Facial nerve monitoring in acoustic tumor surgery JED A. KWARTLER, MD, WILLIAM M. LUXFORD, MD, JAMES ATKINS, MD, and CLOUGH SHELTON, MD,
Newark, New Jersey, Los Angeles, California, and San Antonio, Texas Anatomic and functional preservatlon of the facial nerve during acoustic tumor surgery remainsa primary goal. lntraoperatlve electromyographic facial nerve monitoringwith auditory feedback has enabled the surgeon to more readily achieve this goal. We compared a group of monitored translabyrinthine acoustic tumor removals (N = 89) to a similar unmonltored group (N = 155) in regard to facial nerve function. Function was assessed Immediately postoperatively, at time of discharge, and at 1 year postoperatively using the House six-point scale. Results were grouped as satisfactory, intermediute,or poorand were analyzed by tumor size. Facial nerve results were better at all time intervals In the monitored groups,although the differencewas not statistically significant at the I-year interval. There was no difference between monitored and unmonltored patients In the subgroups with tumors smaller than 2.5 cm in diameter. This study supports the usefulness of intraoperativefacial nerve monitoring in Improving HEAD NECK SURG 1991;104:814.) facial nerve results, particularly in larger tumors. (OTOLARYNGOL
Improvements in diagnosis, instrumentation, and postoperative care have reduced the morbidity associated with acoustic tumor surgery. Surgeons are no longer content to achieve total tumor removal, but have expanded the objectives of posterior fossa surgery to include preservation of cranial nerve functionparticularly the facial and cochlear nerves. Anatomic and functional preservation of the facial nerve has been facilitated by the application of intraoperative monitoring techniques. Ideally, intraoperative monitoring should allow early identification of the facial nerve and provide information on a real-time basis to the surgeon, so that irritation of the nerve can be minimized during manipulation of the tumor. The earliest application of electromyographic (EMG) facial nerve monitoring during acoustic tumor surgery was in 1979 by Delgado et al.' Sugita and Kobayashi' enhanced the use of EMG surface electrodes by applying accelerometers to the face to transduce muscle movement into electrical signals. These signals were From the Division of Otolaryngology, New Jersey Medical School (Dr. Kwartler); the House Ear Clinic, Los Angeles (Drs. Luxford and Shelton); and Wilford Hall Medical Center, San Antonio (Dr. Atkins). Presented at the Annual Meeting of the American Neurotology Society, Palm Beach, Fla., April 27, 1990. Received for publication June 8, 1990; revision received Nov. 3, 1990; accepted Dec. 26, 1990. Reprint requests: William M. Luxford, MD, House Ear Clinic, 2122 West Third St., Los Angeles, CA 90057. 2311127883
played over a loudspeaker, providing real-time auditory feedback. Numerous other authors have subsequently described refinements in the techniques of intraoperative While it seems evident that intraoperative monitoring should improve facial nerve outcome, only two groups have attempted to test this hypothesis in a controlled Harner et al.9 demonstrated the usefulness of intraoperative facial nerve monitoring in 9 1 consecutive patients undergoing acoustic tumor removal by a suboccipital approach. At 3 months postoperatively, 63% of the monitored group demonstrated some facial function, compared to 52% in an unmonitored group. At 1 year, 78% of the monitored group demonstrated some facial function, compared to 65% in an unmonitored group. The differences were statistically significant. When stratified by facial nerve grade and tumor size, there was a reduction in grade VI results at 1 year followup in tumors larger than 2.0 cm. Niparko et al." described the results of 29 patients who underwent translabyrinthine removal of an acoustic neuroma, and compared them to a similar group of 75 unmonitored patients. They believed that monitoring was significantly associated with improved facial function 1 year postoperatively in patients with tumors larger than 2.0 cm. An improvement in outcome was suggested in tumors smaller than 2.0 cm, but was not statistically significant. This article reviews our results in 89 consecutive patients undergoing translabyrinthine removal of an acoustic neuroma with adjunctive facial nerve moni-
814 Downloaded from oto.sagepub.com at The University of Iowa Libraries on June 5, 2016
Volume 104 Number 6 June 1991
Facial nerve monitoring in acoustic tumor surgery 815
Table 1. Facial nerve grade (number of patients) and function (percentage of patients) at three time intervals for monitored (N = 89)” and unmonitored (N = 155)t groups ~~
~~
Immediate Facial grade
Facial function
Unmonitored
N
Satisfactory II
VI
8 (12)
3
24
13
10 (29)
2 p 5 0.05
23
8
16
( 8)
1
5 (23)
20 p
Monitored N (“A)
95
14
(22)
5
Unmonitored N (“A)
(“A)
(65)
18
5
(15) 16
43
(11)
6
Poor
62
Followup
Monitored
14
5
7
V
N
(49)
16
(19)
IV
Unmonitored N (“Yo)
(81)
20
Intermediate
(Yo.)
56 (66)
9
Ill
*
Monitored
N
(“A)
85
I
Discharge
5
10 0.05
11 n.s
Results unavailable at discharge in one patient
t Results unavailable at immediate interval in 12 patients and at discharge in one patient n s , not significant
Table 2. Facial nerve function showing number and percentage of patients at three time intervals for monitored and unmonitored groups, by tumor size Immediate Size
Facial
(cm)
function
Unmonitored
N
Discharge
Monitored
N
(“A)
Unmonitored
(“Yo)
N
Followup
Monitored
N
(“A)
~~
1.O
Satisfactory Intermediate Poor
14 3 0
(82.4) (17.6) ( 0.0)
(73 7) (105) (158)
Satisfactory Intermediate Poor
55 8 4
(82.1) (11.9) ( 6.0)
39 7 3
(79.6) (14.3) ( 6.1)
45 17 8
n.s. 2.5
Satisfactory Intermediate Poor
25 16 18
(“Yo)
N
(“4
13 0 2
(86.7) ( 0.0) (13.3)
18 1 0
(947) (53)
15 0 0
(1000) ( 00) ( 00)
63 5 3
(88 7) ( 70) (42)
44 8
1
( 81 6) ( 163) ( 20)
18 5 2
( 720) ( 200) ( 8 0)
ns
n.s. 1 1 - 2.5
(42.4) 19 (27.1) 3 3 (30.5) p 5 0.02
(643) (24 3) (11 4)
29 8 11
(00) ns
ns (76.0) (12.0) (12.0)
Monitored
N ~
14 2 3
14 0 1
Unmonitored
(“Yo)
17 15 33
(262) 15 (23 0) 3 (50 8) 7 p5001
ns (60.0) (12.0) (28.0)
37 15 13
(569) (23 1 ) (20 0)
ns
n.s., not significant.
toring, compared to a control group of 155 unmonitored patients. METHODS AND MATERIALS
A retrospective chart review of all acoustic tumor surgery performed at the House Ear Clinic between January 1, 1986, and December 31, 1987, was performed. Patients were included in this study if they had undergone a translabyrinthine removal of an acoustic neuroma and had more than 1 year followup, either by
office examination or questionnaire. Patients were excluded from consideration if the tumor was recurrent or the facial nerve was intentionally cut as a result of gross tumor involvement. Two hundred and forty-four (244) patients met these criteria. The patients were divided into two groups on the basis of monitoring status (unmonitored N = 155; monitored N = 89), and further stratified on the basis of tumor diameter (less than 1.O cm; I . 1 cm to 2.5 cm; greater than 2.5 cm). Tumor diameter was determined
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OtolaryngologyHead and Neck Surgery
816 KWARTLER et a1
either by direct radiographic measurement (N = 65) or estimation at surgery (N = 179). The groups were similar in respect to age, sex, and tumor size. For the monitored group, the ages ranged from 19 to 79 years (mean, 53.2 years; SD = 14 years), there were 51 women (57%) and 38 men (43%), and the tumor size ranged from 0.7 to 6.5 cm (mean, 2.2 cm; SD = 1.2 cm). For the unmonitored group, the ages ranged from 18 to 77 years (mean, 50.7 years; SD = 14.3 years). There were 83 women (54%) and 72 men (46%) in this group, and tumor size ranged from 0.5 to 5.0 cm (mean, 2.4 cm; SD = 1.1 cm). EMG facial nerve monitoring was carried out in the operating room using the Nerve Integrity Monitor (NIM-2) (Xomed-Treace, Jacksonville, Fla.). Our technique for monitoring involves insertion of Teflon-coated silver bipolar hookwire electrodes into the superior and inferior orbicularis oris muscle, a monopolar ground electrode into the forehead, and a monopolar electrode into the ipsilateral shoulder to serve as the anode for the facial nerve stimulator probe. A flexible flush-tip monopolar stimulator probe is used to specifically identify and map the facial nerve using a constant-current stimulus, which varies between 0.05 and 3.0 mA. The monitor screen is calibrated to detect EMG activity up to 500 mV. EMG activity is continuously monitored both visually and acoustically throughout the surgery. Facial nerve function was graded using the House six-point scale immediately postoperatively, at the time of discharge, and 1 year postoperatively. A facial nerve grade was unavailable immediately postoperatively in 12 unmonitored patients, and at the time of discharge in one unmonitored and one monitored patient. Statistical analysis using the chi-square test was performed to determine whether intraoperative facial nerve monitoring was significantly associated with a better functional outcome. For analysis, facial nerve function was classified as satisfactory (grades I or II), intermediate (grades 111 or IV), or poor (grades V or VI). RESULTS
Total tumor removal was achieved in all the patients in this series. Overall functional facial nerve results are summarized in Table 1. At all time intervals (immediately postoperatively, at time of discharge, and at 1year followup), intraoperative monitoring resulted in a larger proportion of patients obtaining satisfactory facial nerve function. The relation between monitoring and functional outcome was statistically significant for the first two intervals. The monitored group demonstrated satisfactory facial function in 8 1% immediately
postoperatively, in 65% at discharge, and in 82% at 1year followup. The unmonitored group demonstrated satisfactory facial function in 66%, 49%, and 76%, during these same periods, respectively. Conversely, a poor result was obtained in only 3 of 89 monitored patients (3%) at 1-year followup, compared to 16 of 155 unmonitored patients (10%). When facial nerve outcome was analyzed on the basis of tumor diameter (Table 2), there were no significant differences noted for tumors smaller than 1.0 cm and between 1.1 cm and 2.5 cm. For tumors larger than 2.5 cm, the difference between monitored and unmonitored groups was statistically significant at the immediate and time of discharge intervals. Lack of statistical significance at 1 year may result in part from the small number of patients in some of the categories. DISCUSSION Intraoperative EMG facial nerve monitoring has contributed to a decrease in morbidity caused by traumatic injury to the facial nerve during acoustic tumor surgery. It has been helpful in the early identification of the nerve and recognition of imtation caused by direct trauma and traction. Differentiation of the facial nerve from surrounding soft tissue, tumor, and other cranial nerves has been achieved through the use of stimulator probes. Electrical integrity of the nerve can also be demonstrated at the end of the procedure. A variety of techniques have been advocated for intraoperative facial nerve monitoring. Moller and Jannetta3.4believed that a constant voltage stimulus was necessary to avoid the problem of current shunting caused by cerebrospinal fluid encountered when a constant current stimulus was used. They also suggested that EMG electrodes should be placed in two separate muscle groups innervated by different branches of the facial nerve. Prass and Luders’* attempted to overcome the objection to constant-current stimulation by developing the flush-tip insulated stimulator probe. The effectiveness of a flush-tip probe in limiting current shuntingrelated inaccuracies has been supported by others. Monopolar stimulation has been shown to be more useful than bipolar stimulation in “mapping” the facial nerve and differentiating it from surrounding soft tissue because a bipolar stimulator requires knowledge of the orientation of the nerve.3 Bipolar stimulation is of greater value in distinguishing the facial nerve from other cranial nerves because of the tendency for “stimulus jump” with monopolar stimulation. I ‘ Real-time feedback of EMG information, which allows the surgeon to immediately identify sources of
Downloaded from oto.sagepub.com at The University of Iowa Libraries on June 5, 2016
Volume 104 Number 6 June 1991
Facial nerve monitoring in acoustic tumor surgery 817
facial nerve trauma, has been facilitated by the linking of the EMG output to a loud~peaker.~.’ Several patterns of activity have been described, including bursts or pulses, which represent brief nonrepetitive grouped discharges and are probably not associated directly with facial nerve injury, and trains, which represent repetitive grouped discharges commonly seen in association with lateral-to-medial nerve traction. Our results are consistent with earlier reports on the usefulness of intraoperative EMG facial nerve monitoring. Because of the operative experience of the House Ear Clinic group, good facial nerve results were obtained in small and medium tumors-even without monitoring-and differences in facial function between monitored and unmonitored patients were not significant. Clinically, however, we believe that in many of these cases, monitoring did make tumor removal with facial nerve preservation easier. For larger tumors, the results of monitoring are clearer. Despite the lack of a statistically significant improvement in facial nerve function at 1 year followup for this group, we consider the trend toward improved results to be well established and that this trend will reach significance as more patients are added to this series. We consider facial nerve monitoring an indispensable tool for all our posterior fossa procedures. We wish to acknowledge the assistance of Karen I. Berliner, PhD, House Ear Institute, in the statistical analysis of the data and for review of the manuscript, and Allen Senne, MA, CCC/A, House Ear Clinic, for providing technical data on the monitoring system.
REFERENCES 1. Delgado TE, Buchheit WA, Rosenholtz HR, et al. Intraoperative
2.
3.
4.
5.
6.
7.
8.
9. 10.
11.
12.
monitoring of facial muscle evoked responses obtained by intracranial stimulation of the facial nerve: a more accurate technique for facial nerve dissection. Neurosurgery 1979;4:418-21. Sugita K, Kobayashi S. Technical and instrumental improvements in the surgical treatment of acoustic neurinomas. J Neurosurg 1982;57:747-52. Moller AG, Janetta PJ. Preservation of facial function during removal of acoustic neuromas: use of monopolar constant voltage stimulation. J Neurosurg 1984;61:757-60. Moller AR, Jannetta PJ. Monitoring of facial nerve function during removal of acoustic tumor. Am J Otol 1985;(Suppl): 27-9. Kartush JM, Niparko JK, Bledsoe SC, et al. Intraoperative facial nerve monitoring: a comparison of stimulating electrodes. Laryngoscope 1985;95: 1536-40. Kinney SE, Prass R. Facial nerve dissection by use of acoustic HEAD (loudspeaker) facial EMG monitoring. OTOLARYNGOL NECKSmc 1986;95:458-63. Prass RL, Kinney SE, Hardy RW, et al. Acoustic (loudspeaker) facial EMG monitoring: 11. Use of evoked EMG activity during acoustic neuroma resection. OTOLARYNGOL HEADNECKSURG 1987;97:541-5 1. Benecke JE, Calder HB, Chadwick G . Facial nerve monitoring during acoustic neuroma removal. Laryngoscope 1987;97:687700. Hamer SG, Daube JR, Beatty CW, et al. Intraoperative monitoring of the facial nerve. Laryngoscope 1988:98:209-12. Silverstein H, Smouha EE, Jones R. Routine intraoperative monitoring during otologic surgery. Am J Otol 1988;9:269-75. Niparko JK, Kileny PR, Kemink JL, et al. Neurophysiologic intraoperative monitoring: 11. Facial nerve function. Am J Otol 1989;10:55-61. Rass RL, Luders H. Constant current versus constant voltage stimulation. J Neurosurg 1985;62:622-3.
Downloaded from oto.sagepub.com at The University of Iowa Libraries on June 5, 2016
Newark, New Jersey, Los Angeles, California, and San Antonio, Texas Anatomic and functional preservatlon of the facial nerve during acoustic tumor surgery remainsa primary goal. lntraoperatlve electromyographic facial nerve monitoringwith auditory feedback has enabled the surgeon to more readily achieve this goal. We compared a group of monitored translabyrinthine acoustic tumor removals (N = 89) to a similar unmonltored group (N = 155) in regard to facial nerve function. Function was assessed Immediately postoperatively, at time of discharge, and at 1 year postoperatively using the House six-point scale. Results were grouped as satisfactory, intermediute,or poorand were analyzed by tumor size. Facial nerve results were better at all time intervals In the monitored groups,although the differencewas not statistically significant at the I-year interval. There was no difference between monitored and unmonltored patients In the subgroups with tumors smaller than 2.5 cm in diameter. This study supports the usefulness of intraoperativefacial nerve monitoring in Improving HEAD NECK SURG 1991;104:814.) facial nerve results, particularly in larger tumors. (OTOLARYNGOL
Improvements in diagnosis, instrumentation, and postoperative care have reduced the morbidity associated with acoustic tumor surgery. Surgeons are no longer content to achieve total tumor removal, but have expanded the objectives of posterior fossa surgery to include preservation of cranial nerve functionparticularly the facial and cochlear nerves. Anatomic and functional preservation of the facial nerve has been facilitated by the application of intraoperative monitoring techniques. Ideally, intraoperative monitoring should allow early identification of the facial nerve and provide information on a real-time basis to the surgeon, so that irritation of the nerve can be minimized during manipulation of the tumor. The earliest application of electromyographic (EMG) facial nerve monitoring during acoustic tumor surgery was in 1979 by Delgado et al.' Sugita and Kobayashi' enhanced the use of EMG surface electrodes by applying accelerometers to the face to transduce muscle movement into electrical signals. These signals were From the Division of Otolaryngology, New Jersey Medical School (Dr. Kwartler); the House Ear Clinic, Los Angeles (Drs. Luxford and Shelton); and Wilford Hall Medical Center, San Antonio (Dr. Atkins). Presented at the Annual Meeting of the American Neurotology Society, Palm Beach, Fla., April 27, 1990. Received for publication June 8, 1990; revision received Nov. 3, 1990; accepted Dec. 26, 1990. Reprint requests: William M. Luxford, MD, House Ear Clinic, 2122 West Third St., Los Angeles, CA 90057. 2311127883
played over a loudspeaker, providing real-time auditory feedback. Numerous other authors have subsequently described refinements in the techniques of intraoperative While it seems evident that intraoperative monitoring should improve facial nerve outcome, only two groups have attempted to test this hypothesis in a controlled Harner et al.9 demonstrated the usefulness of intraoperative facial nerve monitoring in 9 1 consecutive patients undergoing acoustic tumor removal by a suboccipital approach. At 3 months postoperatively, 63% of the monitored group demonstrated some facial function, compared to 52% in an unmonitored group. At 1 year, 78% of the monitored group demonstrated some facial function, compared to 65% in an unmonitored group. The differences were statistically significant. When stratified by facial nerve grade and tumor size, there was a reduction in grade VI results at 1 year followup in tumors larger than 2.0 cm. Niparko et al." described the results of 29 patients who underwent translabyrinthine removal of an acoustic neuroma, and compared them to a similar group of 75 unmonitored patients. They believed that monitoring was significantly associated with improved facial function 1 year postoperatively in patients with tumors larger than 2.0 cm. An improvement in outcome was suggested in tumors smaller than 2.0 cm, but was not statistically significant. This article reviews our results in 89 consecutive patients undergoing translabyrinthine removal of an acoustic neuroma with adjunctive facial nerve moni-
814 Downloaded from oto.sagepub.com at The University of Iowa Libraries on June 5, 2016
Volume 104 Number 6 June 1991
Facial nerve monitoring in acoustic tumor surgery 815
Table 1. Facial nerve grade (number of patients) and function (percentage of patients) at three time intervals for monitored (N = 89)” and unmonitored (N = 155)t groups ~~
~~
Immediate Facial grade
Facial function
Unmonitored
N
Satisfactory II
VI
8 (12)
3
24
13
10 (29)
2 p 5 0.05
23
8
16
( 8)
1
5 (23)
20 p
Monitored N (“A)
95
14
(22)
5
Unmonitored N (“A)
(“A)
(65)
18
5
(15) 16
43
(11)
6
Poor
62
Followup
Monitored
14
5
7
V
N
(49)
16
(19)
IV
Unmonitored N (“Yo)
(81)
20
Intermediate
(Yo.)
56 (66)
9
Ill
*
Monitored
N
(“A)
85
I
Discharge
5
10 0.05
11 n.s
Results unavailable at discharge in one patient
t Results unavailable at immediate interval in 12 patients and at discharge in one patient n s , not significant
Table 2. Facial nerve function showing number and percentage of patients at three time intervals for monitored and unmonitored groups, by tumor size Immediate Size
Facial
(cm)
function
Unmonitored
N
Discharge
Monitored
N
(“A)
Unmonitored
(“Yo)
N
Followup
Monitored
N
(“A)
~~
1.O
Satisfactory Intermediate Poor
14 3 0
(82.4) (17.6) ( 0.0)
(73 7) (105) (158)
Satisfactory Intermediate Poor
55 8 4
(82.1) (11.9) ( 6.0)
39 7 3
(79.6) (14.3) ( 6.1)
45 17 8
n.s. 2.5
Satisfactory Intermediate Poor
25 16 18
(“Yo)
N
(“4
13 0 2
(86.7) ( 0.0) (13.3)
18 1 0
(947) (53)
15 0 0
(1000) ( 00) ( 00)
63 5 3
(88 7) ( 70) (42)
44 8
1
( 81 6) ( 163) ( 20)
18 5 2
( 720) ( 200) ( 8 0)
ns
n.s. 1 1 - 2.5
(42.4) 19 (27.1) 3 3 (30.5) p 5 0.02
(643) (24 3) (11 4)
29 8 11
(00) ns
ns (76.0) (12.0) (12.0)
Monitored
N ~
14 2 3
14 0 1
Unmonitored
(“Yo)
17 15 33
(262) 15 (23 0) 3 (50 8) 7 p5001
ns (60.0) (12.0) (28.0)
37 15 13
(569) (23 1 ) (20 0)
ns
n.s., not significant.
toring, compared to a control group of 155 unmonitored patients. METHODS AND MATERIALS
A retrospective chart review of all acoustic tumor surgery performed at the House Ear Clinic between January 1, 1986, and December 31, 1987, was performed. Patients were included in this study if they had undergone a translabyrinthine removal of an acoustic neuroma and had more than 1 year followup, either by
office examination or questionnaire. Patients were excluded from consideration if the tumor was recurrent or the facial nerve was intentionally cut as a result of gross tumor involvement. Two hundred and forty-four (244) patients met these criteria. The patients were divided into two groups on the basis of monitoring status (unmonitored N = 155; monitored N = 89), and further stratified on the basis of tumor diameter (less than 1.O cm; I . 1 cm to 2.5 cm; greater than 2.5 cm). Tumor diameter was determined
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OtolaryngologyHead and Neck Surgery
816 KWARTLER et a1
either by direct radiographic measurement (N = 65) or estimation at surgery (N = 179). The groups were similar in respect to age, sex, and tumor size. For the monitored group, the ages ranged from 19 to 79 years (mean, 53.2 years; SD = 14 years), there were 51 women (57%) and 38 men (43%), and the tumor size ranged from 0.7 to 6.5 cm (mean, 2.2 cm; SD = 1.2 cm). For the unmonitored group, the ages ranged from 18 to 77 years (mean, 50.7 years; SD = 14.3 years). There were 83 women (54%) and 72 men (46%) in this group, and tumor size ranged from 0.5 to 5.0 cm (mean, 2.4 cm; SD = 1.1 cm). EMG facial nerve monitoring was carried out in the operating room using the Nerve Integrity Monitor (NIM-2) (Xomed-Treace, Jacksonville, Fla.). Our technique for monitoring involves insertion of Teflon-coated silver bipolar hookwire electrodes into the superior and inferior orbicularis oris muscle, a monopolar ground electrode into the forehead, and a monopolar electrode into the ipsilateral shoulder to serve as the anode for the facial nerve stimulator probe. A flexible flush-tip monopolar stimulator probe is used to specifically identify and map the facial nerve using a constant-current stimulus, which varies between 0.05 and 3.0 mA. The monitor screen is calibrated to detect EMG activity up to 500 mV. EMG activity is continuously monitored both visually and acoustically throughout the surgery. Facial nerve function was graded using the House six-point scale immediately postoperatively, at the time of discharge, and 1 year postoperatively. A facial nerve grade was unavailable immediately postoperatively in 12 unmonitored patients, and at the time of discharge in one unmonitored and one monitored patient. Statistical analysis using the chi-square test was performed to determine whether intraoperative facial nerve monitoring was significantly associated with a better functional outcome. For analysis, facial nerve function was classified as satisfactory (grades I or II), intermediate (grades 111 or IV), or poor (grades V or VI). RESULTS
Total tumor removal was achieved in all the patients in this series. Overall functional facial nerve results are summarized in Table 1. At all time intervals (immediately postoperatively, at time of discharge, and at 1year followup), intraoperative monitoring resulted in a larger proportion of patients obtaining satisfactory facial nerve function. The relation between monitoring and functional outcome was statistically significant for the first two intervals. The monitored group demonstrated satisfactory facial function in 8 1% immediately
postoperatively, in 65% at discharge, and in 82% at 1year followup. The unmonitored group demonstrated satisfactory facial function in 66%, 49%, and 76%, during these same periods, respectively. Conversely, a poor result was obtained in only 3 of 89 monitored patients (3%) at 1-year followup, compared to 16 of 155 unmonitored patients (10%). When facial nerve outcome was analyzed on the basis of tumor diameter (Table 2), there were no significant differences noted for tumors smaller than 1.0 cm and between 1.1 cm and 2.5 cm. For tumors larger than 2.5 cm, the difference between monitored and unmonitored groups was statistically significant at the immediate and time of discharge intervals. Lack of statistical significance at 1 year may result in part from the small number of patients in some of the categories. DISCUSSION Intraoperative EMG facial nerve monitoring has contributed to a decrease in morbidity caused by traumatic injury to the facial nerve during acoustic tumor surgery. It has been helpful in the early identification of the nerve and recognition of imtation caused by direct trauma and traction. Differentiation of the facial nerve from surrounding soft tissue, tumor, and other cranial nerves has been achieved through the use of stimulator probes. Electrical integrity of the nerve can also be demonstrated at the end of the procedure. A variety of techniques have been advocated for intraoperative facial nerve monitoring. Moller and Jannetta3.4believed that a constant voltage stimulus was necessary to avoid the problem of current shunting caused by cerebrospinal fluid encountered when a constant current stimulus was used. They also suggested that EMG electrodes should be placed in two separate muscle groups innervated by different branches of the facial nerve. Prass and Luders’* attempted to overcome the objection to constant-current stimulation by developing the flush-tip insulated stimulator probe. The effectiveness of a flush-tip probe in limiting current shuntingrelated inaccuracies has been supported by others. Monopolar stimulation has been shown to be more useful than bipolar stimulation in “mapping” the facial nerve and differentiating it from surrounding soft tissue because a bipolar stimulator requires knowledge of the orientation of the nerve.3 Bipolar stimulation is of greater value in distinguishing the facial nerve from other cranial nerves because of the tendency for “stimulus jump” with monopolar stimulation. I ‘ Real-time feedback of EMG information, which allows the surgeon to immediately identify sources of
Downloaded from oto.sagepub.com at The University of Iowa Libraries on June 5, 2016
Volume 104 Number 6 June 1991
Facial nerve monitoring in acoustic tumor surgery 817
facial nerve trauma, has been facilitated by the linking of the EMG output to a loud~peaker.~.’ Several patterns of activity have been described, including bursts or pulses, which represent brief nonrepetitive grouped discharges and are probably not associated directly with facial nerve injury, and trains, which represent repetitive grouped discharges commonly seen in association with lateral-to-medial nerve traction. Our results are consistent with earlier reports on the usefulness of intraoperative EMG facial nerve monitoring. Because of the operative experience of the House Ear Clinic group, good facial nerve results were obtained in small and medium tumors-even without monitoring-and differences in facial function between monitored and unmonitored patients were not significant. Clinically, however, we believe that in many of these cases, monitoring did make tumor removal with facial nerve preservation easier. For larger tumors, the results of monitoring are clearer. Despite the lack of a statistically significant improvement in facial nerve function at 1 year followup for this group, we consider the trend toward improved results to be well established and that this trend will reach significance as more patients are added to this series. We consider facial nerve monitoring an indispensable tool for all our posterior fossa procedures. We wish to acknowledge the assistance of Karen I. Berliner, PhD, House Ear Institute, in the statistical analysis of the data and for review of the manuscript, and Allen Senne, MA, CCC/A, House Ear Clinic, for providing technical data on the monitoring system.
REFERENCES 1. Delgado TE, Buchheit WA, Rosenholtz HR, et al. Intraoperative
2.
3.
4.
5.
6.
7.
8.
9. 10.
11.
12.
monitoring of facial muscle evoked responses obtained by intracranial stimulation of the facial nerve: a more accurate technique for facial nerve dissection. Neurosurgery 1979;4:418-21. Sugita K, Kobayashi S. Technical and instrumental improvements in the surgical treatment of acoustic neurinomas. J Neurosurg 1982;57:747-52. Moller AG, Janetta PJ. Preservation of facial function during removal of acoustic neuromas: use of monopolar constant voltage stimulation. J Neurosurg 1984;61:757-60. Moller AR, Jannetta PJ. Monitoring of facial nerve function during removal of acoustic tumor. Am J Otol 1985;(Suppl): 27-9. Kartush JM, Niparko JK, Bledsoe SC, et al. Intraoperative facial nerve monitoring: a comparison of stimulating electrodes. Laryngoscope 1985;95: 1536-40. Kinney SE, Prass R. Facial nerve dissection by use of acoustic HEAD (loudspeaker) facial EMG monitoring. OTOLARYNGOL NECKSmc 1986;95:458-63. Prass RL, Kinney SE, Hardy RW, et al. Acoustic (loudspeaker) facial EMG monitoring: 11. Use of evoked EMG activity during acoustic neuroma resection. OTOLARYNGOL HEADNECKSURG 1987;97:541-5 1. Benecke JE, Calder HB, Chadwick G . Facial nerve monitoring during acoustic neuroma removal. Laryngoscope 1987;97:687700. Hamer SG, Daube JR, Beatty CW, et al. Intraoperative monitoring of the facial nerve. Laryngoscope 1988:98:209-12. Silverstein H, Smouha EE, Jones R. Routine intraoperative monitoring during otologic surgery. Am J Otol 1988;9:269-75. Niparko JK, Kileny PR, Kemink JL, et al. Neurophysiologic intraoperative monitoring: 11. Facial nerve function. Am J Otol 1989;10:55-61. Rass RL, Luders H. Constant current versus constant voltage stimulation. J Neurosurg 1985;62:622-3.
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