CHAPTER 178 Intraoperative Monitoring of Cranial Nerves in Neuro-otologic Surgery
The advent of sensitive diagnostic techniques and the refinement of microsurgical procedures have brought about a marked reduction in mortality rates for posterior fossa surgery; accordingly, increasing emphasis has been placed on preservation of cranial nerve function. This goal has in turn stimulated the development of techniques for monitoring cranial nerves during surgery. Seventh nerve monitoring during acoustic neuroma surgery has now become routine at most major medical centers, and anatomic preservation of the facial nerve has been achieved in more than 95% of the cases in some series.1 Although facial motility often is compromised in the immediate postoperative period, the long-term prognosis is good if the nerve can be electrically stimulated after tumor removal. Preservation of hearing has been more difficult to achieve because of the more intimate relationship of such tumors with the cochleovestibular nerve but is now often achieved in smaller tumors with the aid of eighth nerve monitoring. Furthermore, the techniques developed for facial nerve monitoring are now readily adapted for monitoring other cranial motor nerves. We have reviewed these topics in greater detail elsewhere.2
The emphasis in this chapter is on the practical aspects of instrumentation, electrode placement, different neurophysiologic techniques used, artifact identification, types of responses encountered, and the relationship between intraoperative recordings and clinical outcome. Somatosensory evoked potential (SEP) recording3 is not treated here, although SEP monitoring can be useful in monitoring large posterior fossa tumors with significant brainstem compression; discussions of SEP monitoring can be found in textbooks by Nuwer4 and Møller.5 This chapter is based primarily on the experience of the senior author with more than 700 posterior fossa procedures, as well as a review of the literature through 2007; also described are the methods currently available for cranial nerve monitoring, emphasizing facial and cochlear nerve monitoring during acoustic neuroma surgery but also including extension of these techniques to other nerves encountered in a variety of skull base procedures.
Successful intraoperative monitoring requires more than simply bringing another piece of equipment into the operating room (OR). The OR, unlike the typical clinical neurophysiology laboratory, presents a time-pressured and electrically hostile environment. Providing technically adequate recordings in the OR requires the services of professional personnel with specialized skills and experience despite the additional costs incurred. Attempts to monitor without such personnel may result in failure, or, even worse, inadequate monitoring, with inaccurate and misleading feedback to the surgeon. Accordingly, a new specialty field of intraoperative neurophysiologic monitoring has evolved, with its own professional organization, the American Society of Neurophysiological Monitoring (ASNM).
Professional certification for surgical neurophysiologists is now offered by two national organizations in the United States. At the technologist level, the American Board of Registered Electrodiagnostic Technologists (ABRET) offers a Certification in Neuro-physiological Intraoperative Monitoring (CNIM). The American Board of Neurophysiologic Monitoring (ABNM) offers board certification to monitoring professionals holding advanced degrees and with a minimum of 3 years’ experience and 300 cases monitored. Because the demand for monitoring services is growing rapidly, there is a growing need for training programs to ensure an adequate supply of qualified personnel.
The basic instrumentation requirements for cranial nerve electromyography (EMG) monitoring are an isolated electrical stimulator that can be precisely controlled at low levels; several low-noise EMG amplifiers; a multichannel display; and an audio monitor with a squelch circuit to mute the output during electrocautery. A system that provides multiple channels is recommended to allow simultaneous monitoring of multiple divisions of the facial nerve independently as well as other cranial nerves.
Auditory brainstem response (ABR) monitoring requires a system that includes high-gain (100 to 500,000 K) differential amplification with multiple band-pass filtering capabilities, acoustic stimulus intensity ranging from threshold to at least 70- to 80-dB hearing level (HL), signal averaging with real-time display of the evolving averages and the input signal, and permanent disk storage with hard copy printout capability.
For complex surgical cases requiring additional EMG monitoring channels or increased ABR averaging capability, or both, several commercial multichannel systems have been developed for intraoperative use. Currently available systems allow multiple independent time bases and functions to operate simultaneously; for example, some channels may be devoted to free-running EMG at slow sweep speeds and others to stimulus-triggered EMG at a faster sweep, and still others can be used for collection of averaged ABRs.
The most commonly used recording electrodes are the platinum or stainless steel subdermal needles designed for EEG; these have a larger uninsulated surface than that of electrodes designed for single-fiber EMG and thus are more likely to detect activity arising anywhere in the monitored muscle. Disposable, presterilized electrodes are available from several sources, and their use is highly recommended.
Intramuscular hook wire electrodes, which are inserted with a hypodermic needle, also have been used.6 These are more delicate and have higher impedance, so the simpler needle electrodes are preferred, except in situations in which the electrodes cannot be easily fastened with tape, as in procedures involving the tongue, or when the use of insulated needles is desirable to avoid crosstalk from overlying muscles. An example of the latter case is recording from extraocular muscles, for which the electrodes must pass through the orbicularis oculi muscle and will therefore respond to facial nerve activity as well.
For monitoring the facial nerve, it is desirable to place the electrodes in at least two different muscles supplied by the nerve. Mechanical trauma to the seventh nerve often causes sustained EMG activity that can make identification of the facial nerve with electrical stimulation difficult. With two or more close-spaced bipolar channels, at least one often will be quiet enough to allow responses to stimulation to be identified without signal averaging even with high tonic EMG activity.
For recording the ABR in hearing conservation procedures, one electrode is placed in the ipsilateral ear canal and another on the forehead or vertex. The placement of the second electrode is not critical so long as it is near the midline. The best electrode option for the ear canal is the Nicolet Tiptrode, which is a compressible foam insert covered with gold foil. The acoustic signal is delivered through a tube in the center of the foam, which provides an acoustic seal, and the foil provides the electrical contact for recording.
Figure 178-1 shows the positioning of recording electrodes for a middle fossa or retrosigmoid craniotomy for acoustic neuroma resection with a goal of hearing preservation. For a translabyrinthine approach, the same configuration is used, with the exception of the earphone and electrodes for ABR recording, because hearing conservation is not possible with this approach. Two channels are devoted to the facial nerve itself, with electrode pairs placed in orbicularis oculi and orbicularis oris muscles. One of the electrodes in the orbicularis oculi pair is placed at the lateral canthus, where it also will record volume-conducted activity in the lateral rectus muscle (from the sixth cranial nerve). One channel is used to record from the masseter or temporalis muscle (V3m) using hookwire electrodes, rather than subdermal needles, to reduce crosstalk, and the fourth channel is connected to electrodes in the ipsilateral trapezius muscle (eleventh nerve). The latter two channels serve two functions: First, larger tumors may expand to involve these nerves, so monitoring may help in their identification and preservation. Second, even with smaller tumors, the extra channels serve as a control for nonsurgical causes of increased EMG activity, particularly light anesthesia.
Figure 178-1. Diagrammatic representation of electrode placement for monitoring acoustic neuroma surgery with attempted hearing conservation. Pairs of needle electrodes are placed in the following muscles: temporalis (supplied by V3m), orbicularis oculi and orbicularis oris (supplied by the seventh nerve), and trapezius (by the eleventh nerve). Note that wires are twisted together to reduce 60-Hz pickup. Broadband click stimuli from a small transducer on the chest are fed through plastic tubing into the ipsilateral ear through a foil-covered sponge insert that also serves as a recording electrode and referred to a needle electrode on the forehead or vertex. An electrocautery ground pad is placed on the arm or shoulder as a signal ground. A flexible-tipped probe is used to stimulate cranial motor nerves, with a needle electrode as the stimulator ground placed in the margin of the craniotomy.
(From Yingling CD. Intraoperative monitoring of cranial nerves in skull base surgery. In: Jackler RK, Brackmann DE, eds. Neurotology. St Louis: Mosby; 1994:967).
Both monopolar and bipolar stimulating electrodes have been used for intraoperative electrical stimulation. In theory, a bipolar electrode can provide more precise localization, with less current spreading from the electrode to adjacent structures. In practice, however, the threshold for bipolar stimulation depends strongly on the orientation of the two contacts with respect to the axis of the nerve.7 Maintenance of a specific bipolar orientation is difficult in the close confines of the posterior fossa. Furthermore, if the stimulus intensity is near the threshold level (see later), a spatial resolution of less than a millimeter is easily obtained even with monopolar stimulation. Monopolar electrodes are therefore more commonly used; however, Schekutiev and Schmid8 and Dankle and Wiegand9 have described coaxial bipolar electrodes that may eliminate the problem of orientation.
The monopolar electrode should be connected to the cathode of the stimulator; the anodal return usually is a needle inserted into the periphery of the wound, preferably on the posterior margin of the incision to minimize stimulus artifact. This is particularly important in recording from extraocular muscles, which yield small-amplitude and short-latency responses that can easily be swamped by electrical artifacts. Several types of monopolar electrodes are available. Prass and Lüders10 developed a malleable electrode, with the insulation continuous to a flush-tip that could be bent so that only the central portion of the tip contacted the desired tissue. They showed that this design minimized the spread of current to adjacent structures. Yingling and Gardi11 developed a probe with a flexible platinum-iridium tip, insulated except for a 0.5-mm ball on the end (Fig. 178-2). This electrode can be used to stimulate within dissection planes or even behind the tumor, out of the surgeon’s view, without concern for inadvertently damaging unseen neural or vascular structures (Fig. 178-3). With this probe, the facial nerve frequently can be located electrically even before it can be seen; dissection can then proceed in the most advantageous manner to avoid neural damage. Kartush and coworkers7 developed a set of Rhoton-type dissecting instruments that are insulated except at the cutting surface, allowing simultaneous dissection with constant stimulation. These “stimulus dissectors” are particularly useful for removing the last portions of tumor capsule that are closely adherent to a nerve.
Figure 178-2. Close-up view of flexible-tipped probe used for intracranial stimulation. The entire probe and the flexible wire are insulated, except for the 0.5-mm ball on the end, to achieve localized stimulation.
Figure 178-3. Surgical view of large acoustic neuroma (retrosigmoid approach) showing use of flexible-tipped probe to locate the facial nerve on the medial surface of the tumor out of direct view. Early identification of the facial nerve “around the corner” on the ventral surface of the tumor helps speed the procedure by allowing rapid removal of the remaining capsule. Tumor is drawn as if transparent to show details of anatomy on the hidden surface.
(Adapted from Yingling CD, Gardi JG. Intraoperative monitoring of facial and cochlear nerves during acoustic neuroma surgery. Otolaryngol Clin North Am. 1992;25:413.)
Little agreement has been achieved regarding the appropriate stimulation parameters to be used in cranial nerve monitoring, which vary considerably from one center to another. Our preference is to use a monopolar constant-voltage stimulator delivering pulses 0.2 ms in duration at a rate of 5 to 10 per second. In our experience, this is an effective and reliable choice that usually evokes a response from normal nerves at a threshold ranging between 0.05 and 0.2 V and averaging approximately 0.1 V.
Although the issue of whether constant-current or constant-voltage stimulators should be used has been a source of continuing controversy, there is no clear advantage of one method over the other. In many cases, however, the features of available equipment will limit the type of stimulation. Whether constant current or constant voltage is used, the more important issue is what actual level of stimulation is most appropriate. Rather than adhering to a simple “set it and forget it” approach, more useful information can be gained by varying the stimulation intensity in different surgical contexts, such as locating a nerve in relation to a tumor or testing the responsiveness of an already identified nerve.
Cortical evoked potentials are notoriously sensitive to many anesthetic agents, so careful adjustment of anesthesia levels is necessary in applications such as spinal cord monitoring with SEPs. Fortunately, the ABR and EMG responses that are used for cranial nerve monitoring are essentially unaffected by any commonly used anesthetics. The major anesthetic consideration is a contraindication to the use of muscle relaxants, because blockade of the neuromuscular junction interferes with monitoring of EMG activity. A few reports12–14 have suggested that partial blockade can be used to prevent patient movement without blocking the ability to elicit EMG responses with facial nerve stimulation. However, in our experience, although electrically evoked EMG is relatively preserved, both spontaneous and mechanically elicited EMG activity tends to be suppressed by these agents. Therefore, no paralytic agents should be used during surgery with cranial nerve monitoring, other than short-acting agents such as succinylcholine given to facilitate intubation. Because the ABR and EMG are not significantly affected by routine concentrations of common anesthetics, no other constraints on anesthetic technique generally are necessary.
Finally, if local anesthetic is used at the incision site, care must be taken to avoid injection near the stylomastoid foramen to avoid anesthetizing the facial nerve. Alternatively, epinephrine 1 : 100,000 without any local anesthetic may be used to aid hemostasis.15
Facial nerve (seventh nerve) monitoring is routinely used in procedures to remove acoustic neuromas and other cerebellopontine angle tumors. Several techniques for facial nerve monitoring have been developed, including the use of sensitive detectors mounted on the face to detect facial nerve activity,16–19 intraoperative EMG, recording of compound nerve action potentials (CNAPs) from the facial nerve,20–22 intraoperative recording of nasal muscle F-wave,23,24 and video analysis to detect facial movements.25,26
Because of its higher sensitivity6,7,11,16,27–46 and easy application, EMG has by far overshadowed the other aforementioned methods and has become the most widely used modality for routine facial nerve monitoring. EMG recordings are used for monitoring cranial motor nerve activity in two distinct ways. First, intracranial electrical stimulation is used to identify and map the course of the nerves with evoked EMG activity and to determine the functional integrity of a nerve. Second, spontaneous EMG activity is continuously monitored to detect changes related to mechanical, thermal, or electrical irritation of the nerves by intraoperative maneuvers such as retraction,47 tumor dissection, use of electrocautery or lasers,48,49 or ultrasonic aspiration.
Electrical stimulation is used in two main ways: (1) to rule out the presence of a nerve in a region to be dissected (using suprathreshold stimulation levels, i.e., 1 V) and (2) to map the precise locations of cranial nerves and determine their functional integrity, using stimulation levels at or just above threshold (i.e., 0.05 to 0.3 V).
Before electrical stimulation begins, correct functioning of the stimulating and recording system must be confirmed as soon as possible to avoid potentially catastrophic false-negative results. The presence of a stimulus artifact is not an unequivocal test; it is possible to have a stimulus artifact with only one lead (either the anodal return or the cathodal stimulator) connected. However, the absence of any artifact usually indicates an open circuit somewhere in the system. To avoid any ambiguity, it is preferable to confirm the operation of the entire system before beginning tumor dissection. In a retrosigmoid approach, the eleventh nerve usually can be stimulated at the jugular foramen as soon as the dura has been opened and the cerebellum retracted; an EMG response in the trapezius muscle confirms that the system is operating correctly. This confirmatory maneuver usually is possible before tumor resection begins, except in operations involving very large acoustic tumors. If the eleventh nerve is not visible at the outset, the stimulating electrode can be placed directly on a muscle and a direct muscular response obtained, although muscle requires higher stimulation levels than those effective in nerve. In translabyrinthine procedures, the facial nerve can be stimulated within the mastoid bone in the course of the labyrinthine dissection (before the tumor is exposed), although the threshold will be higher (usually 0.6 to 1.0 V, although up to 2 V may be needed), depending on the thickness of the overlying bone.
Once system function has been verified, an attempt is made to locate and stimulate the facial nerve. In smaller tumors (cerebellopontine angle component of 1 cm or less), the nerve usually can be located at its brainstem entry and an electrical response confirmed before dissection begins. Once a threshold has been established, the voltage is increased to at least three times the threshold value, and the stimulator is swept across the exposed surface of the tumor to confirm the absence of facial nerve fibers before dissection is begun. With larger tumors, the location of the facial nerve may not be immediately apparent. In such cases, we start with 0.3 V and map the accessible region, and if no response is obtained, we repeat the search at 0.5 and 1.0 V. If no response is obtained at 1.0 V, it can be safely assumed that the facial nerve is not on the exposed surface, and dissection can proceed.
During dissection, the stimulator is used repeatedly to scan the operative field for the presence of facial nerve fibers as the tumor is mobilized, using suprathreshold stimulus intensities as described previously. Once a response is obtained, stimulus intensity is reduced to 0.1 to 0.2 V, and the responsive region is narrowed. When the nerve is in sight, the electrode is placed directly on the nerve, and a threshold is obtained by slowly increasing the stimulus level from zero until a response is obtained. Further stimulation for mapping the location of the nerve is carried out at approximately three to five times this threshold, which should be periodically rechecked as dissection proceeds.
The spatial resolution of electrical mapping with monopolar stimulation is determined by stimulus intensity. For the most accurate localization, the stimulus is kept at a low level. At just suprathreshold levels, the spatial resolution is less than 1 mm, allowing the facial nerve to be easily distinguished from the adjacent vestibulocochlear complex. Conversely, to confirm that the nerve is not in an area about to be cut or cauterized, higher levels of stimulation (up to 1.0 V) are used to reduce the likelihood of false-negative results. As more and more tumor is removed, the course of the facial nerve can be mapped from brainstem to internal auditory canal. Although the nerve may be relatively cylindrical at each end, it frequently is compressed by the tumor in the cerebellopontine angle and may be seen as a broad, flat expanse of fibers splayed across the surface of the tumor. Frequently, the only way to identify the nerve and distinguish it from arachnoid tissue is with electrical stimulation; the nerve may be literally invisible to photons and visible only with electrons!
The primary use of intraoperative stimulation is in localizing and mapping the course of cranial nerves in relation to cerebellopontine angle tumors. However, electrical stimulation also is used to determine changes in the functional status of these nerves and is a useful predictor of postoperative function. In general, facial EMG responses elicited by low-threshold stimulation of the seventh nerve at the brainstem after total tumor resection constitute a good but not infallible predictor of postoperative function, because transient or delayed-onset facial palsies may be seen even when a low threshold is obtained at the end of the operation.50–53 Conversely, a substantially elevated threshold or the inability to elicit a response with stimulation up to 1 V carries a significant likelihood of postoperative facial dysfunction, particularly in the short run.51
Methods based on measurement of the compound muscle action potential (CMAP) amplitude after tumor removal also have been used to quantify the functional status of the seventh nerve.32,54 Absolute amplitude is quite variable between patients, however, and may be partially determined by nonspecific factors such as precise electrode placement, amount of subcutaneous fat,29 facial nerve dimensions, and muscle mass.55 These variations have led some investigators to use ratios rather than absolute values for assessment of postoperative facial nerve function. Taha and associates56 measured the ratio of CMAP amplitudes to stimulation at the brainstem proximally and at the internal auditory meatus distally after tumor excision and found that proximal-to-distal amplitude ratios greater than 2 : 3 were associated with excellent outcome. Similarly, Isaacson and colleagues57 analyzed proximal-to-distal amplitude ratios and reported that a higher ratio reflected good immediate postoperative facial nerve function. Lin and coworkers55 used the ratio between proximal amplitude measurement after tumor dissection and supramaximal amplitude measurement in response to transcutaneous facial nerve stimulation and found that a CMAP ratio greater than 50% of maximum had a 93% positive predictive value when measured at 0.3 mA.
Prediction of long-term rather than short-term facial nerve function is the surgeon’s major concern, because this would allow for better patient counseling and planning of rehabilitative treatment.58 It has been suggested that low threshold recorded at the brainstem after tumor resection is a good predictor of long-term facial nerve function50,59–61; however, the ability of threshold to accurately predict long-term function has been questioned by some.62,63 Neff and colleagues64 evaluated a combination of threshold and amplitude and found that a minimum threshold of 0.05 mA together with a CMAP amplitude greater than 240 µV predicts a good postoperative outcome 1 year postoperatively. Fenton and associates65 concluded that a more reliable predictor of long-term facial nerve is the clinical grade of early postoperative facial function. They also demonstrated that all patients with a recordable EMG response to proximal stimulation after tumor dissection, irrespective of the threshold or amplitude, recovered to a follow-up grade III or better facial nerve function. Although this correlation has been previously reported,53,54,61,66,67 its importance has gone unnoticed. Therefore, it is now accepted that whenever a recordable response to electrical stimulation of whatever amplitude or threshold can be demonstrated, the facial nerve is most likely to show signs of improvement with follow-up, and intervention is therefore not recommended within the first year.65 On the other hand, the absence of response to stimulation at the end of surgery does not doom the patient to a bad outcome. If the nerve is anatomically preserved, even with an immediate postoperative palsy, there is still a good possibility of eventual return of function as functional nerve fibers regenerate. Partial recovery of function in patients with unrecordable responses after surgery has been previously reported.68 The earlier the onset of recovery, the better its quality; however, ifevidence of recovery is lacking at 12 months, then it is unlikely to occur.69
Anatomic identification of the nervus intermedius during cerebellopontine angle surgery is important in order to prevent confusing this nerve with the facial nerve itself. Electrical stimulation of the nervus intermedius during cerebellopontine angle surgery was found to produce a characteristic response in the orbicularis oris channel only: long latency, low amplitude, and higher in threshold than the facial nerve response70 (Fig. 178-4). Initial confusion between the nervus intermedius and a facial nerve strand at the time of stimulation is possible, because the entire course of the facial nerve may not be visually apparent to the surgeon as it passes anterior to the tumor—most common surgical approaches are from the posterior. Furthermore, tumor growth causes the facial nerve to be stretched and widened, so it often cannot be identified as a solitary trunk but rather is seen as a wide ribbon. The surgeon must recognize that stimulation of the nervus intermedius can cause EMG activity in the facial nerve monitoring channels (at least the orbicularis oris) but that the main trunk of the facial nerve may lie in an entirely different location within the cerebellopontine angle (Fig. 178-5). To protect this critical structure, it is imperative for the surgeon to locate the facial nerve itself by stimulation.
Figure 178-4. Responses in orbicularis oris to stimulation of nervus intermedius (top) and facial nerve (bottom). Note smaller scale in nervus intermedius response, which is smaller, of longer latency, and seen only in the lower facial nerve channel.
(From Ashram YA, Jackler RK, Pitts LH, Yingling CD. Intraoperative electrophysiological identification of the nervus intermedius. Otol Neurotol. 2005;26:274.)
Figure 178-5. Anatomic variants in relationship between the nervus intermedius and the facial and vestibulocochlear nerves. A, Nervus intermedius joining nerve VII-VIII complex near brainstem root entry zone. B, Nervus intermedius joining nerve VII-VIII in mid-cerebellopontine angle. C, Nervus intermedius joining nerve VII-VIII near the porus acusticus. D, Nervus intermedius taking a separate course through cerebellopontine angle, where it can be misidentified as the facial nerve unless its unique response characteristics are recognized.
(From Ashram YA, Jackler RK, Pitts LH, Yingling CD. Intraoperative electrophysiological identification of the nervus intermedius. Otol Neurotol. 2005;26:274.)
EMG responses to intracranial stimulation are the most specific indicators of cranial nerve localization and functional status. However, spontaneous EMG activity and EMG responses related to intraoperative events also are useful in preserving neural function. Some patients, particularly those with significant preoperative facial deficits, demonstrate baseline tonic facial EMG activity; this often decreases as the nerve is decompressed with opening of the dura and draining of CSF. Virtually all patients exhibit some mechanically evoked facial EMG activity during tumor dissection, retraction, irrigation, or other intraoperative maneuvers. An increase in EMG activity associated with a particular surgical maneuver often is the earliest indicator of the location of the facial nerve. When such activity is elicited, the stimulator should then be used to search the area in question to positively identify the nerve if possible. Frequently, operative manipulations elicit EMG activity, even if the nerve is not in the immediate area, because of transmission of traction or pressure from the tumor to the nerve. In such cases, a negative response to electrical stimulation indicates that dissection can proceed. In other cases, stimulation after mechanically elicited activity results in identification of the nerve, which can then be precisely localized as described previously.
Finally, ongoing EMG activity often is an indirect indicator of depth of anesthesia, which is of particular concern when no muscle relaxants can be used. A simultaneous increase in spontaneous EMG activity on all channels is unlikely to result from localized dissection. When such a generalized increase occurs, the anesthesiologist should be notified immediately, because overt patient movement often occurs within a few seconds.
Prass and Lüders6 distinguished two types of EMG activity associated with intraoperative events: burst and train activity. These investigators suggested that the phasic “burst” pattern, characterized by short, relatively synchronous bursts of motor unit potentials, corresponded to a single discharge of multiple facial nerve axons, and was associated with direct mechanical nerve trauma, free irrigation, application of pledgets soaked with lactated Ringer’s solution over the facial nerve, or electrocautery. When burst activity occurs, it usually indicates that the facial nerve is being stimulated enough to result in depolarization and production of EMG response but not necessarily to the point of injury.71 It generally is accepted that the occurrence of burst activity of small amplitude (less than 500 µV in amplitude) is not of major concern, and accordingly, the surgeon need not be warned each time such activity occurs.72,73 On the other hand, the occurrence of large-amplitude burst activity (greater than 500 µV in amplitude) during the critical stages of dissection or final stages of drilling usually indicates a degree of facial nerve injury, the extent of which differs with the force and number of impacts.
As described by Prass and Lüders,6 the second pattern, tonic or “train” activity, consisted of episodes of prolonged asynchronous grouped motor unit discharges that lasted up to several minutes. These episodes were most commonly associated with facial nerve traction, usually in the lateral to medial direction. The investigators further divided such train activity into higher-frequency trains (50 to 100 Hz), dubbed “bomber potentials” because of their sonic characteristics, and lower-frequency discharges (1 to 50 Hz), which were more irregular and had a sound resembling popping popcorn. The onset and decline of “popcorn” activity were more gradual than the more abrupt onset and decline of “bomber” activity.
Subsequently, Romstock and coworkers72 classified train activity into three distinct patterns: A, B, and C trains. A trains are characterized by a sinusoidal symmetrical sequence of high-frequency and low-amplitude signals that have a sudden onset; B trains are regular or irregular sequences of repeated spikes or bursts with maximum intervals of 500 msec; and C trains are characterized by continuous irregular EMG responses that have many overlapping components. Although B and C trains did not correlate with postoperative function, the authors suggested a relation between the occurrence of A trains and poor postoperative facial nerve function.
Nakao and colleagues74 classified train activity that occurred during the last stage of tumor resection into an irritable pattern with frequent EMG responses to the slightest stimuli, a silent pattern with few or no EMG responses, a stray pattern with persistent train responses up to 20 minutes despite temporary discontinuance of surgical manipulation, and an ordinary pattern related to mechanical stimulation of the nerve but not easily elicited. These workers found an association between the occurrence of silent or stray EMG patterns and poor postoperative outcome. The occurrence of train activity does not always imply an impending nerve injury but indicates the proximity of the facial nerve to the region of dissection and thus should be considered a reassuring sign denoting an intact and responsive nerve.71 However, train activity of large amplitude (greater than 500 µV in amplitude) has been linked to postoperative nerve injury of variable degree,75,76 and such events require prompt warning to the surgeon. The absence of large-amplitude train activity, on the other hand, does not always mean safe dissection, especially with large tumors producing significant facial nerve compression. The nerve axons become stretched, partially damaged, and less responsive than healthy ones, generating little EMG activity despite significant manipulation.77 Therefore, spontaneous EMG activity may not be a reliable warning sign in large tumors, and the frequent use of electrical stimulation is important to map the tumor surface and measure any change of threshold from baseline to assess the condition of the nerve. Figure 178-6 shows representative samples of different types of EMG activity often encountered in vestibular schwannoma removal.
Figure 178-6. Illustrative examples of three types of electromyographic activity often seen during vestibular schwannoma surgery. A, Dense tonic (sustained) activity, often associated with nerve stretch and demonstrating a sinusoidal pattern. B, Lower tonic activity, called popcorn activity. C, Phasic (transient) burst activity typically associated with direct contact with the nerve. Such events are not of major significance unless they involve large-amplitude trains and occur during critical stages of dissection. D, Burst activity superimposed on ongoing small-amplitude train; it is important not to overlook such events overlapping on background activity, because they may pass unnoticed despite their significance.
(From Yingling CD, Ashram YA. Intraoperative monitoring of cranial nerves in skull base surgery. In: Jackler R, Brackmann DE, eds. Neurotology. 2nd ed. Philadelphia: Elsevier; 2005:958.)
Despite the wide use of intraoperative EMG monitoring, it still has its limitations. A major problem with EMG monitoring is its relatively low specificity. EMG channels can easily pick up artifacts, and the distinction between them and true EMG activity may sometimes be difficult. An example is artifacts produced by bimetallic potentials as a result of contact between surgical instruments made of different metals; because these may be associated with intraoperative events similar to those producing true EMG responses, they can be difficult to recognize. Some useful criteria include the fact that artifacts typically are higher in frequency content than EMG activity and thus sound more “crackly” than true EMG activity, which has more of a “popping” sound; and the tendency for artifacts to appear simultaneously on several channels, which is unlikely for EMG activity (Fig. 178-7). Experienced monitoring personnel are in an optimal position to make such decisions, rather than surgeons, who are focused on the operative field. Another limitation of EMG monitoring is that it is virtually useless during electrocautery, when the facial nerve is potentially at high risk. Attempts to reduce the artifact from bipolar cautery have met with limited success, because such devices generate high-amplitude, broadband noise that is difficult to filter out. Techniques based on detection of motion, which are not subject to electrical interference, such as video monitoring, may provide an important adjunct to EMG monitoring despite their relatively lower sensitivity. In our experience, the practical way to deal with this problem is to use electrical stimulation before bipolar electrocoagulation to confirm that the area to be cauterized is free from facial nerve fibers. The absence of a response to higher levels of electrical stimulation (up to 1 V) in an area about to be cauterized is an indication that electrocautery can proceed safely.
Figure 178-7. Electromyography (EMG) activity versus artifacts. A, Upper trace: Artifact produced by contact of different metallic instruments in surgical field. Note sharp edges on waveforms with exponential decay (may be confused with spike activity). Lower trace: Single electromyographic spike with a low-amplitude EMG background and no exponential decay. B, Upper trace: Regular sinusoidal artifact produced during drilling of the internal auditory canal (IAC). Lower trace: Irregular EMG activity while drilling IAC. C, Upper two traces: Regular artifact with two time scales, 200 msec/cm and 5 msec/cm. Lower two traces: EMG activity on the same two time scales. At 200 msec/cm, it may be difficult to differentiate between true EMG and artifact. However, with the faster 5 msec/cm time base, trace 2 shows that the artifact waveform is regular and synchronized, whereas trace 4 reveals the irregularities that characterize true EMG activity.
(From Yingling CD, Ashram YA. Intraoperative monitoring of cranial nerves in skull base surgery. In: Jackler R, Brackmann DE, eds. Neurotology. 2nd ed. Philadelphia: Elsevier; 2005:958.)