Intraoperative facial nerve monitoring Technical aspects D O U G L A S L. B E C K , MA, and J A M E S E. B E N E C K E , J R . , MD, F A C S , St. Louis, Missouri, and
Los Angeles, California
I n t r a o p e r a t i v e f a c i a l n e r v e monitoring (FNM) Is w i d e l y u s e d as a m e t h o d of assessing the a n a t o m i c a n d p h y s i o l o g i c integrity of t h e f a c i a l n e r v e during p r o c e d u r e s in w h i c h t h e n e r v e m a y b e in j e o p a r d y . T h e d o c t o r s of t h e O t o l o g i c M e d i c a l G r o u p u s e intra o p e r a t i v e F N M during all n e u r o t o l o g l c p r o c e d u r e s . This t e c h n i q u e c a n a l s o b e u s e d for c o n g e n i t a l e a r s u r g e r y a n d c o m p l i c a t e d revision c h r o n i c e a r s u r g e r y . O n t h e basis of e x p e r i e n c e g a i n e d f r o m m o r e t h a n 200 c a s e s in w h i c h F N M has b e e n u s e d , definitive c o n c l u s i o n s c a n b e m a d e In r e g a r d to t e c h n i c a l a s p e c t s . This article will discuss in d i c a t i o n s for I n t r a o p e r a t i v e FNM, e l e c t r o d e d e s i g n , visual a n d a u d i t o r y f e e d b a c k , a n d the particulars a b o u t stimulators. (OTOLARYNGOL HEAD NECK SURG 1 9 9 0 ; 1 0 2 . 2 7 0 . )
Modern
allowed
microsurgical techniques have the neurotologist and neurosurgeon to perform surgery of the cerebellopontine (CP) angle and skullbase safely and with certain priority goals. The preservation of useful life and (when feasible) total tumor removal are the most important goals. Preservation of cranial nerve has become a nearly equally important goal. The facial nerve is at risk in virtually all skullbase and CP angle surgeries. Monitoring the integrity of the fa cial nerve during surgery appears to be a logical means by which the nerve might be preserved in a greater number of cases.
function
Over a period of 12 months we have used and crit ically evaluated a device specifically designed to mon itor FN function intraoperatively. This device is the Nerve Integrity Monitor 2 (NIM-2), manufactured by XOMED (XOMED Inc., Jacksonville, Fla.). The basis for this device and all other FN monitors is assurance that when a physiologically intact facial nerve is stim ulated, the muscles that are innervated by this nerve 1
will contract. Stimulation can be the result of inadver tent surgical manipulation of the nerve, or the inten tional use of a stimulating electrode. The response to this stimulation is the basis for FNM. The technique used must exhibit both sensitivity and specificity.
WHY MONITOR THE FACIAL NERVE? There are several reasons to perform FNM. It im mediately alerts the surgeon when something poten tially harmful is being done to the nerve. FNM also allows for positive identification of the facial nerve when regional anatomy is confusing. After removal of a CP angle tumor, the integrity of the facial nerve can be assessed by electrical stimulation of the nerve. Stimulating along the course of the facial nerve may help identify an area of injury to the nerve. This may have an impact on the surgical procedure. 23
4
MONITORING THE MUSCLE RESPONSE There have been numerous articles in the literature regarding the various techniques of monitoring the mus cle response to facial nerve stimulation. " A detailed description of the different methods is beyond the scope of this article. The method we currently use is a mod ification of previously described techniques. Basically, one wishes to monitor an evoked electromyographic (EMG) response that can be visualized on a screen and can also be played over a loudspeaker to provide im mediate auditory feedback to FN stimulation. Such feedback provides the surgeon with a warning that the facial nerve may be in jeopardy. The EMG is obtained by use of multiple bipolar recording electrodes that are 4
From St. Louis University Medical Center, (Dr. Beck), and the Oto logic Medical Group, Inc. (Dr. Benecke). Supported by funds from the House Ear Institute, an affiliate of the University of Southern California School of Medicine. Presented at the Annual Meeting of the American Academy of Otolaryngology-Head and Neck Surgery, Washington, D C . , Sept. 25-29, 1988. Submitted for publication April 14, 1989; accepted April 27, 1989. Reprint requests: James E. Benecke, Jr., MD, St. Louis University Medical Center, 3635 Vista Ave. at Grand Blvd., St. Louis, MO 63110. 23/1/13506
270
Downloaded from oto.sagepub.com at SAGE Publications on June 21, 2016
Volume 102 Number 3 March 1990
Intraoperative facial n e r v e monitoring t e c h n i c a l a s p e c t s
placed intramuscularly. The use of intramuscular elec trodes provides response specificity. Bipolar electrodes allow for increased sensitivity. The bipolar recording electrodes we use are manu factured in our laboratory in a manner similar to that described by Prass et al. and Prass and Luders. " Teflon-insulated silver wire (part #AG5T, Medwire Corp, Mount Vernon, N.Y.) is sectioned into lengths of about 30 cm and both ends are bared of insulation for a distance of 1.5 mm. One end is inserted into a solder tip filled with 6 0 / 4 0 solder. These steps are ac complished under a microscope. An impedance meter is used to verify electrical continuity. The wider part °f the solder tip and the first 2.5 cm of wire is covered by 3/16-inch heat-shrinkable covering, which adds strain relief to the solder connection. Two strands prepared as above are held together so the solder tips dangle freely, and the bare ends are staggered to an overlap of about 3 mm. The soldered ends are twisted so a uniform twist is achieved along the entire length. The twist is strengthened by placing the pair on a hard surface and running one's finger along the length. Finally, the bare ends of the properly twisted pair are inserted through the sharp end of a 22-gauge hy podermic needle to a distance of 15 mm. The remaining length of the wire with solder tips is folded back over the needle. The protective covering of the needle is replaced, and the entire apparatus is placed in a gas sterilization bag and subsequently gas-sterilized. One pair of bipolar recording electrodes is inserted into the ipsilateral superior orbicularis oris muscle. This is accomplished by inserting the needle and wires into the muscle above the upper lip and carefully withdraw ing the needle so the wires are not pulled out. A second Pair is placed into the ipsilateral inferior aspect of the orbicularis oris muscle. The reference electrode is a standard monopolar subdermal needle electrode and is inserted in the forehead. The anodes for the stimulator are also monopolar subdermal needle electrodes and are always placed in the ipsilateral shoulder. 2
3
STIMULATING THE FACIAL NERVE When selecting an electrode for stimulating the facial nerve, one has to choose between bipolar and monoPolar. The stimulator can use either constant current or constant voltage. Numerous papers in the literature ex tol the virtues of all these parameters. ' - -"-"' To a great extent, whether bipolar or monopolar stimulation 's used, constant current or constant voltage is a matter of preference and experience. 4 6
8
11
271
Our preference is the monopolar stimulator manu factured by XOMED. This is an insulated (flush-tip) monopolar cathode stimulator. Bipolar stimulators require precise placement of the bipolar tip and appropriate stimulator tip orientation in order to elicit a specific response. The flush tip of our stimulator allows accurate placement on the desired tissue. We have not experienced false positives in stimu lating. The stimulator must be directly on the FN to elicit a response. Stimulating the vestibular or coch lear nerves has not resulted in muscle contractions (false positives). There is some disagreement as to the level of current that is optimal for facial nerve stimulation. Our expe rience has taught us flexibility in this regard. When stimulating the facial nerve directly, with no more than epineurium between the probe and the fascicles, we begin stimulating at 0.05 milliamps (mA). In a healthy, intact nerve, this stimulus level produces a robust mus cle response. This is the level used in identifying the facial nerve during a vestibular nerve section. The goal here is to positively locate the facial nerve before sec tioning the vestibular nerve, and 0.05 mA is usually adequate. Current shunting and false-positive responses have not occurred. When FNM is used during CP angle tumor removal (especially acoustic tumor surgery), the stimulus level varies. When uncertainty exists as to where the facial nerve resides relative to the tumor, current levels of 0.15 to 0.20 mA are used to stimulate the tissue in question. We choose to stay at a relatively low level of current to increase specificity. Use of a higher level of current allows one to identify the facial nerve at a greater distance, but one loses selectivity. During tumor removal, frequent electrical stimula tion and close communication between the surgeon and the individual controlling the monitor is necessary to accurately map the course of the FN.
USING THE NIM-2 The NIM-2 has eight DIP switches, which are user controlled. The following are the settings the authors use routinely. Switch 1 is set to constant current. Switch 2 is set to a stimuli rate of 4 per second. Switch 3 is set to a stimulus duration of 100 microseconds. Switch 4 is set to a maximum stimulus output of 1.0 mA. Switch 5 is set at 1.5 millisecond. Switches 6 and 8 are auditory alarms we do not use. Switch 7 is left in the O N position to yield audible EMG. For a more de tailed description of the function of these switches, the reader is referred to the operations manual of the NIM-2.
Downloaded from oto.sagepub.com at SAGE Publications on June 21, 2016
272
OtolaryngologyH e a d a n d N e c k Surgery
BECK and BENECKE, JR.
Other settings we recommend are a time scale of 15 seconds, event capture turned off, and the vertical dis play set at 500 microvolts full-screen. The event thresh old alarm is set in the O F F position. After the patient is under anesthesia, the electrodes are placed and the NIM-2 is set as indicated earlier. It is appropriate to mention that FNM requires the co operation of the anesthesiologist. The use of muscle relaxants after intubation adversely effects the EMG response. We therefore recommend an anesthetic tech nique that does not require the use of muscle relaxants to maintain the proper level of anesthesia. After the patient is connected to the FN monitor, the integrity of the system is assessed preoperatively with the "tap test". One gently taps over the muscle where the bipolar electrode has been placed. An audible "pop" should occur, indicating that the electrode is in the muscle and that the acoustic feedback system is work ing. As an electrical check, the NIM-2 has its own impedance verification system. Typically, impedance values range between 15 and 30 kiloiî, with an im balance of less than 12 kiloO. The ongoing baseline EMG activity should be less than 25 microvolts.
SUMMARY FNM is a technique designed to assist the surgeon in identifying and preserving the facial nerve. The ul timate goal is to permit the patient to enjoy a better facial nerve outcome. We use this technique on all neurotologic and skullbase surgeries. FNM also has great potential in the surgical management of congenital ear disease, in which the location of the facial nerve is generally in question. Complicated revision chronic ear surgery also lends itself to FNM. When the anatomy is confusing, the ability to monitor inadvertant FN ma nipulation and the ability to intentionally stimulate the FN provides an added margin of safety.
REFERENCES 1. Metson R, Thronton A, Nadol JB, Fee WE. A new design for intraoperative facial nerve monitoring. OTOLARYNGOL HEAD NECK SURG 1988;98:258-61.
2. Prass RL, Kinney SE, Hardy RW, Hahn JF. Acoustic (loud speaker) facial EMG monitoring: Part II. Use of evoked EMG activity during acoustic neuroma resection. OTOLARYNGOL HEAD NECK SURG 1987;97:541-51.
3. Prass RL, Luders Η. Acoustic (loudspeaker) facial electromy ographic monitoring: Part I. Neurosurgery 1986;19:392-400. 4. Gantz BJ. Intraoperative facial nerve monitoring. Am J Otol (Supplement) 1985;58-61. 5. Williams JD, Lehman R. Bells against palsy. Am J Otol 1988;9:81-2. 6. Chadwick GM, Calder ΗΒ, Brackmann DE, Benecke JE. Intra operative facial nerve monitoring during acoustic neuroma sur gery. Presented at Evoked Potential Monitoring-State of the Art. Lake Buena Vista, Fla., September 1986. 7. Hamer SG, Daube JR, Ebersold MJ, Beatty CW. Improved preservation of facial nerve function with use of electrical mon itoring during removal of acoustic neuromas. Mayo Clin Proc 1987;62:92-102. 8. Moller AR, Janetta PJ. Monitoring of facial nerve function dur ing removal of acoustic tumors. Am J Otol (Supplement) 1985:27-9. 9. Moller AR, Janetta PJ. Preservation of facial nerve function during removal of acoustic neuromas. J Neurosurg 1984;61:75760. 10. Benecke JE, Calder HB, Chadwick GC. Facial nerve monitoring during acoustic neuroma surgery. Laryngoscope 1987;97:697700. 11. Prass RL. Acoustic EMG monitoring: a prototype of intraoper ative cranial nerve monitoring. Presented at the American Acad emy of Neurology, New York: April 1987. 12. Kartush JM, Niparko IK, Graham MD, Bledsoe SC, Kemink JL. Intraoperative facial nerve monitoring: a comparison of stim ulating electrodes. Laryngoscope 1985;95:1536-40. 13. Babun RW, Ryu JH, McCabe BF. Bipolar localization of the facial nerve in the internal auditory canal. In: WF House, ed. Disorders of the facial nerve. New York: Raven Press, 1982.
Downloaded from oto.sagepub.com at SAGE Publications on June 21, 2016
Los Angeles, California
I n t r a o p e r a t i v e f a c i a l n e r v e monitoring (FNM) Is w i d e l y u s e d as a m e t h o d of assessing the a n a t o m i c a n d p h y s i o l o g i c integrity of t h e f a c i a l n e r v e during p r o c e d u r e s in w h i c h t h e n e r v e m a y b e in j e o p a r d y . T h e d o c t o r s of t h e O t o l o g i c M e d i c a l G r o u p u s e intra o p e r a t i v e F N M during all n e u r o t o l o g l c p r o c e d u r e s . This t e c h n i q u e c a n a l s o b e u s e d for c o n g e n i t a l e a r s u r g e r y a n d c o m p l i c a t e d revision c h r o n i c e a r s u r g e r y . O n t h e basis of e x p e r i e n c e g a i n e d f r o m m o r e t h a n 200 c a s e s in w h i c h F N M has b e e n u s e d , definitive c o n c l u s i o n s c a n b e m a d e In r e g a r d to t e c h n i c a l a s p e c t s . This article will discuss in d i c a t i o n s for I n t r a o p e r a t i v e FNM, e l e c t r o d e d e s i g n , visual a n d a u d i t o r y f e e d b a c k , a n d the particulars a b o u t stimulators. (OTOLARYNGOL HEAD NECK SURG 1 9 9 0 ; 1 0 2 . 2 7 0 . )
Modern
allowed
microsurgical techniques have the neurotologist and neurosurgeon to perform surgery of the cerebellopontine (CP) angle and skullbase safely and with certain priority goals. The preservation of useful life and (when feasible) total tumor removal are the most important goals. Preservation of cranial nerve has become a nearly equally important goal. The facial nerve is at risk in virtually all skullbase and CP angle surgeries. Monitoring the integrity of the fa cial nerve during surgery appears to be a logical means by which the nerve might be preserved in a greater number of cases.
function
Over a period of 12 months we have used and crit ically evaluated a device specifically designed to mon itor FN function intraoperatively. This device is the Nerve Integrity Monitor 2 (NIM-2), manufactured by XOMED (XOMED Inc., Jacksonville, Fla.). The basis for this device and all other FN monitors is assurance that when a physiologically intact facial nerve is stim ulated, the muscles that are innervated by this nerve 1
will contract. Stimulation can be the result of inadver tent surgical manipulation of the nerve, or the inten tional use of a stimulating electrode. The response to this stimulation is the basis for FNM. The technique used must exhibit both sensitivity and specificity.
WHY MONITOR THE FACIAL NERVE? There are several reasons to perform FNM. It im mediately alerts the surgeon when something poten tially harmful is being done to the nerve. FNM also allows for positive identification of the facial nerve when regional anatomy is confusing. After removal of a CP angle tumor, the integrity of the facial nerve can be assessed by electrical stimulation of the nerve. Stimulating along the course of the facial nerve may help identify an area of injury to the nerve. This may have an impact on the surgical procedure. 23
4
MONITORING THE MUSCLE RESPONSE There have been numerous articles in the literature regarding the various techniques of monitoring the mus cle response to facial nerve stimulation. " A detailed description of the different methods is beyond the scope of this article. The method we currently use is a mod ification of previously described techniques. Basically, one wishes to monitor an evoked electromyographic (EMG) response that can be visualized on a screen and can also be played over a loudspeaker to provide im mediate auditory feedback to FN stimulation. Such feedback provides the surgeon with a warning that the facial nerve may be in jeopardy. The EMG is obtained by use of multiple bipolar recording electrodes that are 4
From St. Louis University Medical Center, (Dr. Beck), and the Oto logic Medical Group, Inc. (Dr. Benecke). Supported by funds from the House Ear Institute, an affiliate of the University of Southern California School of Medicine. Presented at the Annual Meeting of the American Academy of Otolaryngology-Head and Neck Surgery, Washington, D C . , Sept. 25-29, 1988. Submitted for publication April 14, 1989; accepted April 27, 1989. Reprint requests: James E. Benecke, Jr., MD, St. Louis University Medical Center, 3635 Vista Ave. at Grand Blvd., St. Louis, MO 63110. 23/1/13506
270
Downloaded from oto.sagepub.com at SAGE Publications on June 21, 2016
Volume 102 Number 3 March 1990
Intraoperative facial n e r v e monitoring t e c h n i c a l a s p e c t s
placed intramuscularly. The use of intramuscular elec trodes provides response specificity. Bipolar electrodes allow for increased sensitivity. The bipolar recording electrodes we use are manu factured in our laboratory in a manner similar to that described by Prass et al. and Prass and Luders. " Teflon-insulated silver wire (part #AG5T, Medwire Corp, Mount Vernon, N.Y.) is sectioned into lengths of about 30 cm and both ends are bared of insulation for a distance of 1.5 mm. One end is inserted into a solder tip filled with 6 0 / 4 0 solder. These steps are ac complished under a microscope. An impedance meter is used to verify electrical continuity. The wider part °f the solder tip and the first 2.5 cm of wire is covered by 3/16-inch heat-shrinkable covering, which adds strain relief to the solder connection. Two strands prepared as above are held together so the solder tips dangle freely, and the bare ends are staggered to an overlap of about 3 mm. The soldered ends are twisted so a uniform twist is achieved along the entire length. The twist is strengthened by placing the pair on a hard surface and running one's finger along the length. Finally, the bare ends of the properly twisted pair are inserted through the sharp end of a 22-gauge hy podermic needle to a distance of 15 mm. The remaining length of the wire with solder tips is folded back over the needle. The protective covering of the needle is replaced, and the entire apparatus is placed in a gas sterilization bag and subsequently gas-sterilized. One pair of bipolar recording electrodes is inserted into the ipsilateral superior orbicularis oris muscle. This is accomplished by inserting the needle and wires into the muscle above the upper lip and carefully withdraw ing the needle so the wires are not pulled out. A second Pair is placed into the ipsilateral inferior aspect of the orbicularis oris muscle. The reference electrode is a standard monopolar subdermal needle electrode and is inserted in the forehead. The anodes for the stimulator are also monopolar subdermal needle electrodes and are always placed in the ipsilateral shoulder. 2
3
STIMULATING THE FACIAL NERVE When selecting an electrode for stimulating the facial nerve, one has to choose between bipolar and monoPolar. The stimulator can use either constant current or constant voltage. Numerous papers in the literature ex tol the virtues of all these parameters. ' - -"-"' To a great extent, whether bipolar or monopolar stimulation 's used, constant current or constant voltage is a matter of preference and experience. 4 6
8
11
271
Our preference is the monopolar stimulator manu factured by XOMED. This is an insulated (flush-tip) monopolar cathode stimulator. Bipolar stimulators require precise placement of the bipolar tip and appropriate stimulator tip orientation in order to elicit a specific response. The flush tip of our stimulator allows accurate placement on the desired tissue. We have not experienced false positives in stimu lating. The stimulator must be directly on the FN to elicit a response. Stimulating the vestibular or coch lear nerves has not resulted in muscle contractions (false positives). There is some disagreement as to the level of current that is optimal for facial nerve stimulation. Our expe rience has taught us flexibility in this regard. When stimulating the facial nerve directly, with no more than epineurium between the probe and the fascicles, we begin stimulating at 0.05 milliamps (mA). In a healthy, intact nerve, this stimulus level produces a robust mus cle response. This is the level used in identifying the facial nerve during a vestibular nerve section. The goal here is to positively locate the facial nerve before sec tioning the vestibular nerve, and 0.05 mA is usually adequate. Current shunting and false-positive responses have not occurred. When FNM is used during CP angle tumor removal (especially acoustic tumor surgery), the stimulus level varies. When uncertainty exists as to where the facial nerve resides relative to the tumor, current levels of 0.15 to 0.20 mA are used to stimulate the tissue in question. We choose to stay at a relatively low level of current to increase specificity. Use of a higher level of current allows one to identify the facial nerve at a greater distance, but one loses selectivity. During tumor removal, frequent electrical stimula tion and close communication between the surgeon and the individual controlling the monitor is necessary to accurately map the course of the FN.
USING THE NIM-2 The NIM-2 has eight DIP switches, which are user controlled. The following are the settings the authors use routinely. Switch 1 is set to constant current. Switch 2 is set to a stimuli rate of 4 per second. Switch 3 is set to a stimulus duration of 100 microseconds. Switch 4 is set to a maximum stimulus output of 1.0 mA. Switch 5 is set at 1.5 millisecond. Switches 6 and 8 are auditory alarms we do not use. Switch 7 is left in the O N position to yield audible EMG. For a more de tailed description of the function of these switches, the reader is referred to the operations manual of the NIM-2.
Downloaded from oto.sagepub.com at SAGE Publications on June 21, 2016
272
OtolaryngologyH e a d a n d N e c k Surgery
BECK and BENECKE, JR.
Other settings we recommend are a time scale of 15 seconds, event capture turned off, and the vertical dis play set at 500 microvolts full-screen. The event thresh old alarm is set in the O F F position. After the patient is under anesthesia, the electrodes are placed and the NIM-2 is set as indicated earlier. It is appropriate to mention that FNM requires the co operation of the anesthesiologist. The use of muscle relaxants after intubation adversely effects the EMG response. We therefore recommend an anesthetic tech nique that does not require the use of muscle relaxants to maintain the proper level of anesthesia. After the patient is connected to the FN monitor, the integrity of the system is assessed preoperatively with the "tap test". One gently taps over the muscle where the bipolar electrode has been placed. An audible "pop" should occur, indicating that the electrode is in the muscle and that the acoustic feedback system is work ing. As an electrical check, the NIM-2 has its own impedance verification system. Typically, impedance values range between 15 and 30 kiloiî, with an im balance of less than 12 kiloO. The ongoing baseline EMG activity should be less than 25 microvolts.
SUMMARY FNM is a technique designed to assist the surgeon in identifying and preserving the facial nerve. The ul timate goal is to permit the patient to enjoy a better facial nerve outcome. We use this technique on all neurotologic and skullbase surgeries. FNM also has great potential in the surgical management of congenital ear disease, in which the location of the facial nerve is generally in question. Complicated revision chronic ear surgery also lends itself to FNM. When the anatomy is confusing, the ability to monitor inadvertant FN ma nipulation and the ability to intentionally stimulate the FN provides an added margin of safety.
REFERENCES 1. Metson R, Thronton A, Nadol JB, Fee WE. A new design for intraoperative facial nerve monitoring. OTOLARYNGOL HEAD NECK SURG 1988;98:258-61.
2. Prass RL, Kinney SE, Hardy RW, Hahn JF. Acoustic (loud speaker) facial EMG monitoring: Part II. Use of evoked EMG activity during acoustic neuroma resection. OTOLARYNGOL HEAD NECK SURG 1987;97:541-51.
3. Prass RL, Luders Η. Acoustic (loudspeaker) facial electromy ographic monitoring: Part I. Neurosurgery 1986;19:392-400. 4. Gantz BJ. Intraoperative facial nerve monitoring. Am J Otol (Supplement) 1985;58-61. 5. Williams JD, Lehman R. Bells against palsy. Am J Otol 1988;9:81-2. 6. Chadwick GM, Calder ΗΒ, Brackmann DE, Benecke JE. Intra operative facial nerve monitoring during acoustic neuroma sur gery. Presented at Evoked Potential Monitoring-State of the Art. Lake Buena Vista, Fla., September 1986. 7. Hamer SG, Daube JR, Ebersold MJ, Beatty CW. Improved preservation of facial nerve function with use of electrical mon itoring during removal of acoustic neuromas. Mayo Clin Proc 1987;62:92-102. 8. Moller AR, Janetta PJ. Monitoring of facial nerve function dur ing removal of acoustic tumors. Am J Otol (Supplement) 1985:27-9. 9. Moller AR, Janetta PJ. Preservation of facial nerve function during removal of acoustic neuromas. J Neurosurg 1984;61:75760. 10. Benecke JE, Calder HB, Chadwick GC. Facial nerve monitoring during acoustic neuroma surgery. Laryngoscope 1987;97:697700. 11. Prass RL. Acoustic EMG monitoring: a prototype of intraoper ative cranial nerve monitoring. Presented at the American Acad emy of Neurology, New York: April 1987. 12. Kartush JM, Niparko IK, Graham MD, Bledsoe SC, Kemink JL. Intraoperative facial nerve monitoring: a comparison of stim ulating electrodes. Laryngoscope 1985;95:1536-40. 13. Babun RW, Ryu JH, McCabe BF. Bipolar localization of the facial nerve in the internal auditory canal. In: WF House, ed. Disorders of the facial nerve. New York: Raven Press, 1982.
Downloaded from oto.sagepub.com at SAGE Publications on June 21, 2016
