27.1 Introduction

Facial nerve monitoring has become standard in neurotologic procedures, distinct from its more adjunctive role in other subspecialties in otolaryngology. Many of the early studies evaluating the benefits of neuromonitoring were retrospective and utilized historical matched cohorts. Control groups commonly consisted of operative cases performed before adoption of facial nerve monitoring and were compared to the same surgeon’s outcomes following routine nerve monitor use. From a methodologic perspective, these studies are plagued by confounding factors such as improved skill of the surgeon over time and the benefits of other advances in skull base surgery occurring over the study period. Still, the evidence from these studies ethically precludes a randomized trial. Based on the existent data, the National Institutes of Health Consensus Statement on Acoustic Neuroma explicitly recommends the routine use of intraoperative facial nerve monitoring.s. Literatur

With improvements in imaging modalities facilitating earlier detection of vestibular schwannomas (VSs), preservation of facial nerve function has become a primary consideration in the management of patients undergoing microsurgical resection. Two different, but complementary, techniques for facial nerve monitoring are utilized for posterior fossa skull base surgery and will be described within this chapter. Continuous facial electromyography (EMG) is in widespread use among centers conducting otologic and neurotologic procedures, while transcranial motor evoked potential (TCMEP) monitoring is utilized less frequently. These techniques may be used in a variety of ways to facilitate facial nerve preservation and to aid in decision making during VS resection.

27.2 History and Overview

The earliest and most primitive form of facial nerve monitoring is direct observation of facial nerve twitching from muscle contraction after mechanical stimulation of the facial nerve. This remains a technique commonly used among head and neck surgeons in parotid gland dissection and in settings where equipment for facial nerve monitoring is unavailable. It is only of historical interest for microsurgery of the posterior fossa. Intraoperative facial nerve EMG was first introduced in 1979.s. Literatur Facial EMG allows for early identification and confirmation of the facial nerve either distally within the internal auditory canal or proximally at the brainstem, and provides good spatial resolution to map the nerve course at low stimulation levels. Suprathreshold stimulation levels may also be used before dissecting a particular region of the tumor capsule to confirm the nerve is not in close proximity. Continuous EMG testing detects nerve stimulation in real time, providing the surgeon with valuable feedback during the course of dissection by monitoring spontaneous EMG activity from mechanical manipulation. Finally, direct electrical stimulation of the proximal nerve following tumor resection provides some prognostic value for estimating postoperative facial nerve function and the likelihood of recovery even in the presence of immediate postoperative facial paralysis.

TCMEP monitoring of the facial nerve aims to provide a “functional” assessment of cranial nerve integrity during cranial base surgery without the need for direct proximal stimulation by the surgeon. It has been found to be of greatest help in the setting of large tumors where tumor dissection (Fig. 27‑1 ) is often necessary prior to definitive identification of the facial nerve.s. Literatur

27.3 Monitoring Setup

Although the advent of facial nerve monitoring represents a tremendous advance in skull base surgery, there are a number of considerations critical to its effective use. Anesthetic agents that cause long-term neuromuscular blockade may interfere to varying degrees with monitoring and should be avoided. Typically, anesthetic induction with endotracheal intubation is performed using short-acting agents, namely succinylcholine, so that neuromuscular blockade will clear early before critical portions of the surgery are underway. It is important that anyone providing neuroanesthesia be comfortable managing such patients without the use of long-term muscles relaxants. Additionally, injection of xylocaine and other similar pharmacologic agents, particularly in the vicinity of the stylomastoid foramen, may induce local muscular blockade of the facial nerve, interfering with the interpretation of the facial EMG. Further discussion regarding anesthetic considerations during neuromonitoring for VS microsurgery can be found in Chapter 26.

Although an indispensable tool, there are ways in which intraoperative monitoring can be counterproductive and detrimental to surgical outcomes. Facial nerve monitoring can in no way take the place of a fundamental anatomic knowledge and of sound microsurgical dissection technique. Potential problems with intraoperative monitoring may be mitigated by routine use and specialized technical training among those involved in setting up the equipment. The ability to troubleshoot the equipment in real time, recognize when it is not working appropriately, and proceed with the operation is an important aspect of its use.

The use of multiple EMG channels improves the sensitivity of EMG monitoring, and therefore, it is recommended to monitor at least two branches of the facial nerve. Subdermal electrodes are preferred to surface electrodes, which are more susceptible to artifact and easily displaced during surgery. Electrodes for facial EMG are most commonly placed in the orbicularis oculi and orbicularis oris muscles. Electrodes should be secured well with adhesive tape, and the orbicularis oculi subdermal needle electrode should be oriented pointing away from the globe to avoid ocular injury. Placing a small loop of redundancy in the electrode wire near the insertion site reduces the risk of accidental displacement if the proximal wires are inadvertently pulled. Additionally, a ground electrode is placed over the sternum and a return electrode for monopolar stimulation is placed on the contralateral shoulder.

Using multiple channels allows for an “internal control” for troubleshooting potential intraoperative problems. For example, increased activity in one channel and silence in other leads may indicate nonsurgical factors or faulty electrode placement. In addition, multiple leads allow for simultaneous monitoring of multiple divisions of the facial nerve, which may be particularly useful for a very splayed nerve. Mechanical trauma to the nerve may induce high tonic EMG activity, making response to direct stimulation difficult to detect. In this case, having additional electrodes will often allow for audible responses to direct stimulation on the quieter channels.

We have chosen to include the “tap test” as part of the surgical “time-out” procedure. This is performed by gently tapping on the electrodes and surrounding face to elicit EMG activity on the monitoring system. It is important to understand what information can and cannot be gleaned from this. It can confirm for the surgeon (1) the volume of the loudspeaker is at an adequate level and (2) the integrity of the equipment’s connectivity. However, this response represents an artifact, and is not a compound muscle action potential (CMAP) emanating from the facial nerve. Ultimately, the surgeon or a technologist with experience in neurophysiologic intraoperative monitoring should be responsible for proper setup.

27.4 Intraoperative Use

False-positives or false-negatives resulting from an incorrect setup can be more detrimental than not using monitoring at all, as this can mislead the surgeon and may result in false assurance during dissection. At the authors’ center, an electrophysiologist is responsible for monitoring equipment setup and is present throughout the case to help monitor, interpret, and troubleshoot EMG monitoring. Their presence is necessary for TCMEP and other intraoperative neuromonitoring techniques, although most EMG systems can be programmed for surgeons who may not have this luxury. Adjustable stimulating probes allow the surgeon to change the stimulation levels directly and the EMG machine settings can be programmed to provide audible feedback about stimulation parameters.

When present, the electrophysiologist should be situated in the line of sight of the surgeon and have a video monitor to visualize tumor dissection. EMG activity should be monitored via loudspeaker for acoustic feedback and an oscilloscope for visual feedback. Before stimulation, it is important for the surgeon and electrophysiologist to communicate about parameters including the stimulation level and the baseline TCMEP value. TCMEP stimulation itself results in activity detected on the facial EMG and noticeable contraction of the patient’s musculature. Therefore, the electrophysiologist should confirm with the surgeon each time before employing its use to ensure it is an appropriate time in the case. A “squelch circuit” is another important feature routinely employed that allows for muting of the artifact produced with electrocautery.

27.5 Facial Electromyography

Free-running EMG, or so-called passive monitoring, involves recording CMAPs through needle electrodes in the facial muscles activated through nonelectrical stimulation of the nerve, be it mechanical or stretch activity. Continuous EMG provides the surgeon real-time immediate feedback during facial nerve dissection. Spontaneous neurotonic firing and increased irritability may suggest risk of permanent injury and can direct the surgeon to stop their current approach and work elsewhere until EMG activity returns to baseline. One should also note that if the patient is “becoming light” from inadequate anesthesia during the procedure, facial muscle fasciculation and resultant EMG activity may be one of the earlier signs—preceding gross physical movement or changes in the bispectral (BIS) monitor sometimes used to determine the depth of anesthesia.

Stimulating the nerve electrically or with gentle mechanical stimulation may result in a burst potential that represents a short synchronous firing of a single discharge across multiple axons. Intriguingly, sharp cutting or laceration of a facial nerve is less likely to elicit neurotonic activity, as compared to traction, compression, or gentle manipulation.s. Literatur Traction on the nerve from mechanical stretch leading to a prolonged irregular series of EMG activity is referred to as a train potential. Trains represent tonic activity, often from multiple asynchronous discharges from different motor units that may last seconds to minutes. Two train patterns have been described—high-frequency trains (50–100 Hz) present acoustically similar to an airplane engine and are called “bomber potentials,” while low-frequency trains (1–50 Hz) acoustically resemble popping popcorn. Trains may result from mechanical tension in the form of pressure or stretch from tumor dissection. This feedback may direct the surgeon to alter their surgical strategy to reduce the risk of permanent facial nerve injury. Unfortunately, there is often a delay in seconds to minutes between the inciting event and the development of train activity. Similar to burst potentials, train activity may also result from thermal cautery and cold irrigation.

27.6 Direct Stimulation

Direct electrical stimulation of the facial nerve is often termed “active” monitoring. Intraoperatively, direct electrical stimulation may be used for definitive nerve identification, confirmation of neural integrity, and mapping of the nerve location and course. Early use of the stimulating probe within the case allows for confirmation of correct equipment setup and reassurance that the system is responding appropriately. Before employing active stimulation, it may be checked in surrounding soft tissue rather than directly in the dissection field.

Both monopolar and bipolar electrodes can be used for direct facial nerve stimulation. Bipolar probes theoretically provide greater stimulation specificity and precision compared to monopolar probes. However, in practice, bipolar electrodes are cumbersome to use in the tight confines of the posterior fossa. In addition, stimulation efficiency is dependent on the orientation of the electrode tips in relation to the facial nerve, adding additional complexity.s. Literatur In contrast, monopolar probes are compact and also provide submillimeter spatial resolution when using low stimulation settings. Furthermore, monopolar stimulation at higher current levels is invaluable for tumor mapping, where high sensitivity is required. Today, most available probes are insulated and monopolar in design, with the Prass probe (Medtronic Xomed, Jacksonville, FL) being most widely used. Kartush and colleagues developed a set of dissecting instruments with a noninsulated cutting surface allowing for simultaneous stimulation and dissection (Neurosign Magstim Co., Carmarthenshire, UK). This avoids the need to constantly change instruments when stimulation is desired.

Stimulation may be performed using either a constant-current or constant-voltage system. While still a topic of controversy, intraoperative skull base neuromonitoring is most commonly performed using constant-current stimulators. The primary limitation of constant-current stimulation is the potential for current shunting through surrounding fluid, including cerebrospinal fluid (CSF) and irrigant, and thus, the possibility of false-negative stimulation. However, the use of insulated flush-tip stimulating probes largely resolves this shortcoming.

The pulse parameters most commonly adjusted during VS microsurgery include the amplitude of the current (milliamp, mA) and the length of time the current is applied (pulse duration, microseconds [µs]). To characterize the magnitude of the pulse charge, both parameters must be known. For example, the charge delivered to a nerve using 0.1 mA and a 50-µs pulse duration is less than that using 0.1 mA with a 100-µs pulse duration. Usual parameters when stimulating the facial nerve directly may vary between 0.05 and 0.2 mA, and 50- and 200-µs pulse duration.

Stimulation parameters are tailored to the task at hand. Suprathreshold levels are typically employed before the nerve is definitively identified. By using higher stimulation settings, the surgeon can confirm that the nerve is not in proximity when bipolar coagulating or resecting tumor capsule. Before engaging the tumor, the entire dorsal surface of the tumor capsule should always be stimulated at higher levels to ensure the facial nerve is not splayed over the back side of the tumor and to confirm that the tumor is not in fact a facial nerve schwannoma, both of which are uncommon. The use of successively lower stimulation settings may help localize or triangulate the facial nerve at the brainstem in the case of a larger tumor with significant brainstem compression. Minimum settings are generally used to directly stimulate a previously identified nerve, to assess overall neural integrity, or to precisely locate a conduction block if proximal stimulation is lost. While preferences vary from surgeon to surgeon, there is a theoretical advantage to resecting the cerebellopontine component of the tumor before the intracanalicular portion because a nerve will always stimulate distal to an acute injury. If a significant conduction block develops within the internal auditory canal before the facial nerve is found proximally, dissection at the brainstem, cistern, and porus may be more challenging.

27.7 Transcranial Motor Evoked Potentials

TCMEP monitoring yields information about the integrity of the entire course of the facial nerve without requiring direct proximal nerve stimulation (Fig. 27‑2 ). The general technique for motor evoked potentials elsewhere in the body has been used in intraoperative monitoring of spinal cases, but its application to facial nerve surgery is more complex. Prior studies have validated the technique and suggest some utility in prognosticating facial nerve outcomes.s. Literatur This technique is used routinely at our center in VS surgery and is felt to be most useful for larger tumors where early identification of the facial nerve at the brainstem may be difficult. Of note, TCMEP is more sensitive to inhalational anesthetic agents than EMG because of their impact on cortical excitability, and measurements must be interpreted with this in mind. Reference to the BIS monitor provides a measure of the depth of anesthesia.

The exact setup for this technique is beyond the scope of this chapter, but is described in Cosetti et als. Literatur and Cueva.s. Literatur The authors stimulate from corkscrew electrodes for the cranial electrode montage and record from separate subdermal bipolar electrodes placed in the facial muscles. TCMEPs of the facial nerve are subject to artifact as the strength needed to stimulate the appropriate area of the cortex through the skull may also lead to peripheral motor responses from the face (Fig. 27‑3 ). Careful setup and interpretation of the measured response can recognize and avoid this pitfall.

An absent direct stimulation response but affirmed baseline TCMEP response may provide the surgeon important feedback that the thin nerve remains intact and, thus, confidence to continue the dissection. Surgeons can become “paralyzed” by absent or overly active EMG monitoring responses. It is our opinion that we are able to accomplish complete resections more often with large tumors due to the added information provided by TCMEP monitoring.