Chapter 11 Intraoperative Cranial Nerve Monitoring

Jack M. Kartush and Alice Lee

firmation by an assistant’s hand. As early as 1898, Introduction Krauze described galvanic stimulation of the facial nerve during an acoustic neurectomy, with visual con- The increasing availability and sophistication of firmation of the response.2 In the 1940s, Olivecrona3 intraoperative cranial nerve monitoring (IOM) made a concerted attempt to routinely preserve the within the last few decades has opened a new era facial nerve during acoustic tumor resections using in the pursuit of functional neural preservation dur- a facial nerve stimulator and a nurse whose respon- ing microsurgery. The use of intraoperative elec- sibility was to observe the patient for facial contrac- tromyographic (EMG) facial nerve monitoring has tions. At times, the surgery was performed under a been a standard of care for the resection of acoustic local anesthetic to allow the patient to move their neuromas and other cerebellopontine angle tumors face upon demand. During the mid-1960s, Parsons, for over a decade.1 Parallel applications are now Jako, and Hilger independently reported on dedi- routinely being used to monitor cranial nerves dur- cated facial nerve monitors that were intended for ing other otolaryngology procedures involving the use during otologic and parotid surgery.4–6 Jako’s parotid gland, thyroid, neck, middle ear, and mas- device was distinguished by a mechanical trans- toid. Although monitoring is becoming more com- ducer placed along the patient’s cheek that detected monly used for these procedures, polls suggest that facial contractions. Delgado et al7 were the first to there is currently no consensus on the role of IOM as report on intraoperative electromyographic (EMG) being a standard of care in these settings. Nonethe- facial nerve monitoring during cerebellopontine less, it is interesting to note that, although litigation angle (CPA) surgery. Using surface electrodes, they 20 years ago for iatrogenic facial palsy focused on reviewed a printout of the EMG tracings. Sugita and whether or not monitoring was used, litigation today Kobayashi8 modified this technique by using accel- focuses on whether or not it was used correctly. Thus, erometers to transduce facial motion into electrical a key objective of this chapter is to examine common energy, but commented on one false-positive error pitfalls in intraoperative monitoring because “poor due to inadvertent stimulation of the motor portion monitoring is worse than no monitoring.” of the trigeminal nerve. More importantly, they used a loudspeaker to provide acoustic feedback that allows the surgeon to interpret the evoked responses History in context to ongoing surgical events. Prass and Luders9 subsequently correlated specific patterns of EMG activity to surgical manipulations. Silverstein The concept of intraoperative nerve monitoring has et al10 enhanced Jako’s motion-detector device (WR been in existence for over a century. Early forms of Electronics, St Paul, MN) and recommended its use intraoperative facial nerve monitoring consisted of for many otologic procedures. Kartush and Prass11 visual confirmation of facial motion or tactile con- developed the “Nerve Integrity Monitor” (NIM)

165 166 Practical Neurotology and Skull Base Surgery

with Nicolet Company (Madison, WI) in 1984 that Neuromonitoring Basics was subsequently purchased by Xomed (Medtronic) and has become the most commonly used device in the United States.11 Many other companies now pro- Goals of Monitoring duce “multimodality” monitoring devices that can monitor up to 32 channels of responses not only of The goals of intraoperative neural monitoring have EMG but also EEG making them ideal for complex been well established. First and foremost is to assist spine and brain monitoring. Their sophisticated in identifying the nerve of interest. Second, infor- multimodality features, however, add a high degree mation gleaned from responses during monitoring of complexity and, therefore, are typically only used assists in the detection of injury to the nerve dur- by specially trained technologists. Many devices ing dissection. Third, stimulation proximal to the are available that are acceptable for cranial nerve manipulated nerve post dissection can provide a monitoring. This chapter uses the NIM for many predictive assessment of nerve function. examples simply because of the senior author’s 25 years of experience with it: including its strengths and weaknesses. A few other dedicated devices for Responsibility of Monitoring cranial EMG monitoring include the Magstim Neu- rosign, the WR Electronics Silverstein, WR Electron- There are two aspects of monitoring. One is the ics Brackmann and the IOM Solutions Nerveana technical component, which consists of using and (Figure 11–1). setting up the monitoring equipment correctly and

Figure 11–1. Numerous dedicated EMG nerve monitors are commercially available. (Reprinted with permission, courtesy of Jack M. Kartush, MD) Intraoperative Cranial Nerve Monitoring 167 understanding the inherent properties of the system receive real-time information through the internet. to avoid an erroneous setup (eg, no muscle relax- While there is little need for such remote monitor- ation, correct electrode placement, low impedance, ing for basic EMG modalities if the surgeon is well etc). The other aspect is the interpretive component. trained, complex procedures (EEG for carotidec- Is the person performing the monitoring able to dis- tomy, transcranial motor evoked potentials [MEP] tinguish between a true response versus an artifac- for scoliosis surgery) benefit greatly from a well- tual one? When problems occur, can they perform trained team of local technologist and remote neu- appropriate troubleshooting to identify and correct rologist/neurophysiologist. Today, most hospitals the issue at hand? With something relatively simple, seek monitoring service companies who specialize such as facial nerve monitoring, it is often possible in these complex procedures and have the requi- for the surgeon to perform both components, assum- site information technology (IT) resources to ensure ing that he or she has undergone the proper training appropriate hardware, software, and systems man- to do so. Many features of the NIM were specifically agement to optimize connectivity from the OR to the designed to allow for surgeon interpretation such as remote monitoring professional. auditory representation of the EMG through both raw EMG and intelligently designed alarms that alert the surgeon to significant responses as well as Monitoring Pitfalls potential problems such as electrode displacement or high impedance. Another important feature is Monitoring is an adjunct, not a replacement for confirming current flow during stimulation with anatomical knowledge and surgical skill. When both visual and auditory displays. In fact, a surgeon neurophysiologic responses appear to conflict with trained in monitoring is unequivocally the ideal per- anatomy and surgical findings, one must either son to interpret the responses as he or she can imme- troubleshoot for a solution or disregard the monitor. diately correlate them to the surgical events. Even As the senior author (JMK) has stated for decades, the best trained technologist, neurophysiologist, “Poor monitoring is worse than no monitoring.” or neurologist will have difficulty separating true Monitoring can be likened to going into a minefield responses from artifact without the benefit of seeing with a minesweeper that is malfunctioning. Under and understanding the surgical events in real time. this circumstance, it is better to disregard the mine- In contrast, complex types of monitoring that sweeper rather than to depend on something that require the averaging of small potentials such as is unreliable. There are many possible pitfalls even auditory brainstem recording (ABR) and somatosen- for relatively simple monitoring modalities such as sory evoked potentials (SSEP) typically require too facial nerve recording. Just as physicians learn to much time and effort for the surgeon, which could generate a differential diagnosis for medical dis- distract from the operation. Furthermore, many sur- orders, so must the well-trained surgeon develop geons simply are not trained in performing the tech- consistent protocols and a differential diagnosis for nical or interpretive aspects of these more complex intraoperative monitoring problems if they are to monitoring. Thus, either the surgeon has undergone perform both technical and interpretive components sufficient training to manage both the technical and without a technologist or neurophysiologist. As the interpretive aspects of nerve monitoring or a well- use of monitoring has become increasingly routine trained technician, neurophysiologist, or neurolo- in otolaryngology, neurosurgery, and spine surgery, gist should assist in monitoring. Unfortunately, poor surgical residency programs should begin to include reimbursement in this country has made it prohibi- monitoring courses in their core curriculum, and test tive for neurologists to spend hours or an entire day for competency in this area just as they do in other in one operating room. Thus, with a trained technol- critical areas. General surgeons are just beginning to ogist in the operating room, professional interpre- adopt monitoring for thyroidectomy and, without tation is increasingly being performed by remotely any prior experience in monitoring, it is critical that located neurophysiologists and neurologists, who they actively pursue IOM training. A great deal of 168 Practical Neurotology and Skull Base Surgery

effort has gone into the training of IOM technolo- box that may have scores of different input sockets, gists and reading professionals over the last 20 years. but software allocation of stimulus and recording Organizations such as the American Society of Neu- pathways can lead to misallocation of the signals. rophysiologic Monitoring (ASNM) provide training For example, in SEP monitoring, we have seen stim- courses for surgeon, technologists, neurophysiolo- ulation of the left leg when the right arm was the gists, and neurologists as well as a means to certify intended target. Similarly, during ABR recording, levels of monitoring interpretative competence (eg, software misallocation may inadvertently lead to the American Board of Neurophysiologic Monitoring sound signal being conveyed to the right ear trans- [ABNM]). In contrast, the American Board of Reg- ducer, when the left ear is the intended side. These istration of Electroencephalographic and Evoked and other details are addressed later in the chapter. Potential Technologists (ABRET) has focused on a certification exam for technologists, Certification for Intraoperative Monitoring (CNIM). Thus, surgeons Facial Nerve Monitoring who choose to perform technical and/or interpre- tive aspects of monitoring should take advantage of these educational resources. Clinical Application There are numerous technical and interpre- tive pitfalls that can lead to false-positive and false- Although the indications for facial nerve monitor- negative errors. A false-positive error occurs when ing are expanding, its benefit has been most appar- a signal is interpreted as being a true response from ent during resection of acoustic neuromas given the the nerve of interest when in fact it was generated high incidence of postoperative facial palsy prior to by artifact or another neuromuscular generator. the use of monitoring. These benign tumors typi- Examples include (a) mistaking a response from a cally originate from the superior vestibular nerve pharyngeal constrictor while seeking a vocal fold in the internal auditory canal. The proximity of response during thyroidectomy, or (b) mistaking the adjacent facial nerve accounts for its routine a response from masseter and temporalis muscles involvement by tumor displacement or microscopic while seeking a facial nerve response during acous- infiltration. Most clinicians prefer an EMG-based tic tumor surgery adjacent to the root entry zone of system using intramuscular needle electrodes to the trigeminal nerve. maximize recording sensitivity and specificity. The A false-negative error occurs when a response high sensitivity of electromyographic facial nerve from the nerve of interest is missed. There are multi- monitoring allows electrical stimulation of the nerve ple causes of such an error. Long-lasting neuromus- to be supplemented by identifying low amplitude cular blockade and topical anesthetics can abolish mechanical-evoked potentials caused by blunt sur- or impair EMG responses. Not understanding that gical trauma. The specificity of EMG monitoring monitoring is disabled during electrocautery can reduces the chance for false-positive errors that can result in the nerve being burned and yet no response occur with a motion detector-based system. Sugita will be detected during cautery. The experienced and Kobayashi8 reported on one case in which the surgeon learns that when cautery must occur in close facial nerve was inadvertently cut because stimula- proximity to a nerve, it should be done at the lowest tion of the trigeminal nerve resulted in contraction possible setting and then the nerve should be electri- of the muscles of mastication. The motion detector cally stimulated immediately following cautery (or other was not able to differentiate between this movement risky maneuver) to confirm that it has not been injured. versus contraction of the facial musculature. Incorrect setup of electrodes or monitoring Retrospective studies indicate improved facial parameters can result in both false negative and nerve outcome with intraoperative monitoring par- false-positive errors. Errors with multimodality ticularly with large acoustic tumors.12–15 Although a devices can occur in numerous additional ways that controlled, prospectively randomized study would may be difficult to detect because electrodes cannot be most convincing, such a study is unlikely to only be placed in the wrong input of an interface take place because most of today’s surgeons who Intraoperative Cranial Nerve Monitoring 169 have adopted monitoring believe that there is an who responded to a query on the type of monitor- unequivocal benefit and are reluctant to withhold ing used, reported using intraoperative monitoring this modality from their patients. It is surprising that without the use of nerve stimulators, whereas 21.5% after years of reluctant acceptance, some now refer used nerve stimulators only without EMG monitor- to the analogy that there has never been a controlled ing. There was no statistically significant association study for skydiving with and without parachutes . . . between the use of monitoring in current practice Because the incidence of nerve injury in chronic and a history of inadvertent permanent facial nerve ear surgery is markedly less than that seen in acous- injury. The authors concluded that there was a grow- tic neuroma surgery, it is more difficult to show if ing trend of facial nerve monitoring during parotid injury could have been avoided with monitoring surgery despite the lack of any published prospec- due to the need for an extremely large sample size. tive, randomized clinical studies to examine the effi- In a recent survey conducted to identify facial nerve cacy of its use in this setting. IOM practice in the United States in this setting, the An interesting corollary is the gradual accep- authors found that although 75% of the 223 respon- tance of pulse oximetry in anesthesia. At first many dents had access to monitoring, only 32% thought anesthesiologists and nurse anesthetists were reluc- that it was a requirement.16 The standard of care is tant to accept “yet another gimmick” that would often defined as what the average provider would take away from the “art” of anesthesiology. Many have done under similar circumstances. Those who took pains to vocally indicate that it was not a performed more otologic surgeries than other types standard of care. However, as pulse oximetry has of surgeries were more likely to use the monitoring become less expensive, widely available and a use- for chronic ears. ful real-time measure to assess oxygenation, it has The use of monitoring in middle ear and mas- indeed become a standard of care, even though no toid disease, especially in the setting of revision controlled study has a shown a statistical difference surgery, is justified anatomically. Noss et al17 retro- in outcomes. Like pulse oximetry, we predict that the spectively reviewed 262 cases and found that while use of intraoperative neuromonitoring will increase a dehiscent facial nerve was visualized during 10% simply because it provides “critical information of of the primary surgeries and in 20% of the revision key structures at risk in the operative field.” Neu- surgeries, an electrophysiological dehiscence was romonitoring has become accepted in many surgi- detected through the use of stimulation in 53% of the cal fields including brain, spine and thyroid. One of primary surgeries and in 96% of the revision cases. the last remaining fields that we predict will finally A stimulation threshold of <1 volt was concluded to adopt monitoring is in pelvic surgery where the be a more useful criterion of dehiscence than clini- risks of impotence and incontinence during prosta- cal observation under an operating microscope. The tectomy, hysterectomy and colorectal oncologic sur- cost-effectiveness of intraoperative facial nerve mon- gery remain problematic. itoring in both primary and revision surgeries for middle ear and mastoid disease was demonstrated by Wilson et al.18 The estimated additional cost of Setup and Recording Technique $222.73 to $528.00 to the otologic surgery for moni- toring and an audiologist/technician was offset by Long-acting muscle relaxants should be avoided. the avoidance of the high management costs of facial Although low doses may not have a significant effect nerve paralysis. on the response to electrical stimulation, they dimin- Lowry et al19 surveyed US otolaryngologists to ish the ability to monitor small amplitude responses determine the patterns of use of facial nerve moni- associated with mechanical evoked potentials of the toring during parotid gland surgery. Of the 1,548 facial nerve. Therefore, judging depth of paralysis respondents, the more parotidectomies performed using a “Train of Four” peripheral nerve stimulator per year (>10 and >20), the more likely monitoring at a relatively high current setting is not an accurate was used (79% and 60%, respectively). Four hun- measure of the facial nerve’s ability to respond to dred and ninety-two of the 627 (78.5%) surgeons minor surgical trauma.