorithm to focus on temporal patterns and improve intra-session network comparison.

An inter-subject or population-trained network was produced to determine if a single network could be used to detect swallowing across all subjects. TDANN training was performed ten times each on standardized and non-standardized data from a series

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of twelve subjects (seven healthy sessions, five dysphagic sessions). When the best of each of these population-trained networks was applied to the full dataset of 40 sessions, a median ACCmax of 0.932 was met for standardized data and a median ACCmax

of 0.920 was obtained for non-standardized data. ROC curves of the standardized

network applied to individual datasets is shown in the final column of Figure 4-10, and

also on Figure 4-11. The effect of these networks on the full compiled data set of 40 subjects is presented in Figure 4-12.

Figure 4-12 : ROC Curve of the best population-trained TDANNs, both of non- standardized and standardized data. The network with the greatest ROC area is the one trained on standardized data.

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Discussion Increasing the number of input delays of the neural network in effect makes the system review a longer period of time for each analytical output period. This allows the pattern of swallowing and the change over time to be used as a computational factor.

Analysis showed that the optimal number of delays was between five and seven, signifying the prior 500-700 ms of data is incorporated into each prediction. Figure 4-6 showed that further increases in the number of delays caused no significant increase in predictive ability, as measured by accuracy or ROC area. Similar trials run adjusting the number of hidden layer neurons showed that beyond 40 neurons, there was no significant increase in algorithm performance.

While our mouthpiece was capable of recording from seven separate sensors, there are advantages to a detection device that requires fewer sensors. Each sensor increases the occupied space in the oral cavity, requires additional computation, and draws additional power. Toward this end, the simulations were run with varying numbers of sensors. Training of the networks were performed with selections of the sensors including the full seven, the T-shaped set of five sensors promoted by Ono and

Hori, the midline three sensors, the midline anterior and posterior sensors only, and the single posterior sensor, seen in Figure 4-7 (Kazuhiro Hori et al. 2009, 2). Welch’s t-test showed that there was no significant difference between five and seven sensors

(p>0.05). These results signify that for a detection system, if size and power become an issue, the system can be reduced to a five-sensor device without losing significant accuracy.

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One significant problem with the neural network was the recognition of low-

amplitude signals. In some subjects, an individual or group of swallows would induce

significantly lower pressure than other swallows in that subject, resulting in poor

training or detection. Dry swallows, swallows of only saliva, were the most common

example of this. Analysis of the amplitudes of these swallows showed that the peak

pressure on the anterior midline sensor during dry swallows was on average 37%

weaker than during 10 mL water swallows. This is reflected in previous research that

reported smaller range of movement during dry swallows (Hamlet 1989).

In the first four subjects, swallowing materials, especially the cracker had a

tendency to stick under the pressure sensors and prevent complete swallowing. The

residue would irritate the subjects, leading to the premature cessation of the study

session. Early sensor platforms were thick and poorly shaped to the curved palate, creating crevices between the mouthpiece and sensor platform and increasing surface

area. As more pressure sensor platforms were created, the thickness was reduced with

increased skill of the researchers, and improved wax methods allowed for adherence of

the sensors to the mouthpiece without crevices.

Optimizing Thresholds for FES systems The analysis of the ROC curves brought interesting questions as to the optimal

threshold for the users. The threshold that had the highest accuracy usually had both a

few false positives and a few false negatives. This is due to the equation ascribing each

correct response, whether a true positive or true negative, equal balance. The goal of

the palatometer is to serve as a controller for an FES device to protect the airway during

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swallowing. An accidental recognition (false positive) would cause inopportune

activation, but would not lead to potential harm to the body, whereas a missed swallow

(false negative) may result in aspiration. Toward this end, greater emphasis should truly

be placed on correctly obtaining a high true positive rate, or maximizing selectivity. The proper threshold, therefore, would place the ROC curve in a position on the upper axis of the box (TPR=1) and maximize the specificity (minimize the FPR).

When the 40 individual session networks were applied to their representative datasets and thresholded to the levels of a perfect sensitivity (TPR=1), the median of the

maximum specificities that could be obtained from each subject was 0.839. If the

sensitivity was allowed to drop to 0.95, the median maximum specificity was 0.952. This

means 5% of the true positives were missed, while 5% of non-swallows were detected

as swallows. When the intra-subject network and the population-trained network were

applied to each experiment and the selectivity was restricted to 0.95, the median

maximum specificities were 0.946 and 0.863. The effect of lowering the minimum

sensitivity for the intra-session, intra-subject, and population-trained networks are

displayed in Figure 4-13.The individually trained network is the best fit, and the intra-

subject network is not significantly different, but the population-trained network still

obtains a high specificity. Networks trained to fit the specific subject are generally more

accurate. While the accuracy at this position is likely lower than a threshold that

balances false positives and negatives, the optimal functional effect of the system is

derived from a threshold that obtains a selectivity value greater than 95%.

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Figure 4-13 : Barplot of the median maximum specificity from the output of self- trained networks, intra-subject networks, and the population-trained network.

The current palatometer can be used to record pressure patterns clinically, and the combination of this device with other clinical devices should be used to learn more about the swallow. Video fluoroscopy swallowing studies while wearing the palatometer would allow for improved knowledge of the bolus propulsion process, as pressure in the mouth could be correlated directly to the location of the tongue and bolus.

Articulography and other imaging systems would allow for similar analysis of tongue location during swallowing and relationship between position and pressure. Of particular interest, due to the new capabilities of this device, is the position of the tongue and palate during the recording of negative pressure. Analysis of the relationship between oral pressure, tongue position, and tongue velocity would allow for

129 understanding of the dynamic relationships and improvement of oral modeling. The measurement of sealed oral compartments and the pressures induced would provide interesting data on the capabilities of the mouth.

Prior studies have been performed to develop methodologies of automatically detecting the swallow. Amft and Troster’s system, which incorporated infra-hyoid EMG and audio recording, was able to obtain a true positive rate of 0.84, but the precision was only 0.18 due to the significant number of false positives (Amft and Troster 2006).

The population-trained TDANN in this study was able to obtain a true positive rate of

0.90 with a precision of 0.80, an improvement on both counts. The oral palatometer has higher specificity and selectivity detection of swallows. In addition to improved accuracy, the palatometer detects the swallow during the oral phase, earlier in the deglutition process than the EMG/microphone system. The palatometer and detection algorithms have significant potential as clinical and research tools for swallowing studies.

All current palatometry designs, including this one, require a physical connection to the exterior to transfer the pressure signal to the recording equipment. While the wires used in this system are smaller and more flexible than air tubing, future designs should incorporate a wireless system. A battery-powered transmitter could be incorporated into the mouthpiece and used to send the signal to the computer or other recording device. This would allow for more user flexibility, as the device would not have to remain tethered to a box on a countertop. Recording could be performed by a

130 tablet or smartphone with wireless technology, enabling the entire system to be portable. The FES stimulator could receive the signal directly from the mouthpiece, creating a closed-loop system that is less obtrusive than current devices.

Conclusion The TDANN is effectively trained using a selection of data, creating an algorithm that has higher sensitivity and specificity than any existing detection method. The larger the training set, the better the network, especially if some of the desired subject’s data is included within the training and validation. Lowering the threshold to ensure a sufficient TPR will increase FPR, but this is what will improve protection for an FES device. A properly used detection algorithm will over-activate, as oral protection (true positives) is the most important factor. Further study of the pressure pattern of each individual swallow and development of a more advanced algorithm will allow for detection of the specific type of swallow occurring. The algorithm may also specify tongue position during rest and isometric pressure, detections that could be used for rehabilitation and training. Time delay artificial neural network detection of oral transport phase is a step that will allow for further understanding of the swallowing process.

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Chapter 5 - Discussion and Conclusions

Overview of Dissertation Providing new methods for aiding airway protection is important for treating

patients with dysphagia. The purpose of this research was to quantify and combine two

methods of applying stimulation to replicate airway protection mechanisms and develop

an automatic controller to recognize swallowing. The performance of novel stimulation

methods was tested in pre-clinical models and analyzed by video evaluation of

functional output. The developed algorithm was applied to recorded palatometry data

to determine the effectiveness over time of a control signal based on training data. Our

hypotheses were that the novel stimulation methods would generate significant

laryngeal elevation and vocal fold closure, and that simultaneous stimulation would not

contraindicate either output. Additionally, we hypothesized that the palatometry signal

could be used as a control signal to sensitively and selectively recognize swallowing.

As discussed in Chapter Two, laryngeal elevation stimulation paradigms were

tested in pre-clinical surgical methods to determine the amount of motion that could be induced in relation to previously tested methods. Chapter Three discusses our determination of the optimal location of transtracheal stimulation, and it was shown

that bilateral transtracheal stimulation remains effective during hyolaryngeal elevation

induced by XII/C1 stimulation. Palatometry proved to be an effective recording source

for selectively and specifically detecting swallowing, as presented in Chapter Four. These

developments serve as improvements on existing FES devices and can be used as tools for rehabilitation and long term care.

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Contribution/Relationship to the Literature Protection of the airway is critical for health, especially in stroke and Parkinson’s

disease patients, as these conditions often result in disability of functional protective

mechanisms and dysphagia (Troche et al. 2010; Heuschmann et al. 2004). Prior stimulation methods have been applied to improve laryngeal elevation and vocal fold closure, to varying degrees of effectiveness. Surface stimulators are clinically prescribed by physicians and therapists, even though the resulting elevation has been shown to be minimal at best, or depressive at worst (Logemann 2007; Humbert et al. 2006).

Temporarily inserted intramuscular electrodes induce up to 50% of natural laryngeal

elevation, and while this is said to be clinically effective, there has been no translation to

clinical use (Burnett et al. 2003). Nerve cuff stimulation, specifically using a selective

FINE, enables targeted activation of desired muscles (D.J. Tyler and Durand 2002). Prior

studies had selectively stimulated the oral muscles of tongue movement, meanwhile

demonstrating that selectivity of the geniohyoid muscle was possible (Paul B. Yoo,

Sahin, and Durand 2004). This study used a more proximal placement and was able to

effectively stimulate two of the primary muscles used during laryngeal elevation without

the contrary or negative impact of other oral muscles. The magnitude of motion induced

was significantly higher than that induced by intramuscular electrodes, and while this

was a canine model, the increase in elevation is expected to be on par with that which occurs during natural swallowing.

Vocal fold closure, another critical protective mechanism during swallowing and lost in many dysphagics, has been induced by prior researchers to varying degrees of

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efficiency. Surface stimulation had been shown to be minimally effective, while

implanted nerve cuffs induced complete closure (Humbert et al. 2008; Broniatowski et al. 2010). Because many patients require a tracheostomy, we used this access to determine if an effective method of stimulation could be applied without requiring further surgery or implantation. Transtracheal stimulation had been applied previously, initially for determining RLN validity during thyroidectomy. Further studies showed it was effective at adjusting vocal fold position, but the optimal position and necessary stimulation parameters needed to be known if a device was going to be used clinically.

This study recorded the optimal angular positions of stimulation and measured the variability, while visualizing the extent of vocal fold closure. The analysis shows that the optimal stimulation location is in the expected range of the nerve position and that stimulation under 5 mA is sufficient to induce complete vocal fold closure.

Palatometry has been recorded before for research studies of oral capabilities and as a clinical tool to understand the capabilities of users with speech impediments and oral deficiencies. Clinically available devices use air-filled bulbs, while electric

sensors have been used more recently to increase the number of sensors that can fit

and reduce the profile of the device (Kazuhiro Hori et al. 2009; Kieser et al. 2007). We

developed a low-profile palatometry system which recorded positive and negative

pressure concurrent with respiration and EMG of submental muscles. Previous studies used this data to learn more about the typical swallowing pattern, some going as far as to say there was no “typical” swallowing pattern (Kennedy et al. 2009). Our research was the first to utilize any form of recognition algorithm to detect swallowing

135 automatically from palatometry. The population-trained time-delay neural network selectively recognizes swallowing, allowing for a threshold that can be adjusted to optimize the algorithm for each subject. This generalized algorithm has potential for use in research and clinical trials to understand the activity of swallowing throughout the day.

Future Studies

The domain of this interdisciplinary research spans across the fields of neural engineering, otolaryngology, and speech and language pathology, and has different and valuable impact in each domain. The most important takeaway from this research is the increase in measurable effect that can be induced by stimulation, and what remains to be found is whether this quantitative improvement causes a functional improvement in the target population. Properly manipulated stimulation will induce laryngeal elevation and vocal fold closure, but human studies must be performed to determine if this improves protection of the airway, reducing aspiration, and reducing aspiration pneumonia, the true functional goal of these stimulation methods. Our hypothesis is that the techniques developed here will cause this functional improvement. This hypothesis will have to be tested through a series of further studies, moving from canine to human subjects, considering a wide variety of dysphagia symptoms.

Stimulation Given the results of the combined laryngeal elevation and vocal fold adduction studies, the information learned should be applied to translation of the results into

136 application in the clinic. This should be performed in two steps. Initially, a less-invasive design should be used to allow testing for effect and clinical validity of combined stimulation. Transtracheal stimulation can be immediately applied to patients whom already have undergone tracheostomy. One primary question that could not be answered in the pre-clinical model is what amount of sensation is induced by transtracheal stimulation. Electrical stimulation has not been applied directly to the trachea in aware subjects, and it is unknown whether current will induce sensations of pain, tingling, or even a cough reflex at the current levels being used to induce vocal fold closure. The angular positions determined by our research and comparison to known human anatomical data has been used to professionally produce electrified tracheostomy tubes. Specifications were given to a manufacturer and a trial run of 20 tracheal tubes was produced. An IRB for human studies has been written and approved for experimentation at University Hospitals in Cleveland, Ohio. The study will apply various stimulus patterns while laryngoscopy is recorded and the subjects return information on levels of sensation. It is expected that transtracheal stimulation will be able to be provided without inducing significant pain, allowing vocal fold closure.

If transtracheal stimulation proves to be effective at producing vocal fold closure without restrictive sensation or other negative response, then there are a few ways it could be used as a clinical tool. As the professionally made devices currently are designed, they could serve as a clinical device used during rehabilitation, temporarily placed and stimulated to measure vocal fold motion and exercise weakened muscles.

Electrified tracheal tubes with attached stimulators could be provided as a vocal fold

137 adductor system for dysphagics. Alternately, the electrified transtracheal tube could be adapted into an electrified transtracheal stent, a device inserted semi-permanently into the trachea through the mouth, removing the need for a tracheostomy. This device would require transmission of signal and power to the tracheal stent via either a lead or wireless transmission, potentially implanted during an outpatient visit.

Laryngeal elevation by nerve stimulation needs to next be intraoperatively tested in humans to determine both the extent of laryngeal elevation and optimize the location. Using anatomy and histology, a proper electrode and placement could be applied temporarily in an experimental subject undergoing an unrelated head and neck surgery. EMG recordings would allow for determination of the effect of various electrode locations, and video of the induced motion would allow for measurement of the elevatory effect. Prior to and after the study, the subject’s natural range of motion could be measured using intramuscular stimulation and natural initiation of the swallow for contrast, with the hypothesis being that nerve stimulation can obtain at least as much elevation as intramuscular stimulation.

Palatometry The palatometer, as a low-profile recording device, has potential for many clinical uses, especially if the recording process can be reduced to a wireless platform.

Making the palatometer wireless would enable mobilization of the user and allow use outside the clinic. Wireless signals could be sent to a designed receiver or picked up directly by the Bluetooth antenna already in computers or smart phones. Clinicians could use the signals to diagnose, assign exercise, and test rehabilitation progress.

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Programs using the wireless palatometer could include simple oral data presentation,

interactive visual instruction to the user for training, games to practice tongue motion, or even serving as a controller for other devices such as a television or remote control car.

A wireless system would require the sensors, a power source, transmitter, and

other required electronic components to all be placed in the mouth. While one perk of

electronic sensors used in this study was reducing the size of the device, the addition of

these other components will significantly increase the size of the oral equipment. The

mouth has a few possible locations for placement of the components, primarily in the

arch or labial cheek. Minimization of the size of these components will be critical, but

the benefits of developing a wireless system by fitting the components into the space is great.

The neural network can be incorporated into existing clinical recording processes to aid clinicians with detecting swallowing. Currently, while recording palatometric data, clinicians must manually record timing of observed swallows, something that can be done automatically with the computer program. While it will not be perfectly accurate, at least the program will point out positions of likely swallowing to inform the clinician of observations they otherwise may have missed due to small exterior motions, multiple swallows, or even looking away from the subject.

To further understand the palatometry recordings, the device needs to be used in conjunction with oral imaging devices. The palatometer could be worn while

139 undergoing videofluoroscopic swallowing studies or articulography, enabling timing of the pressure signals with tongue and bolus position. The timing of the phases of swallowing would be able to be measured in conjunction with the visualization of bolus flow. This further information could then be incorporated into a more complex TDANN that allows for detection of tongue position, swallowing phase, and even bolus size and material. Increased output from the TDANN would allow for increased applications of the device.

Application of Research

Rehabilitation Once transtracheal stimulation has been shown to be effective and accepted in human studies, it can then begin to be used in rehabilitation environments. Current rehabilitation methods, such as VitalStim, use application of electrical stimulation or vibration as a method of providing sensory information to the body during swallowing.

The idea behind this sensory signaling is that reinforcement of the feedback loop that occurs during a healthy swallow will increase the body’s ability to relearn lost capabilities through the idea of neuroplasticity. While increased sensation may aid the central nervous system’s response over a long time period, the application of transtracheal stimulation or intramuscular stimulation for laryngeal elevation will have the immediate effect of aiding the protective mechanisms occurring during swallowing.

The contractions induced by these methods will induce sensory recordings that will be felt by the brain, similar to what is occurring during existing neuroplasticity trials.

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Because the body is feeling elevation instead of some unrelated vibration in the same area, we hypothesize that the sensory rehabilitative effect will be increased.

Within the rehabilitation population, there are subjects who retain the ability to

induce laryngeal elevation and vocal fold closure, but the magnitude or velocity of these motions is reduced. The paresis that these subjects endure reduces their capabilities to safely swallow. EMG recordings of the laryngeal muscles can determine when they are activating, even when they are activating only a small amount. A feedback system could be used to reinforce the activity of these muscles, using either the EMG signal from the desired muscles or the palatometer as the command source to assist the weakened muscles. This muscular assist would allow for both protection and rehabilitation, as the

patient would relearn the abilities of this muscle through the sensation and exercise of

the muscle. Increased contraction instructs the brain how much it is meant to contract,

allowing for relearning of the possible motion.

Implantation For those patients who do not regain safe swallowing following rehabilitation

therapy, application of a complete FES system for laryngeal manipulation would allow

for external control of the muscles that have lost central control. If oral transport phase

patterns remain, a wireless palatometer would be used for detecting swallow. This

trigger would then be sent to the stimulator, which would have leads to stimulate both

for laryngeal elevation and vocal fold closure. A fully implanted stimulation system

would apply selective nerve stimulation to nerve cuffs on the recurrent laryngeal nerve

and to either a single cuff on the XII/C1 complex or a pair of cuffs on distal branches to

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the desired muscles. Bilateral stimulation, as demonstrated in Chapters 2 and 3, was

shown to have no significant increase on laryngeal elevation or vocal fold closure.

Unilateral implantation would thus be sufficient for a protection system.

If the nerve implant system is considered too invasive by the physician, an

externalized stimulation system using transtracheal stimulation and intramuscular

stimulation could be used for protections. A single stimulator placed around the neck

could provide current to an electrified tracheal tube and implanted intramuscular

electrodes. The stimulator would receive the activation command from a wireless

palatometer, allowing for closed loop control.

Overall Conclusions Dysphagia is a significant problem that can be treated by application of FES. This

research presents new methods of applying FES to induce protective mechanisms that

occur during swallowing to prevent aspiration. Nerve cuff stimulation proved to be more effective than either surface or intramuscular stimulation, providing a new tool for the clinician to aid swallowing protection. Transtracheal stimulation, a variation of surface stimulation, can seal the glottis as necessary during swallowing, meaning this non-surgical method can be used in a significant patient population. Laryngeal elevation and vocal fold adduction stimulation can be applied simultaneously to provide increased airway protection, laying the groundwork for clinical trials of a fully implanted stimulator. Palatometry serves as an accurate detector of deglutition, enabling its use as

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the natural command source of an FES stimulation system. The contractions generated

by these methods next need to be translated into a full system for clinical use to reach

the largest patient population and allow for maximal impact to reduce aspiration pneumonia.

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