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Technique and pitfalls of facial nerve monitoring
TECHNIQUE AND PITFALLS OF FACIAL NERVE MONITORING THOMAS J. HABERKAMP, MD, SAMUEL C. LEVINE, MD
The routine u s e of facial nerve monitoring is controversial. Several investigators advocate the routine use of facial nerve monitoring devices; however, there are no studies to date that prove their effectiveness. There is little in the literature discussing the proper technique and the pitfalls of monitoring. This discussion is critical in making the techniques of facial nerve monitoring safe and effective. Only when more standardized techniques are adopted can the question of usefulness be answered.
Iatrogenic facial nerve injury is one of the most devastating complications of ear surgery. The rate of injury is reported at between 1 in 3,000 and 1 in 100, d e p e n d i n g on the experience of the surgeon and the p r o c e d u r e being performed. ~ The role that facial nerve monitoring plays in routine otologic and neurotologic surgeries is undefined and highly controversial. Although neurophysiologic monitoring of the facial nerve is a relatively new development, there is a long-standing practice of having an assistant observe or palpate the face for twitching while dissecting close to the facial nerve. A l t h o u g h using an assistant gave the surgeon a sense of security, there is no proof that it was effective. Unfortunately, such m a n u a l techniques are highly d e p e n d e n t on the skill, and the attentiveness level of the assistant monitoring the facial movements. Beginning in the late 1970s to the early 1980s, a n u m b e r of neurophysiologic monitoring devices began to appear commercially. 2 These devices consisted of both simple strain gauges to detect facial m o v e m e n t physically, and e l e c t r o m y o g r a p h y (EMG) devices to measure the electrical activation of the muscles. In reality, these devices m a y be looked u p o n as a simple extension of the use of an assistant to monitor the facial function. Facial nerve monitoring devices were quickly accepted for use in otoneurologic procedures, particularly in the removal of acoustic neuromas. 3'4 Some authors have been able to show rapid i m p r o v e m e n t in the ability to preserve facial nerve function in acoustic n e u r o m a s u r g e r y d On the other hand, their use in routine otologic surgeries remains highly controversial. M a n y advocate the routine use of facial nerve monitoring for both otologic and neurotologic procedures. 6-s There is no consensus that monitors should be routinely used in otologic surgery, 9 and some have voiced concerns that facial nerve monitors m a y increase the risk of facial nerve injuries b y giving a false sense of security. In fact, there are no studies to date that show statistically i m p r o v e d or diminished results from the rou-
tine use of facial nerve monitoring. Because facial n e r v e injury is relatively rare and the rate of injury is highly variable, 1 it m a y p r o v e difficult to design adequate studies proving efficacy. Concerns voiced about the routine use of facial n e r v e monitoring include (1) surgeons will learn to rely on the monitor rather than anatomic k n o w l e d g e to protect the nerve, (2) the nerve m a y be inadvertently severed if the monitor indicates a falsely negative stimulation, (3) the feedback given b y the monitor is not rapid e n o u g h to p r e v e n t injury if the nerve is encountered, and (4) the technology adds cost to the procedure. It is apparent that this controversy will not be resolved in the near future. One must r e m e m b e r that it is rare for the facial nerve to have a significantly anomalous course. 1° The most c o m m o n a n o m a l y is a dehiscent facial nerve, which is present in u p to 55% of cases, and m a y p u t the facial nerve at risk for injury. 1° An abnormal course for the facial nerve is seen with increased f r e q u e n c y w h e n other anomalies of the temporal bone are seen. Bellucci noted an anomalous nerve in 13% of patients operated on for aural dysplasias, and Jahrsdoerfer noted anomalies in 24% of cases with minor malformations of the ear. 1° On the other hand, if one is only going to use the monitor only for difficult cases, these anomalies are not always predictable. M a n y reports about anomalous facial nerves are case reports in which the a n o m a l y is u n e x p e c t e d l y encountered. House f o u n d that a significant n u m b e r of facial nerve injuries occurred in routine tympanoplasty, n A l t h o u g h the index of suspicion must be raised w h e n other anomalies of the temporal bone are present, one must always be vigilant regarding facial nerve anomalies. We believe that greater emphasis needs to be given to adequate training in the p r o p e r use of the facial n e r v e monitoring devices. As with any medical device, injuries can occur from i m p r o p e r use. Only w h e n a r m e d with k n o w l e d g e of p r o p e r monitoring techniques, as well as the pitfalls monitoring, will full benefit be gained.
From the Department of Otolaryngology and Bronchoesophagology, Rush Medical College, Rush-Presbyterian-St. Luke's Medical Center, Chicago, IL; and the Department of Otolaryngology, The University of
TECHNIQUE
Minnesota, Minneapolis, MN. Address reprint requests to Thomas J. Haberkamp, MD, Department
General Monitoring Technique
of Otolaryngology and Bronchoesophagology, Rush-Presbyterian-St. Luke's Medical Center, 1653 W Congress Pkwy, Chicago, IL 60612. Copyright © 2001 by W.B. Saunders Company 1043-1810/01/1203-0013535.00/0 doi: 10.1053/otot.2001.25845
The most i m p o r t a n t principle we can state is that the technique of facial nerve monitoring is not a replacement for p r o p e r anatomic dissection; rather, it is an adjunct. The nerve is identified both anatomically and electrically w h e n
OPERATIVE TECHNIQUES IN OTOLARYNGOLOGY--HEAD AND NECK SURGERY, VOL 12, NO 3 (SEPT), 2001: PP 157-161
1 57
facial nerve monitoring is used. In practice the nerve is usually first identified anatomically, with subsequent testing of the functional integrity of the nerve. The nerve is safest if the course of the nerve is completely defined. It is also safest to identify the nerve in an area that is free of disease, and trace it through the diseased areas. As a result, the anatomic dissection must be initially performed as if the monitor were not being used. However, if the nerve monitor unexpectedly indicates facial nerve stimulation, this should be investigated. The landmarks to the facial nerve are well defined in the mastoid and the middle ear, and should be used. 12'13 During this dissection, the monitor may give warning of anatomic malformations or variations in anatomy that put the facial nerve at risk. There are 2 commonly encountered clinical situations. At times during the dissection, the monitor unexpectedly indicates facial nerve stimulation. This may represent an anatomic variant, or a false-positive stimulation. The surgeon must first ask; what action set off the monitor? Stimulation may be able to confirm a true response. If stimulation cannot confirm a response, the structure cannot be sacrificed unless the true nerve is found (false-negatives are discussed later). When the dissection potentially encounters the nerve, the first thing to do is to anatomically trace it to an adjacent anatomic landmark. One may go to a different anatomic landmark, identify the nerve there, and attempt to trace the nerve to the area in question. For example, if soft tissue in the mastoid posterior to the semicircular canal is encountered and stimulates adversely during dissection, one should proceed to either trace the presumed nerve to the stylomastoid foramen, or back toward the semicircular canal or middle ear. An alternative is to go to the digastric or the cochleariformis process to identify the nerve and trace it to or through the tissue in question. This type of dissection will resolve the question in nearly all cases, either identifying the true location of the nerve or confirming that the anomaly is the nerve. In the second situation, the nerve is anatomically identified, but does not stimulate. This can be a particularly dangerous situation. The first thing to do in this situation is to confirm that the monitor is properly functioning. In our experience, the most common situations in which false-negatives occur are when paralytic agents are given without the surgeon's knowledge, when the volume has been turned off for the monitor, and when electrical interference prevents the monitor from detecting a response. It is also possible that the nerve is already injured. One should first determine if the monitor is properly functioning. When technical causes of monitor dysfunction are ruled out, the nerve may be stimulated in another area where it can be identified. Proximal stimulation of the nerve will confirm contiguity of the nerve. If one cannot confirm nerve function (and monitor function) with stimulation, the dissection should be performed as if the monitor were not present. One must take care to avoid injuring the structure in question. When the nerve is fully dissected, one can again attempt stimulation at alternative sites. At times one will be able to show that the structure in question is anatomically separate from the nerve. If the structure in question is in contiguity with the nerve, proximal stimulation is helpful to confirm the nerve was not injured during the initial encounter. If nerve integrity cannot be confirmed with the monitor, it does not mean the nerve was injured. Nerve decompression or exploration should only be considered if both stimulation at and proximal to the potential site of injury fails, and the anatomic dissection suggests violation of the nerve sheath. 158
Otherwise, watchful waiting until the patient is in recovery is wise. There are multiple reasons for false-positive results, which will be discussed later in the report. To summarize, one must never sacrifice a structure that appears to be the nerve unless the real nerve is both anatomically and electrically confirmed. Also, one must never perform dissection without full orientation to the anatomy. The monitor will not protect the surgeon during blind dissection, and it will not protect the nerve from overly aggressive dissection.
EQUIPMENT There are 2 general types of nerve-monitoring devices. The first device consists of a strain gauge that is clipped to the cheek to detect facial movement (Silverstein nerve monitor, WR Electronics, Minneapolis, MN). The second type evaluates the EMG response of the facial nerve; this includes many devices. The latter type of monitor may be equipped with an alarm to detect the EMG activity, generally giving a beep when electrical activity consistent with an EMG is detected. The surgeon or monitoring technician may then further analyze the tracings. Both types of devices are now equipped with probes to provide electrical stimulation to the nerve. This will allow mapping of the nerve position, test functional nerve continuity, and evaluate thresholds. Finally, both types of devices allow passive monitoring of nerve activity during dissection; this allows the potential for the monitor to alert the surgeon if the anatomic dissection adversely stimulates the nerve. Strain G a u g e D e v i c e s
These devices consist of a strain gage that is placed on the cheek with one arm of the detector intraorally placed, and the other on the outer cheek (Fig 1). The cheek should be slightly bunched into the gage, and the tension is adjusted so the device touches the skin but does not compress the tissues. These devices are relatively inexpensive and simple to set up. Because the device is mechanical, it is not prone to electrical interference. However, it is prone to mechanical interference. As a result, setting up the device and the draping of the patient so the drapes do not directly lie on the detector will lessen false-positive responses caused by movement of the drapes.
FIGURE 1. Set up of the strain gauge device. The paddles of the sensor are placed with the cheek between them, with the tissues slightly bunched up. TECHNIQUES OF FACIAL NERVEMONITORING
EMG Devices
These devices actually monitor the electrical activity of the facial muscles. All use bipolar electrodes to this effect. They are prone to electrical interference from such devices as power supplies and other stimulators. At times, a 60 Hz signal is seen from the electrocardiogram (EKG) used with anesthesia. Metal-to-metal contact, such as touching a retractor with a dissector, may also produce a false positive. EMG monitoring devices respond at a lower level of stimulation to the nerve. 14 The increased sensitivity comes at the price of more frequent false-positive responses. 14 Because the EMG response is recorded, the tracings can be analyzed, which will aid in the differentiation of true from false-positive responses. A true biphasic response indicates neural activity, static discharges, and metal-to-metal contact, generally producing a spiked monopolar activity (Figs 2, 3). Analysis of latency of response may differentiate stimulation of adjacent cranial nerves from the facial nerve, most commonly the 5 th n e r v e . One must be careful in setting up the device to keep the electrodes, wires, and the preamplifier connection box (Fig 4) isolated from electrical sources, such as microscope power supplies and EKG electrodes. Bipolar electrodes can also be placed near each other (near field) (Fig 5), or farther apart (far field) (Fig 6). Near-field placement will be less prone to interference or to picking up activity from other muscles, but will also be less sensitive if other adjacent branches of the nerve are stimulated. Far-field responses can monitor large areas, but are more prone to pick up other activity. The impedance must be checked after placement, according to the manufacturer recommendations for the specific electrodes and devices. It is common practice to test the electrodes by tapping the face after placement of the devices. We must emphasize that this does not confirm that the device is working; it only will confirm that the volume of the alert signal is not turned off. Mechanical responses may be seen even if the patient is fully paralyzed. The analysis of a response must begin with consideration for what activity elicited the response. One must first
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FIGURE 3. Three types of activity in acoustic neuroma surgery that may also be seen in other surgeries. (A) Dense tonic activity often seen if the nerve is stretched or otherwise stimulated. (B) Burst activity indicating nerve irritability. (C) Phasic activity may be seen with irrigation or nontraumatic nerve contact with instruments.
consider if the activity before the response could have elicited a true response. If a soft tissue structure was encountered, it may represent a true response. If one was not in the field, it is likely that it was a false-positive response. When a positive response is seen, one may proceed with several steps. Metal-to-metal contact will often set off the monitor, particularly with the EMG devices. This may be seen if the retractors are touched with a dissector or drill. In these cases, the response is a sharp discharge, monopolar, and of variable latency. Also, the devices are equipped with an alarm to signal a response, and a speaker is set up to give an audible indication of electrical activity. Frequently false-positive responses elicit neither a bipolar response on the oscilloscope nor the background audible response, which may be a popping, rumbling, or thumping sound. A true-positive response should produce an alarm, an audible response on the speaker, and a bipolar wave on the oscilloscope.
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FIGURE 2. (A) Various artifact responses seen in EMG monitoring. Note the sharp, spiked, and generally monopolar response. The responses are simultaneous in all channels. One must also correlate the activity that produced the response to the interpretation. (B) Various EMG responses, showing spontaneous activity. The true EMG is more rounded and bipolar. There is also less synchrony between channels. HABERKAMP AND LEVINE
FIGURE 4. The preamplifier connector box with the electrodes placed in it. The box is best placed away from sources of electrical interference, such as microscope light sources. 159
False-negative responses present more problems because they m a y give the surgeon a false sense of security that the nerve is not at risk. W h e n anatomic dissection encounters the probable nerve, but it does not stimulate, the surgeon must first check the integrity of the monitor itself. One m u s t initially be certain the volume was not turned off for the device, and that all electrodes are in place and active. Neuromuscular blockade presents special problems; complete blockade is contraindicated if nerve monitoring is to be used. We do not advocate partial paralysis, which is particularly problematic if the surgeon is unaware of it. The literature suggests that supramaximal stimulability of the facial nerve is maintained during all levels of partial blockade. 15 However, one m u s t maintain the ability to detect even minor mechanical stimulation of the nerve. This will be effected by partial blockade. Technically, partial blockade m a y be compatable with facial nerve monitoring; however, the surgeon must be aware of the partial blockade and take this into account during the procedure. Another c o m m o n cause of false-negative stimulation is reversal of the stimulator connection and the ground electrode. This can be recognized by analyzing the set-todelivered stimulus ratio, which is reduced. Generally, the delivered response is at the set level, +0.02 mA. W h e n the stimulator connections are reversed, the delivered response is generally only 0.02 m A total. The nerve is often stimulable through bone, requiring approximately 1.0 m A stimulus intensity per 1-mm thickness of bone. 7 Soft tissue on the nerve is frequently stimulable, both electrically and mechanically. Once again, the settings required to electrically stimulate the nerve through this tissue depend on the thickness of the tissue, the actual proximity to the nerve, and the electrical properties of the tissue (granulation stimulates more readily that scar). It is c o m m o n to mechanically stimulate soft tissue attached to the nerve and find that it stimulates poorly. One should avoid traumatizing this tissue and find a proximal area to stimulate the nerve. If the nerve can be stimulated normally proximal to this area, it has not been injured. Unless the nerve appears to be injured anatomically, we do not recommend exploration of the nerve or decompression. This does not m e a n the nerve should not be anatomically identified and fully
!;
FIGURE 6. Far-field placement. This placement will monitor most facial nerve branches, but will also readily pick up masseter and temporalis activity.
traced; it simply means the b o n y canal need not be decompressed simply for poor stimulability if the nerve otherwise appears intact. CONCLUSIONS
Facial nerve monitoring remains a controversial technique for use in routine otologic cases. Routine use of monitoring has several potential benefits. As with any technique, there is a learning curve. If one is going to use the monitor for complex or difficult cases, we believe the experience gained with routine monitoring will improve the reliability of the technique in difficult cases. We have experienced a significant reduction in the number of unexplained false-positives, as well as false-negatives, as experience was gained. We believe that the most important principle in monitoring is that it is an adjunct to, not a replacement for, proper anatomic dissection and gentle surgical technique. Remembering this will avoid m a n y of the potential pitfalls of the technique. Although m a n y of the devices are expensive, the costs of the consumables used in an individual case are small. More routine use should reduce the costs of monitoring. The question of using monitoring technicians is not addressed here. There are a n u m b e r of advantages, as well as disadvantages, to using a technician. This choice should be left to the individual surgeon, as should the decision whether to use the device. REFERENCES
FIGURE 5. Near-field placement of the facial electrodes. Note the monitored field covers only a small number of facial muscles, but also is less likely pick up masseter activity. 160
1. SchuringAG: Iatrogenicfacialnerve injury. Am J Otol 9:432-433,1988 2. Kinney SE, Prass RL: Facial nerve dissection by use of acoustic (loudspeaker) facial EMG monitoring. Otolaryngol Head Neck Surg 95:458-463, 1986 3. Benecke JE, Jr., Calder HB, Chadwick G: Facial nerve monitoring during acoustic neuroma removal. Laryngoscope97:697-700, 1987 4. Uziel A, BenezechJ, Frerebeau P: Intraoperative facial nerve monitoring in posterior fossa acoustic neuroma surgery. Otolaryngol Head Neck Surg 108:126-134, 1993 5. Kwartler JA, Luxford WM, Atkins J, et ah Facial nerve monitoring in acoustictumor surgery.OtolaryngolHead Neck Surg 104:814-817,1991 6. Olds MJ, Rowan PT, Isaacson JE, et ah Facial nerve monitoring TECHNIQUES OF FACIAL NERVE MONITORING
7.
8. 9.
10.
among graduates of the Ear Research Foundation. Am J Otol 18:507511, 1997 Silverstein H, Smouha E, Jones R: Routine identification of the facial nerve using ~lectrical stimulation during otological and neurotological surgery. Laryngoscope 98:726-730, 1988 Silverstein H, Smouha EE, Jones R: Routine intraoperative facial nerve monitoring during otologic surgery. Am J Otol 9:269-275, 1988 Roland PS, Meyerhoff WL: Intraoperative electrophysiological monitoring of the facial nerve: Is it standard of practice? Am J Otolaryngol 15:267-270, 1994 Proctor B: Canals of the temporal bone, anatomic variations and anomalies involving the facial canal, in Surgical anatomy of the ear and temporal bone. New York, NY, Theime, 1989, pp 100-111
HABERKAMP AND LEVINE
11. House JW: Iatrogenic facial paralysis. Ear Nose Throat J 75:720-723, 1996 12. Pulec JL: The facial nerve: How to find it. Ear Nose Throat J 72:677685, 1993 13. Pulec JL: Total facial nerve decompression: Technique to avoid complications. Ear Nose Throat J 75:410-415, 1996 14. Bendet E, Rosenberg SI, Willcox TO, et al: Intraoperative facial nerve monitoring: A comparison between electromyography and mechanical-pressure monitoring techniques [letter]. Am J Otol 20:793-799, 1999 15. Ho LC, Crosby G, Sundaram P, et ah Ulnar train-of-four stimulation in predicting face movement during entracranial facial nerve stimulation. Anesth Analg 69:242-244, 1989
161
The routine u s e of facial nerve monitoring is controversial. Several investigators advocate the routine use of facial nerve monitoring devices; however, there are no studies to date that prove their effectiveness. There is little in the literature discussing the proper technique and the pitfalls of monitoring. This discussion is critical in making the techniques of facial nerve monitoring safe and effective. Only when more standardized techniques are adopted can the question of usefulness be answered.
Iatrogenic facial nerve injury is one of the most devastating complications of ear surgery. The rate of injury is reported at between 1 in 3,000 and 1 in 100, d e p e n d i n g on the experience of the surgeon and the p r o c e d u r e being performed. ~ The role that facial nerve monitoring plays in routine otologic and neurotologic surgeries is undefined and highly controversial. Although neurophysiologic monitoring of the facial nerve is a relatively new development, there is a long-standing practice of having an assistant observe or palpate the face for twitching while dissecting close to the facial nerve. A l t h o u g h using an assistant gave the surgeon a sense of security, there is no proof that it was effective. Unfortunately, such m a n u a l techniques are highly d e p e n d e n t on the skill, and the attentiveness level of the assistant monitoring the facial movements. Beginning in the late 1970s to the early 1980s, a n u m b e r of neurophysiologic monitoring devices began to appear commercially. 2 These devices consisted of both simple strain gauges to detect facial m o v e m e n t physically, and e l e c t r o m y o g r a p h y (EMG) devices to measure the electrical activation of the muscles. In reality, these devices m a y be looked u p o n as a simple extension of the use of an assistant to monitor the facial function. Facial nerve monitoring devices were quickly accepted for use in otoneurologic procedures, particularly in the removal of acoustic neuromas. 3'4 Some authors have been able to show rapid i m p r o v e m e n t in the ability to preserve facial nerve function in acoustic n e u r o m a s u r g e r y d On the other hand, their use in routine otologic surgeries remains highly controversial. M a n y advocate the routine use of facial nerve monitoring for both otologic and neurotologic procedures. 6-s There is no consensus that monitors should be routinely used in otologic surgery, 9 and some have voiced concerns that facial nerve monitors m a y increase the risk of facial nerve injuries b y giving a false sense of security. In fact, there are no studies to date that show statistically i m p r o v e d or diminished results from the rou-
tine use of facial nerve monitoring. Because facial n e r v e injury is relatively rare and the rate of injury is highly variable, 1 it m a y p r o v e difficult to design adequate studies proving efficacy. Concerns voiced about the routine use of facial n e r v e monitoring include (1) surgeons will learn to rely on the monitor rather than anatomic k n o w l e d g e to protect the nerve, (2) the nerve m a y be inadvertently severed if the monitor indicates a falsely negative stimulation, (3) the feedback given b y the monitor is not rapid e n o u g h to p r e v e n t injury if the nerve is encountered, and (4) the technology adds cost to the procedure. It is apparent that this controversy will not be resolved in the near future. One must r e m e m b e r that it is rare for the facial nerve to have a significantly anomalous course. 1° The most c o m m o n a n o m a l y is a dehiscent facial nerve, which is present in u p to 55% of cases, and m a y p u t the facial nerve at risk for injury. 1° An abnormal course for the facial nerve is seen with increased f r e q u e n c y w h e n other anomalies of the temporal bone are seen. Bellucci noted an anomalous nerve in 13% of patients operated on for aural dysplasias, and Jahrsdoerfer noted anomalies in 24% of cases with minor malformations of the ear. 1° On the other hand, if one is only going to use the monitor only for difficult cases, these anomalies are not always predictable. M a n y reports about anomalous facial nerves are case reports in which the a n o m a l y is u n e x p e c t e d l y encountered. House f o u n d that a significant n u m b e r of facial nerve injuries occurred in routine tympanoplasty, n A l t h o u g h the index of suspicion must be raised w h e n other anomalies of the temporal bone are present, one must always be vigilant regarding facial nerve anomalies. We believe that greater emphasis needs to be given to adequate training in the p r o p e r use of the facial n e r v e monitoring devices. As with any medical device, injuries can occur from i m p r o p e r use. Only w h e n a r m e d with k n o w l e d g e of p r o p e r monitoring techniques, as well as the pitfalls monitoring, will full benefit be gained.
From the Department of Otolaryngology and Bronchoesophagology, Rush Medical College, Rush-Presbyterian-St. Luke's Medical Center, Chicago, IL; and the Department of Otolaryngology, The University of
TECHNIQUE
Minnesota, Minneapolis, MN. Address reprint requests to Thomas J. Haberkamp, MD, Department
General Monitoring Technique
of Otolaryngology and Bronchoesophagology, Rush-Presbyterian-St. Luke's Medical Center, 1653 W Congress Pkwy, Chicago, IL 60612. Copyright © 2001 by W.B. Saunders Company 1043-1810/01/1203-0013535.00/0 doi: 10.1053/otot.2001.25845
The most i m p o r t a n t principle we can state is that the technique of facial nerve monitoring is not a replacement for p r o p e r anatomic dissection; rather, it is an adjunct. The nerve is identified both anatomically and electrically w h e n
OPERATIVE TECHNIQUES IN OTOLARYNGOLOGY--HEAD AND NECK SURGERY, VOL 12, NO 3 (SEPT), 2001: PP 157-161
1 57
facial nerve monitoring is used. In practice the nerve is usually first identified anatomically, with subsequent testing of the functional integrity of the nerve. The nerve is safest if the course of the nerve is completely defined. It is also safest to identify the nerve in an area that is free of disease, and trace it through the diseased areas. As a result, the anatomic dissection must be initially performed as if the monitor were not being used. However, if the nerve monitor unexpectedly indicates facial nerve stimulation, this should be investigated. The landmarks to the facial nerve are well defined in the mastoid and the middle ear, and should be used. 12'13 During this dissection, the monitor may give warning of anatomic malformations or variations in anatomy that put the facial nerve at risk. There are 2 commonly encountered clinical situations. At times during the dissection, the monitor unexpectedly indicates facial nerve stimulation. This may represent an anatomic variant, or a false-positive stimulation. The surgeon must first ask; what action set off the monitor? Stimulation may be able to confirm a true response. If stimulation cannot confirm a response, the structure cannot be sacrificed unless the true nerve is found (false-negatives are discussed later). When the dissection potentially encounters the nerve, the first thing to do is to anatomically trace it to an adjacent anatomic landmark. One may go to a different anatomic landmark, identify the nerve there, and attempt to trace the nerve to the area in question. For example, if soft tissue in the mastoid posterior to the semicircular canal is encountered and stimulates adversely during dissection, one should proceed to either trace the presumed nerve to the stylomastoid foramen, or back toward the semicircular canal or middle ear. An alternative is to go to the digastric or the cochleariformis process to identify the nerve and trace it to or through the tissue in question. This type of dissection will resolve the question in nearly all cases, either identifying the true location of the nerve or confirming that the anomaly is the nerve. In the second situation, the nerve is anatomically identified, but does not stimulate. This can be a particularly dangerous situation. The first thing to do in this situation is to confirm that the monitor is properly functioning. In our experience, the most common situations in which false-negatives occur are when paralytic agents are given without the surgeon's knowledge, when the volume has been turned off for the monitor, and when electrical interference prevents the monitor from detecting a response. It is also possible that the nerve is already injured. One should first determine if the monitor is properly functioning. When technical causes of monitor dysfunction are ruled out, the nerve may be stimulated in another area where it can be identified. Proximal stimulation of the nerve will confirm contiguity of the nerve. If one cannot confirm nerve function (and monitor function) with stimulation, the dissection should be performed as if the monitor were not present. One must take care to avoid injuring the structure in question. When the nerve is fully dissected, one can again attempt stimulation at alternative sites. At times one will be able to show that the structure in question is anatomically separate from the nerve. If the structure in question is in contiguity with the nerve, proximal stimulation is helpful to confirm the nerve was not injured during the initial encounter. If nerve integrity cannot be confirmed with the monitor, it does not mean the nerve was injured. Nerve decompression or exploration should only be considered if both stimulation at and proximal to the potential site of injury fails, and the anatomic dissection suggests violation of the nerve sheath. 158
Otherwise, watchful waiting until the patient is in recovery is wise. There are multiple reasons for false-positive results, which will be discussed later in the report. To summarize, one must never sacrifice a structure that appears to be the nerve unless the real nerve is both anatomically and electrically confirmed. Also, one must never perform dissection without full orientation to the anatomy. The monitor will not protect the surgeon during blind dissection, and it will not protect the nerve from overly aggressive dissection.
EQUIPMENT There are 2 general types of nerve-monitoring devices. The first device consists of a strain gauge that is clipped to the cheek to detect facial movement (Silverstein nerve monitor, WR Electronics, Minneapolis, MN). The second type evaluates the EMG response of the facial nerve; this includes many devices. The latter type of monitor may be equipped with an alarm to detect the EMG activity, generally giving a beep when electrical activity consistent with an EMG is detected. The surgeon or monitoring technician may then further analyze the tracings. Both types of devices are now equipped with probes to provide electrical stimulation to the nerve. This will allow mapping of the nerve position, test functional nerve continuity, and evaluate thresholds. Finally, both types of devices allow passive monitoring of nerve activity during dissection; this allows the potential for the monitor to alert the surgeon if the anatomic dissection adversely stimulates the nerve. Strain G a u g e D e v i c e s
These devices consist of a strain gage that is placed on the cheek with one arm of the detector intraorally placed, and the other on the outer cheek (Fig 1). The cheek should be slightly bunched into the gage, and the tension is adjusted so the device touches the skin but does not compress the tissues. These devices are relatively inexpensive and simple to set up. Because the device is mechanical, it is not prone to electrical interference. However, it is prone to mechanical interference. As a result, setting up the device and the draping of the patient so the drapes do not directly lie on the detector will lessen false-positive responses caused by movement of the drapes.
FIGURE 1. Set up of the strain gauge device. The paddles of the sensor are placed with the cheek between them, with the tissues slightly bunched up. TECHNIQUES OF FACIAL NERVEMONITORING
EMG Devices
These devices actually monitor the electrical activity of the facial muscles. All use bipolar electrodes to this effect. They are prone to electrical interference from such devices as power supplies and other stimulators. At times, a 60 Hz signal is seen from the electrocardiogram (EKG) used with anesthesia. Metal-to-metal contact, such as touching a retractor with a dissector, may also produce a false positive. EMG monitoring devices respond at a lower level of stimulation to the nerve. 14 The increased sensitivity comes at the price of more frequent false-positive responses. 14 Because the EMG response is recorded, the tracings can be analyzed, which will aid in the differentiation of true from false-positive responses. A true biphasic response indicates neural activity, static discharges, and metal-to-metal contact, generally producing a spiked monopolar activity (Figs 2, 3). Analysis of latency of response may differentiate stimulation of adjacent cranial nerves from the facial nerve, most commonly the 5 th n e r v e . One must be careful in setting up the device to keep the electrodes, wires, and the preamplifier connection box (Fig 4) isolated from electrical sources, such as microscope power supplies and EKG electrodes. Bipolar electrodes can also be placed near each other (near field) (Fig 5), or farther apart (far field) (Fig 6). Near-field placement will be less prone to interference or to picking up activity from other muscles, but will also be less sensitive if other adjacent branches of the nerve are stimulated. Far-field responses can monitor large areas, but are more prone to pick up other activity. The impedance must be checked after placement, according to the manufacturer recommendations for the specific electrodes and devices. It is common practice to test the electrodes by tapping the face after placement of the devices. We must emphasize that this does not confirm that the device is working; it only will confirm that the volume of the alert signal is not turned off. Mechanical responses may be seen even if the patient is fully paralyzed. The analysis of a response must begin with consideration for what activity elicited the response. One must first
B
C
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20 ms
FIGURE 3. Three types of activity in acoustic neuroma surgery that may also be seen in other surgeries. (A) Dense tonic activity often seen if the nerve is stretched or otherwise stimulated. (B) Burst activity indicating nerve irritability. (C) Phasic activity may be seen with irrigation or nontraumatic nerve contact with instruments.
consider if the activity before the response could have elicited a true response. If a soft tissue structure was encountered, it may represent a true response. If one was not in the field, it is likely that it was a false-positive response. When a positive response is seen, one may proceed with several steps. Metal-to-metal contact will often set off the monitor, particularly with the EMG devices. This may be seen if the retractors are touched with a dissector or drill. In these cases, the response is a sharp discharge, monopolar, and of variable latency. Also, the devices are equipped with an alarm to signal a response, and a speaker is set up to give an audible indication of electrical activity. Frequently false-positive responses elicit neither a bipolar response on the oscilloscope nor the background audible response, which may be a popping, rumbling, or thumping sound. A true-positive response should produce an alarm, an audible response on the speaker, and a bipolar wave on the oscilloscope.
B
FIGURE 2. (A) Various artifact responses seen in EMG monitoring. Note the sharp, spiked, and generally monopolar response. The responses are simultaneous in all channels. One must also correlate the activity that produced the response to the interpretation. (B) Various EMG responses, showing spontaneous activity. The true EMG is more rounded and bipolar. There is also less synchrony between channels. HABERKAMP AND LEVINE
FIGURE 4. The preamplifier connector box with the electrodes placed in it. The box is best placed away from sources of electrical interference, such as microscope light sources. 159
False-negative responses present more problems because they m a y give the surgeon a false sense of security that the nerve is not at risk. W h e n anatomic dissection encounters the probable nerve, but it does not stimulate, the surgeon must first check the integrity of the monitor itself. One m u s t initially be certain the volume was not turned off for the device, and that all electrodes are in place and active. Neuromuscular blockade presents special problems; complete blockade is contraindicated if nerve monitoring is to be used. We do not advocate partial paralysis, which is particularly problematic if the surgeon is unaware of it. The literature suggests that supramaximal stimulability of the facial nerve is maintained during all levels of partial blockade. 15 However, one m u s t maintain the ability to detect even minor mechanical stimulation of the nerve. This will be effected by partial blockade. Technically, partial blockade m a y be compatable with facial nerve monitoring; however, the surgeon must be aware of the partial blockade and take this into account during the procedure. Another c o m m o n cause of false-negative stimulation is reversal of the stimulator connection and the ground electrode. This can be recognized by analyzing the set-todelivered stimulus ratio, which is reduced. Generally, the delivered response is at the set level, +0.02 mA. W h e n the stimulator connections are reversed, the delivered response is generally only 0.02 m A total. The nerve is often stimulable through bone, requiring approximately 1.0 m A stimulus intensity per 1-mm thickness of bone. 7 Soft tissue on the nerve is frequently stimulable, both electrically and mechanically. Once again, the settings required to electrically stimulate the nerve through this tissue depend on the thickness of the tissue, the actual proximity to the nerve, and the electrical properties of the tissue (granulation stimulates more readily that scar). It is c o m m o n to mechanically stimulate soft tissue attached to the nerve and find that it stimulates poorly. One should avoid traumatizing this tissue and find a proximal area to stimulate the nerve. If the nerve can be stimulated normally proximal to this area, it has not been injured. Unless the nerve appears to be injured anatomically, we do not recommend exploration of the nerve or decompression. This does not m e a n the nerve should not be anatomically identified and fully
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FIGURE 6. Far-field placement. This placement will monitor most facial nerve branches, but will also readily pick up masseter and temporalis activity.
traced; it simply means the b o n y canal need not be decompressed simply for poor stimulability if the nerve otherwise appears intact. CONCLUSIONS
Facial nerve monitoring remains a controversial technique for use in routine otologic cases. Routine use of monitoring has several potential benefits. As with any technique, there is a learning curve. If one is going to use the monitor for complex or difficult cases, we believe the experience gained with routine monitoring will improve the reliability of the technique in difficult cases. We have experienced a significant reduction in the number of unexplained false-positives, as well as false-negatives, as experience was gained. We believe that the most important principle in monitoring is that it is an adjunct to, not a replacement for, proper anatomic dissection and gentle surgical technique. Remembering this will avoid m a n y of the potential pitfalls of the technique. Although m a n y of the devices are expensive, the costs of the consumables used in an individual case are small. More routine use should reduce the costs of monitoring. The question of using monitoring technicians is not addressed here. There are a n u m b e r of advantages, as well as disadvantages, to using a technician. This choice should be left to the individual surgeon, as should the decision whether to use the device. REFERENCES
FIGURE 5. Near-field placement of the facial electrodes. Note the monitored field covers only a small number of facial muscles, but also is less likely pick up masseter activity. 160
1. SchuringAG: Iatrogenicfacialnerve injury. Am J Otol 9:432-433,1988 2. Kinney SE, Prass RL: Facial nerve dissection by use of acoustic (loudspeaker) facial EMG monitoring. Otolaryngol Head Neck Surg 95:458-463, 1986 3. Benecke JE, Jr., Calder HB, Chadwick G: Facial nerve monitoring during acoustic neuroma removal. Laryngoscope97:697-700, 1987 4. Uziel A, BenezechJ, Frerebeau P: Intraoperative facial nerve monitoring in posterior fossa acoustic neuroma surgery. Otolaryngol Head Neck Surg 108:126-134, 1993 5. Kwartler JA, Luxford WM, Atkins J, et ah Facial nerve monitoring in acoustictumor surgery.OtolaryngolHead Neck Surg 104:814-817,1991 6. Olds MJ, Rowan PT, Isaacson JE, et ah Facial nerve monitoring TECHNIQUES OF FACIAL NERVE MONITORING
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among graduates of the Ear Research Foundation. Am J Otol 18:507511, 1997 Silverstein H, Smouha E, Jones R: Routine identification of the facial nerve using ~lectrical stimulation during otological and neurotological surgery. Laryngoscope 98:726-730, 1988 Silverstein H, Smouha EE, Jones R: Routine intraoperative facial nerve monitoring during otologic surgery. Am J Otol 9:269-275, 1988 Roland PS, Meyerhoff WL: Intraoperative electrophysiological monitoring of the facial nerve: Is it standard of practice? Am J Otolaryngol 15:267-270, 1994 Proctor B: Canals of the temporal bone, anatomic variations and anomalies involving the facial canal, in Surgical anatomy of the ear and temporal bone. New York, NY, Theime, 1989, pp 100-111
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11. House JW: Iatrogenic facial paralysis. Ear Nose Throat J 75:720-723, 1996 12. Pulec JL: The facial nerve: How to find it. Ear Nose Throat J 72:677685, 1993 13. Pulec JL: Total facial nerve decompression: Technique to avoid complications. Ear Nose Throat J 75:410-415, 1996 14. Bendet E, Rosenberg SI, Willcox TO, et al: Intraoperative facial nerve monitoring: A comparison between electromyography and mechanical-pressure monitoring techniques [letter]. Am J Otol 20:793-799, 1999 15. Ho LC, Crosby G, Sundaram P, et ah Ulnar train-of-four stimulation in predicting face movement during entracranial facial nerve stimulation. Anesth Analg 69:242-244, 1989
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