Am J Otolaryngol 10:181-187, 1989 Effect of Superior Laryngeal Nerve Stimulation on Phonation in an In Vivo Canine Model GERALD S. BERKE,MD, DENNIS M. MOORE, MD, BRUCER. GERRATT, PHD, DAVID G. HANSON, MD, AND MANUEL NATIVIDAD We investigated the effect of variation in superior laryngeal nerve stimulation (SLNS) on vocal fold vibration. Photoglottography (PGG), electroglottography (EGG), and subglottic pressure (Psub) were measured in seven mongrel dogs using an in vivo canine model of phonation. The PGG, EGG, and Psub signals were examined at three SLNS frequencies (100 Hz, 130 Hz, and 160 Hz) using a constant rate of air flow. Increasing SLNS, which causes a contraction of the cricothyroid muscle, produced a marked increase in F,, little change in Psub, an increase in the open quotient, and a decrease in the closed quotient of the glottal cycle. AM J OTOLARYNGOL 10:181-187. 0 1989 by W.B. Saunders Company. Although theoretical models have been impor- termed “the external tensor” of the vocal cord. tant in providing information about phonatory Through action potentials carried in the external control mechanisms, verification of current branch of the superior laryngeal nerve (SLN), CT theories will require the use of physiologic prep- activation produces a lengthening and thinning arations. This investigation was designed to ex- of the cords. Human studies using high-speed amine the effect of changes in vocal fold mass laryngeal photography during phonation have and tension on vocal fold vibration, using an in demonstrated a lengthening and thinning of the vivo canine model of phonation. The effect of cords with rising Fo,l as well as an increase in different levels of superior laryngeal nerve stim- electromyographic activity of the CT muscle.2*3-5 ulation (SLNS) and recurrent laryngeal nerve It has been shown that stimulation of the SLN in stimulation (RLNS) under conditions of constant dogs produced F, increases from 135 Hz to 540 air flow was studied photoglottographically and Hz.~ Lengthening and thinning the vocal folds electroglottographically, and subglottic pressure affects F, by decreasing the effective vibrating (Psub) was measured. This article reports the re- mass. In addition, lengthening the vocal cord sults of SLNS. causes elongation of the vocalis muscle (VOC). It has also been shown that passive and active ten- LARYNGEAL CONTROL FACTORS sion in the VOC were greatly enhanced by exter- nal stretchinga CT activation then elevates F, by The fundamental frequency (F,) of phonation lengthening the VOC, thereby increasing the depends on the effective mass and stiffness of stiffness and reducing the mass per unit area of the vocal folds interacting with transglottal pres- the vocal folds. sure. The cricothyroid muscle (CT) has been IN VW0 CANINE MODEL Received November 6, 1988, from the UCLA Division of The dog has been the principal animal model Head and Neck Surgerv.Wadsworth VA Larvnaeal Phvsiol- ogy Laboratory, UCLA School of Medicine, L&Angeles. Ac- for laryngeal studies. The canine larynx is simi- cepted for publication November 23, 1988. lar to the human larynx in size and vocal fold Supported by a Veterans Administration National Merit histology; however, the upper portion of the vo- Review grant and a grant from the National Institutes of Health. cal fold has a thicker and looser lamina propria Address correspondence and reprint requests to Gerald S. than humans, resulting in increased thickness of Berke, MD, Laryngeal Physiology-Laboratory, West Los An- the vocal fold.’ The canine larynx also has a geles VA Medical Center, Los Angeles CA 90073. Q 1989 by W.B. Saunders Company. postglottic space in some animals, and, during 0196-0709/89/1003-0003$5.00/O phonation, there is a posterior V-shaped chink 181 SUPERIOR LARYNGEAL NERVE STIMULATION behind the arytenoids. Anastomotic fibers run- lower margin closing.14 Simultaneous monitor- ning between the superior and recurrent laryn- ing of PGG and EGG signals provides informa- geal nerves in the dog (Galen’s nerve) are be- tion for peak glottal opening and glottal closure lieved to be sensory in nature. Longitudinal elas- similar to high-speed laryngeal photography.15 ticity curves of the epithelium, ligament, and muscle of the canine larynx have different ten- Experimental Preparation sion-length slopes than those for humans, but their overall shape is similar. The cricoid and Our experimental setup was described in a thyroid cartilages are more angulated and previous study and is similar to that of prior in shorter in dogs; the ventricles are considerably vivo canine studies.l”s6 Each dog was anesthe- larger and the vocal ligament is not well-defined. tized with 2 mL ketamine by intramuscular in- In spite of these differences, much information jection followed by intravenous pentobarbital concerning the mechanics of vocal fold vibration until loss of the cornea1 reflex was achieved. The has been derived from studies of excised canine animal was then placed supine on an operating larynges. table (Fig 1) and direct laryngoscopy was per- Some studies have suggested that excised la- formed to confirm normal laryngeal anatomy. A rynges do not reproduce physiologic conditions T-mm oral endotracheal tube was inserted, of vocal fold tension and mass during vibration through which the animal breathed spontane- with sufficient accuracy.g-ll The in vivo canine ously. A midline incision was made from the model appears to be a more physiologically valid mandible to the sternum. The strap and sterno- preparation for studying vocal fold vibration cleidomastoid muscles were retracted laterally than the excised larynges because blood flow to expose the larynx and trachea. The external and intrinsic laryngeal muscular tension are branch of the superior laryngeal nerves were iso- maintained while preventing postmortem tissue lated at their entrance into the CT muscle. A degeneration. These are critically important fac- gauze/silver electrode was applied to the nerves tors to consider when applying the results from and insulated from the surrounding tissue. The models of vocal fold vibration to human phona- recurrent laryngeal nerves were isolated 5 cm tion. inferior to the larynx. Electrodes were applied in the same fashion. Ground electrodes were su- METHODS tured to the trachea and connected to the anode of the nerve stimulator. Electrical isolation be- Subjects tween RLNS and SLNS was verified by direct observation. Maximal stimulation of the recur- Seven adult male mongrel dogs (weighing 25 rent laryngeal nerves, to the point at which the to 30 kg) were selected. Each dog was screened strap muscles were noted to contract (approxi- by direct laryngoscopy to assess its suitability as mately 9 volts), was not observed to produce a subject for the experiment. Dogs with long contraction of the cricothyroid muscle. In addi- necks were preferred for ease of preparation. tion, no lengthening or thinning of the vocal cords occurred during maximal RLNS. Isolated Glottographic Techniques maximal stimulation of the superior laryngeal nerves to the point at which the strap muscles Glottography has proven useful in the study of were observed to contract did not demonstrate temporal events during vocal fold movement.l’ tensing or bulging of the vocalis muscle on direct Photoglottography (PGG) uses a photoelectric laryngoscopic observation. No arytenoid adduc- transducer to measure transillumination of light tion or phonation could be elicited by maximal through the glottis during phonation.13 As the SLNS. EGG electrodes (Synchrovoice, Briarcliff vocal folds vibrate, the intensity of light trans- Manor, NY) were placed in direct contact with mitted through the glottis reflects the cross- the thyroid cartilage while the reference elec- sectional area of the glottis over time. Electro- trode was sutured to the skin. A l.O-cm button glottography (EGG) is a technique measuring the was placed to suspend the epiglottis anteriorly impedance of a small electric current across the through the thyrohyoid membrane to improve neck in the vicinity of the vocal folds. Changes in visualization of the vocal folds. A distal trache- impedance are modulated by changes in lateral otomy was performed and an endotracheal tube American vocal fold contact area, and the differentiated passed to permit the animal to breathe sponta- Journal of EGG signal (dEGG) can provide temporal infor- neously. A more proximal tracheotomy was per- formed, through which a cuffed tracheotomy Otolaryngology mation on points of upper margin opening and 182 BERKE ET AL Figure 1. Diagrammatic representation of experimental preparation. (Reprinted with permission.17) 3* PHOTOSENSOR Recurrent laryngeal n. tube was placed with its tip resting 10 cm below tion, an air flow of approximately 375 cc/set is the glottis. A catheter-tipped pressure trans- required to develop a Psub of at least 20 cm H,O, ducer was inserted through this upper tracheot- and to match target frequencies of from 80 to 160 omy. The cuff on the superiorly directed tube Hz. The air was bubbled through 5 cm H,O for was inflated to just seal the trachea. Air flow, warming and humidification, and the tempera- obtained from the UCLA physical plant, was ture in the animal’s trachea was measured at 15- passed through the cephalad tracheotomy tube. minute intervals to assure a constant air flow The rate of air flow was measured with a flow- temperature of 37°C. The PGG light sensor (Cen- meter (Gilmont Instruments, model F1500, Great tronics OSD 50-2, Mountainside, NJ) was placed Neck, NY) and kept at a constant rate of 375 cc/ on the animal’s trachea approximately 3 cm be- set throughout the study. Unlike human phona- low the larynx. A xenon light source and fiberop- tion, which can be induced with Psub in the tic cable provided supraglottic illumination for range of 6 to 10 cm H,O, canine phonation re- the PGG.