P J. Kirkpatrick, FR.C.S., G. Watters, M.B., Ch.B.,A. J. Strong, FR.C.S., J. R. Walliker, M.A.,

and M. J. Gleeson, ER.C.S.

Prediction of Facial Nerve Function

After Surgery for Cerebellopontine

Angle Tumors: Use of a Facial

Nerve Stimulator and Monitor

The aims of surgery for benign tumors of the cere-

bellopontine angle now include total resection with accep-

tably low mortality and morbidity, and preservation offacial nerve function. In response to these expectations,various nerve stimulators have been used to monitor facialnerve function intraoperatively,1-4 and results showingexcellent postoperative facial nerve function have beenreported.5 It is claimed that such devices enable the sur-

geon to identify the seventh nerve earlier during surgery,

assist in distinguishing the nerve from tumor and othernon-neural soft tissues, and facilitate tumor excision byallowing mapping of regions of tumor distant from thenerve.6 It has also been suggested that facial nerve stimu-lators can predict postoperative nerve function with accu-

racy.7 In this series an intraoperative nerve stimulatorand monitor have been used in 22 patients, and the re-

liability of the technique in predicting facial nerve func-tion examined.

171

Skull Base Surgery, Volume 1, Number 3, July 1991 Department of Neurosurgery, Maudsley Hospital, London, and Departments of Clinical Physicsand Engineering and of Otolaryngology, Guys Hospital, London, United Kingdom Reprint requests: Dr. Kirkpatrick, Neurosurgical Unit of theMaudsley, Guys and King's College Hospitals, DeCrespigny Park, London SE5 8AZ, United Kingdom Copyright X) 1991 by Thieme MedicalPublishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved.

SKULL BASE SURGERYNOLUME 1, NUMBER 3 JULY 1991

PATIENTS AND METHODS

Patients

The cases of 22 patients undergoing surgery fortumors in the cerebellopontine angle have been analyzedretrospectively over a period of4 years (Table 1). Two diedpostoperatively: patient 17 died 7 days after operationfollowing an apparently unrelated spontaneous, contra-lateral, supratentorial intracerebral hemorrhage. The sec-ond death (patient 18) resulted from a brainstem hemor-rhage 4 weeks after subtotal removal of an invasivemalignant ependymoma. In patient 22 the translabyrin-thine approach to a 3.5 cm acoustic neuroma had to beabandoned following an unexplained hypoxic episode thatcaused cerebellar swelling. A complete removal wasachieved at a later date using the suboccipital route. Afourth patient (2) was lost to follow-up.

The remaining 18 patients available for follow-up(Table 1) were grouped according to the size of the tumor(maximum dimension measured on preoperative com-puted tomograms) into small (up to 2.5 cm) and large(over 2.5 cm) tumors.8

Su rgery

All operations were conducted by one or both of thesurgeons (M.I.G. and A.J.S.). The surgical approach was

Case123456789

101112131415161718

19202122

Age(yr)566421316167555150162255553229326842

44464654

Table 1.Date of

Operation04.12.8622.01.8714.01.8828.01.8809.06.8818.08.8810.11.8812.01.8911.02.8909.03.8907.04.8920.04.8921.04.8924.08.8909.10.8916.11.8918.01.9001.03.90

29.03.9010.05.9017.05.9005.07.90

dictated by the clinical and radiologic criteria, principallythe size of the tumor, the width of the petrous angle, theheight of the jugular bulb in relation to the internal audi-tory meatus, pneumatization of the petrous bone, and thepresence or absence of middle ear sepsis. Thirteen of the18 patients underwent a combined otological (M.J.G.) andneurosurgical (A.J.S.) translabyrinthine approach. Theremaining five patients were submitted to a suboccipitalresection (A.J.S.).

Facial Nerve Stimulator and Monitor

All operations were carried out using a speciallydeveloped monitoring system that consists of the follow-ing major components:

1. An electrical stimulator delivering 1 msec widecapacitor-coupled constant current pulses at arepetition rate of 35 Hz. Output current is vari-able up to 5 mA peak, although the most com-monly used values were in the range 0.5 to 1.0mA. The stimulator was electrically isolatedfrom all other circuits to minimize artifact gener-ation.

2. A bipolar stimulating electrode with bayonethandle (see acknowledgments). An advantage ofthis was that the spread of stimulation currentcould be controlled by varying the separationbetween the electrodes. This permitted initialcoarse and then fine localization of the nerve.

Data on Patients Undergoing Surgery for Cerebellopontine Angle Tumors

PathologyAcousticAcousticAcousticAcousticAcousticAcousticAcousticAcousticAcousticAcousticAcousticAcousticAcousticAcousticAcoustic

EpendymomaAcousticMalignant

schwannomaAcoustic

MeningiomaAcousticAcoustic

TumorSize (cm)

3.51.73.23.53.02.01.51.52.0

>4.03.03.50.52.02.01.5

>4.0>4.0

1.52.01.53.5

SurgicalApproach

SuboccipitalTranslabyrinthineTranslabyrinthineTranslabyrinthineTranslabyrinthineTranslabyrinthineTranslabyrinthine

SuboccipitalSuboccipitalSuboccipitalSuboccipital

TranslabyrinthineTranslabyrinthineTranslabyrinthineTranslabyrinthineTranslabyrinthineTranslabyrinthine

Suboccipital

TranslabyrinthineTranslabyrinthineTranslabyrinthineTranslabyrinthine

Follow Up(months)

43N/A3130242320181016158

151

128

Died at 7 daysDied at 4 wk

Facial Nerve Function (atLatest Follow Up)

III(lost)

IIIVIIIIV

I11VIIVIVVIIV

II11

VI (at 6 days)VI (at 4 wk)

4 13 112 1

(Tumor removal later complete via thesuboccipital route)

172

FACIAL NERVE STIMULATOR-KIRKPATRICK, WATTERS, STRONG, ET AL

3. Three platinum needle electrodes of 0.25 mmdiameter and 10 mm length. One electrode wasinserted into the obicularis oris above the upperlip and acted as a reference, and a pair wereinserted into the obicularis oris below the lip todetect muscle activity. Our early experiencewith this system indicated that silver/silver chlo-ride gel surface electrodes were unreliable forthis application, partly because of poorer elec-tromyography (EMG) to interference ratio, andpartly because of a tendency to become dis-lodged or to dry out when applied to the face.(Reliable monitoring electrodes are absolutelyessential because of their inaccessibility understerile drapes close to the operating field.)

4. An electrically isolated preamplifier situatedclose to the monitoring electrodes for minimumnoise pick-up with protection against, and fastrecovery from, the effects of diathermy.

5. A filtered 10W audio amplifier and loudspeakerdesigned to maximize the audibility of facialEMG signals while minimizing that of inter-ference and noise. Auditory feedback gave awarning to the surgeon of inadvertent mechani-cal stimulation of the nerve (see later). EMGresponses to bipolar diathermy close to the nervecould be distinguished from artifacts.

6. Light-emitting diode "bar-graph" display ofmean EMG voltage using long decay time con-stants. The long decay allowed the surgeon tolook up from the microscope and check whetheror not an unexpected sound had been emittedfrom the monitor.

7. Voice and EMG tape recorder interface whichswitched the tape motor on for 1 minute afterEMG signals had exceeded a preset threshold.

The system exceeds the electrical safety requirements ofBS5724.

Surgical Use ofthe Facial Nerve Stimulator

Discharge PatternsBy means of auditory feedback from the nerve

stimulator/monitor to the surgeon, different patterns ofEMG discharge could be recognized. On direct electricalstimulation of the nerve, a sharp "burst" of evoked poten-tials was heard that lasted for the duration of the appliedstimulus and rapidly declined after removal. Elicitation ofa burst response indicated that the nerve was electricallyintact distal to the stimulation site. If the nerve was sub-mitted to excessive traction during dissection, the stimula-tor would produce a spontaneous "train" of dischargesthat would start during the period of traction and continuefor up to 60 seconds after release. Train responses were

encouraged the surgeon to redirect his approach. A thirdpattern of discharge was recognized in patient 12 wherethe nerve was transected. In this case spontaneous inter-mittent discharges produced "popping" sounds that wereheard throughout the entire procedure. This signal mayhave been an indicator of severe nerve damage. Similardischarge patterns have been noted by others and aredescribed in detail elsewhere.9

Surgical ApplicationAnesthesia was induced with thiopentone, and relax-

ation for tracheal intubation was produced with a competi-tive neuromuscular blocker. Anesthesia was maintainedwith 66% nitrous oxide with added isoflurane. The lungswere ventilated mechanically to produce a moderate fall inend-tidal carbon dioxide. By the time surgical exposure ofthe facial nerve was achieved, muscle power had returnedas assessed by a peripheral nerve stimulator. If necessary,muscle relaxation was reversed using intravenous neostig-mine and glycopyrronium. Omnopon 20 mg was also givenintravenously to reduce spontaneous respiratory effort toenable mechanical ventilation of the lungs at concentra-tions of isoflurane of about 1.5%. Continuous lumbardrainage of cerebrospinal fluid (CSF) was established.

During the translabyrinthine approach,'0 once thecortical bone overlying the middle cranial fossa and sig-moid sinus had been removed and the mastoid air systemexenterated, the nerve stimulator was used to help locatethe mastoid segment of the facial nerve, thereby aidingskeletonization of this structure. The device was able toevoke facial EMG responses on stimulation of the nervethrough a thin layer of bone and allowed identification ofthe nerve before the fallopian canal could be seen. Follow-ing this, the semicircular canals were sequentially re-moved and the limits of the internal auditory meatusidentified. The dura was then opened, and the intracranialsegment of the facial nerve identified and dissected fromthe tumor using the nerve stimulator intermittently at lowcurrent strength to determine the precise course of thenerve. Where a suboccipital approach was undertaken,the nerve stimulator allowed earlier identification of theposition of the facial nerve. Dissection and removal of thetumor could then proceed confidently by directing thesurgeon's dissection to areas remote from the nerve untilthe anatomic disposition of the nerve became fully apparent.

Postoperative Follow-Up

Patients were recalled and assessed at various timesafter operation, which ranged from 3 to 43 (mean, 16.4)months (Table 1). Facial nerve function was graded clini-cally according to House"I (Table 2) by a single observer(PJ.K.). Facial nerve function could not be examinedretrospectively after a uniform postoperative interval.However, analysis of the case notes and outpatient lettersindicated that those with facial weakness did not showinterpreted as a warning of impending nerve damage and 173

SKULL BASE SURGERYNOLUME 1, NUMBER 3 JULY 1991

Table 2. House Facial Nerve Grading System*Grade Definition

I Normal Normal facial function in all areas11 Mild dysfunction Gross: slight weakness noticeable on close inspection; may have very slight syn-

kinesis. At rest: normal symmetry and tone. Motion: forehead, moderate to goodfunction; eye, complete closure with minimum effort; mouth, slight asymmetry

Ill Moderate dysfunction Gross: obvious but not disfiguring difference between the two sides; noticeable but notsevere synkinesis, contracture and/or hemifacial spasm. At rest: normal symmetryand tone. Motion: forehead, slight to moderate movement; eye, complete closurewith effort; mouth, slightly weak with maximum effort

IV Moderately severe dysfunction Gross: obvious weakness and/or disfiguring asymmetry. At rest: normal symmetry andtone. Motion: forehead, none; eye, incomplete closure; mouth, asymmetric withmaximum effort

V Severe dysfunction Gross: only barely perceptible motion. At rest: asymmetry. Motion: forehead, none;eye, incomplete closure; mouth, slight movement

VI Total paralysis No movement*Adapted from House and Brackmann."1

significant recovery beyond six months, which constitutedthe minimum time for the follow-up of these patients.Three patients with good facial function (grades I/II) im-mediately after operation developed transient mild facialweakness that recovered within a few days. There was nolate deterioration in facial function. The three most recentpatients in the series all maintained good facial function 1month after surgery and therefore were included in thestudy despite follow-up times of less than 6 months.

RESU LTS

The ages of the 18 patients who were eligible forfollow-up ranged from 16 to 67 years (mean, 42.7; me-dian, 44). Tumor pathology is listed in Table 1 (16 acousticneuromas, one petrous apex meningioma, and one myxo-papillary ependymoma). Tumor removal was consideredto be macroscopically complete in all cases. The facialnerve was identified and judged to be anatomically intactin 15 of the 18 cases (83.3%), partially divided in two(11.1%), and completely divided in 1 (5.6%).

Small Tumor Group

In 8 of the 11 patients (72.7%), intraoperative facialnerve stimulation after complete tumor resection indicatedneural conduction in an anatomically intact nerve. Subse-quent postoperative facial assessment showed complete(grade I) recovery in five, and near complete (grade II)recovery in three. The remaining three (27.3%) patients inthe small tumor group had reduced EMG responses tostimulation of the facial nerve, which was anatomicallyintact in two cases and partially divided in one. Follow-updemonstrated a corresponding grade IV weakness in thefirst two and a complete facial palsy (grade VI) in the third.The presence of mild facial weakness or facial twitchingpreoperatively did not appear to affect outcome adversely

174 (Table 3).

Large Tumor Group

Intraoperative recordings showed normal responsesto facial nerve stimulation in three of the seven (42.9%)cases, all of which had an anatomically intact nerve at theend of the procedure. Of these three cases, two developeda moderately severe (grade III) and the other a severe(grade V) facial weakness. In four of the large tumorgroup, (57.1%) the response to nerve stimulation was poor(in three) or absent (in one). The absent response in onecase and subsequent complete facial paralysis (grade VI) 8months postoperatively was predicted since complete di-vision of the nerve was suspected. The remaining threedeveloped a moderately severe facial palsy, grade III intwo cases and grade IV in one.

DISCUSSION

The importance of being able to identify the facialnerve reliably during surgery from cerebellopontine angletumors and to predict its eventual functional outcome isrecognized. Immediate intra- or postoperative recognitionthat facial nerve paralysis is irreversible would certainlyaid patient counseling, help indicate whether tarsorrhaphyshould be temporary or permanent, and aid planning ofearly nerve grafting procedures. Electroneurography hasbeen used by some but has recognized limitations in thatan absent response does not preclude eventual functionalrecovery. 12 Measurement of the response to intraoperativefacial nerve stimulation has been reported to be of predic-tive value,7 but our initial experience has shown this to betrue only for the surgery of small (up to 2.5 cm) acoustictumors. Thus, in the small tumor group we have found thatfacial nerve conductance at the end of tumor removalpredicted good facial recovery in 100%. The three patientswith small tumors who showed a poor response to facialnerve stimulation all developed moderately severe facialweakness.

In contrast, results from the large tumor group

FACIAL NERVE STIMULATOR-KIRKPATRICK, WATTERS, STRONG, ET AL

Table 3. Results of Surgery

Patient Facial Nerve at Operation* Facial Nerve FunctionNo. Identified Intact Functional Preoperative Postoperative

Tumor size 0-2.5 cm7 Y Y Y S1Weak Gradel

14 Y Y Y Normal Grade15 Y Y Y Normal Grade19 Y Y Y Normal Grade21 Y Y Y Normal Grade8 Y Y Y (Occasional twitch) Grade II

16 Y Y Y Normal Grade II20 Y Y Y Si Weak Grade II6 Y Y N Normal Grade IV

13 Y Y N Normal Grade IV9 Y Partial N Normal Grade VI

Tumor size >2.5 cm1 Y Y N Normal Grade III3 Y Y Y Normal Grade III5 Y Y Y Normal Grade III

10 Y Y N S1 Weak Grade IV11 Y Partial N Normal Grade IV4 Y Y Y Normal Grade V

12 Y N N Normal Grade VI

*Function assessed by intraoperative facial nerve stimulation (see Methods). Y: yes; N: no.

showed no reliable indication of eventual facial function.In acoustic tumor surgery, facial nerve outcome is in-versely related to the size of the tumor.3,13-18 The in-creased chance of neural disruption during lengthy, diffi-cult procedures for larger tumors is not the only reason forthis observation, since there is a poor correlation betweenstructural integrity of the nerve and its postoperative func-tion.13 It is possible that distortion of the nerve by largetumors precludes accurate intraoperative identification,and claims of anatomic integrity may not be reliable.

Demonstration of a normal EMG response to intra-operative facial nerve stimulation was expected to providean accurate method of determining facial nerve integrity.However, despite apparent identification of an anatomi-cally and electrically intact facial nerve during the re-moval ofthree of the large acoustic tumors, the outcome offacial function was still poor. It is possible that the stimula-tion intensities and frequencies used may not have beenoptimal for the detection of subtle changes in nerve con-duction that could indicate damage. The use of differentstimulation techniques may help to predict facial nerveoutcome more accurately in patients with large tumors.3Other factors are believed to influence the stimulationresponse. First, current may be shunted from the stimulat-ing electrode by surrounding soft tissue, irrigating fluid,or CSF Second, it is possible that the stimulation itselfmay in some way injure the nerve. This would be moresignificant for bigger tumors where stimulation is usedmore often.

Intraoperative nerve recording is not without disad-vantage. The technique demands that the patient is anes-thetized without full muscle relaxation and is therefore notunder strict ventilatory control. In one patient the proce-

dure was abandoned following an unexplained period ofrelative arterial desaturation that necessitated full paral-ysis and controlled ventilation because of cerebellar swell-ing. This episode may have been related to bronchialhypersensitivity that provoked coughing, or alternativelyresulted from mild anaphylaxis following blood transfu-sion. A second procedure was required 6 weeks laterduring which a large tumor was excised via the suboccipi-tal route under full neuromuscular relaxation and withoutthe use of the nerve stimulator. Chronic chest disease maytherefore constitute a relative contraindication to intra-operative nerve monitoring.

SUMMARY

Monitoring surgically and electrically evoked facialEMG responses allows the surgeon to locate and map thenerve and continually assess nerve conductance. Ouranalysis of postoperative facial nerve function indicatesthat intraoperative nerve monitoring predicts functionaloutcome after surgery for small tumors. Although thetechnique has so far been unable to determine outcome forlarge tumors, we would like to emphasize that we havecome to regard EMG monitoring of facial nerve functionas indispensable for the localization of the nerve trunk, asan indicator of inadvertent disturbance during surgery,and for verification of continued function. The modifica-tion to the anesthetic method has proved entirely satisfac-tory except in one case, and we would strongly advocatethe use of this technique for any procedure involving thefacial nerve. 175

SKULL BASE SURGERYNOLUME 1, NUMBER 3 JULY 1991

REFERENCES

1. Delgado TE, Buchheit WA, Rosenholtz HR, et al: Intraoperativemonitoring of facial muscle evoked responses obtained by intra-cranial stimulation of the facial nerve: A more accurate tech-nique for facial nerve dissection. Neurosurgery 4:418-421, 1979

2. Sugita K, Kobayashi S: Technical and instrumental improvementsin the surgical treatment of acoustic neuromas. J Neurosurg 57:747-752, 1982

3. Moller AR, Janetta PJ: Preservation of facial function duringremoval of acoustic neuromas. J Neurosurg 61:757-760, 1984

4. Zini C, Gandolfi A: Facial-nerve and vocal-cord monitoring duringotoneurosurgical operations. Arch Otolaryngol Head Neck Surg113:1291-1293, 1987

5. Benecke JE, Calder HB, Chadwick G: Facial nerve monitoringduring acoustic neuroma removal. Laryngoscope 97:697-700,1987

6. Niparko JK, Kileny PR, Kemink JL, Lee HM, Graham MD:Neurophysiologic intraoperative monitoring: II. Facial nervefunction. Am J Otol 10:55-61, 1989

7. Silverstein H, Smouha E, Jones R, Sarasota FL: Routine identi-fication of the facial nerve using electrical stimulation duringotological and neurotological surgery. Laryngoscope 98:726-730, 1989

8. Pulec J, House WF, Britton B: A system of management of acousticneuroma based on 364 cases. Trans Am Acad OphthalmolOtolaryngol 75:48-55, 1971

9. Kartush JM: Electroneurography and intraoperative monitoring incontemporary neurotology. Otolaryngol Head Neck Surg 101:496-503, 1989

10. Fisch U: Transmastoid translabyrinthine access to the internalmeatus. In: Naumann HH (ed): Head and Neck Surgery, vol 3.Stuttgart: Thieme, 1982, pp 505-543

11. House JW, Brackmann DE: Facial nerve grading system. Oto-laryngol Head Neck Surg 93:146-147, 1985

12. Croxson GR, Moffat DG, Hardy DG, Baguley DM: Role of post-operative electroneuronography in predicting facial nerve re-covery after acoustic neuroma removal: A pilot study. J Laryn-gol Otol 103:60-62, 1989

13. Tator CH, Nedzelski JM: Facial nerve preservation in patients withlarge acoustic neuromas treated by a combined middle transten-torial translabyrinthine approach. J Neurosurg 57:1-7, 1982

14. Shiobara R, Takayuki 0, Kanzaki J, Toya S: A modified extendedmiddle cranial fossa approach for acoustic nerve tumours. JNeurosurg 68:358-365, 1988

15. Symon L, Bordi LT, Compton JS, Sabin IH, Sayin E: Acousticneuroma: A review of 392 cases. Br J Neurosurg 3:343-348,1989

16. Thomsen J, Tos M, Harmsen A: Acoustic neuroma surgery: Re-sults of translabyrinthine tumour removal in 300 patients. Dis-cussion of choice of approach in relation to overall results andpossibility of hearing preservation. Br J Neurosurg 3:349-360,1989

17. Hardy DG, Macfarlane R, Baguley D, Moffat DA: Surgery foracoustic neuroma J Neurosurg 71:799-804, 1989

18. Moffat DA, Croxson GR, Baguley DM, Hardy DG: Facial nerverecovery after acoustic neuroma removal. J Laryngol Otol 103:169-172, 1989

The facial nerve monitor and stimulator was developed inthe Department of Clinical Physics and Bioengineering at Guy'sHospital, London. We are grateful to J. Baker, Clinical ResearchCentre, Northwick Park Hospital, for construction of the stimu-lating electrode, and to Dr. Ray Towey for excellent anesthesiaand description of the anesthetic method.

REVIEWER'S COMMENTS

The report by Kirkpatrick et al on the prediction of facial nerve function after surgery foracoustic neuroma nerve tumors using an EMG stimulator monitor parallels in many ways theexperiences related by surgeons in this country at a recent meeting of the American Neuro-Otologic Society. More and more surgeons are employing facial nerve monitoring for theiracoustic tumor surgery. At the Ear Research Foundation, we use not only the EMG monitors (NIMand Brackmann), but also the WR-S8 monitor stimulator which uses an ultrasensitive strain gauge.We found that the strain gauge monitor is more useful in the temporal bone for localizing andpreserving the facial nerve, for the EMG monitors seem to be somewhat overly sensitive in thisregion. We have also found that the combination of the two monitors in the cerebellopontine anglehas enabled us to distinguish better between significant EMG discharge patterns and artifact.Although the authors describe using stimulation at the end of the case to determine electricalintegrity of the nerve, they do not relate the specific levels of stimulation that was used. This mayexplain the cases in which the nerve is stimulated postoperatively but patients developed asignificant weakness. We have found that the level of the stimulation required at the brainstem toproduce facial contraction is helpful in predicting the facial nerve outcome. Specifically, when thefacial nerve stimulates at the lowest possible setting, which correlates to 0.05 mA, the patient'spostoperative facial nerve function is usually normal in the recovery room. After acousticneuroma resection, however, delayed weakness may occur even if the nerve stimulates at the lowestcurrent setting. Since the level of current necessary to generate a contraction is increased above0.3 mA, the postoperative facial nerve function tends to be worse. We have also found that theanatomic integrity alone is not a good predictor for postoperative facial nerve function. This isindeed an important area for further research and has important implications for facial nerverehabilitation after acoustic neuroma surgery.

Herbert Silverstein, M.D.176