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   Table of Contents      
ORIGINAL ARTICLE
Year : 2019  |  Volume : 5  |  Issue : 3  |  Page : 128-138

Electrophysiological Monitoring of Fifth and Seventh Cranial Nerves in Cerebellopontine Angle Lesions


1 Associate Professor, Department of Neuro surgery, G. B. Pant Hospital, New Delhi, India
2 Department of Pathology, Aryogya Dham Hospital, Gwalior, India
3 Assistant Professor, Department of Anesthesiology, G R Medical College, Gwalior, India
4 Director, Department of Neuro surgery, GIPMER, New Delhi, India
5 Professor, Department of Neuro surgery, GIPMER, New Delhi, India
6 Department of Obstetrics and Gynaecology, Maharaja Agrasen Hospital, New Delhi, India
7 Clinical Process Specialist, IQVIA Research (India) Pvt Ltd, Mumbai (Thane), India

Date of Submission12-Aug-2019
Date of Decision31-May-2019
Date of Acceptance01-Sep-2019
Date of Web Publication17-Dec-2019

Correspondence Address:
Shakti Singhal
Assistant Professor, Department of Anesthesiology, G R Medical College, Gwalior- 474009
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/mamcjms.mamcjms_58_18

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  Abstract 


Introduction: Neural preservation is particularly imperative in the surgical management of acoustic neuromas. Intraoperative neuromonitoring is essential in structural and functional preservation of these cranial nerves during the operations. Intraoperative stimulus threshold and response amplitude measurements from trigeminal and facial nerves can predict the functional outcome postoperatively. Materials and Methods: A prospective study was conducted on 30 patients over a period of one year from March 2011 to April 2012 in the Govind Ballabh Pant Hospital, New Delhi, India. Results: There were 27 (90%) cases of vestibular schwannomas, one of meningioma (3.33%), and two of epidermoids (total cases n = 30). Among these, 11 (36.66%) were male and 19 (63.33%) were female patients, and 37% of patients in our study were in the age group of 20 to 30 years. Among the vestibular schwannomas, 16 were right sided and 11 were left sided. Hearing loss was the main presenting symptom (56%) followed by tinnitus (27%), facial pain (10%), and gait instability (7%) in our study. Conclusion: Intraoperative neurophysiological monitoring showed strong support in the value of identification of facial and trigeminal nerves, prevention of injury, and value of prognosis. Stimulus thresholds, response amplitude, and proximal-to-distal amplitude ratio can assist the surgeon during tumor dissection.

Keywords: Amplitude, cerebellopontine angle tumors, neuromonitoring


How to cite this article:
Singhal G, Agrawal G, Singhal S, Shrivastav A, Jagetia A, Singhal D, Gangil J. Electrophysiological Monitoring of Fifth and Seventh Cranial Nerves in Cerebellopontine Angle Lesions. MAMC J Med Sci 2019;5:128-38

How to cite this URL:
Singhal G, Agrawal G, Singhal S, Shrivastav A, Jagetia A, Singhal D, Gangil J. Electrophysiological Monitoring of Fifth and Seventh Cranial Nerves in Cerebellopontine Angle Lesions. MAMC J Med Sci [serial online] 2019 [cited 2020 Apr 8];5:128-38. Available from: http://www.mamcjms.in/text.asp?2019/5/3/128/273284




  Introduction Top


The aims of surgery in the cerebellopontine angle (CPA) have changed from tumor resection and prolongation of life to the anatomical and functional preservation of the cranial nerves (CNs).[1],[2] This evolution in operation may be precisely illustrated by Moskowitz and Long,[3] who divided the surgical treatment of vestibular schwannomas (VSs) into four distinct phases, namely, the pioneer era (1890–1925), the curative era (1925–1960), the magnification era (1960–1974), and the recent era (from 1975). Although significant advances have been observed in the recent era, facial nerve (FN) anatomical preservation during VS surgery is currently around the range of 95% and FN functional preservation is in the low range of 70%.

Immediate postoperative facial paresis can be observed in 13% to 23.7% depending on the tumor location.[4] Management of CPA epidermoid tumors has improved during the past 30 years; however, postoperative facial weakness still occurs in 5% to 8.3% of patients. Similarly, trigeminal neurinomas remain challenging entities, and their surgical treatment may lead to postoperative facial paralysis in up to 37.5% of cases.[5] Therefore, in general, facial weakness is still a complication of major concern in patients undergoing CPA surgery.[6],[7],[8],[9] In this interim, several developments regarding radiological diagnosis, the introduction of the operative microscope and microsurgical techniques, advances in the field of neuroanesthesia, as well as intraoperative neuromonitoring were responsible for significant reductions in the morbidity and mortality in patients suffering from CPA tumors.[7],[10],[11],[12],[13] Intraoperative neuromonitoring may demonstrate improvements in structural and functional preservation of these CNs during the operations.


  Aims and Objectives Top


This article evaluates whether the intraoperative stimulus threshold and response amplitude measurements from trigeminal nerve and FN can predict the functional outcome postoperatively.


  Materials and Methods Top


Study design

Prospective study (from March 2011 to April 2012).

Setting

Department of Neurosurgery, Govind Ballabh Pant Hospital, New Delhi, India.

Study population

Thirty patients.

Intervention

  1. Fifth and seventh cranial nerve (CN) electrophysiological monitoring was performed during surgery with the idea to preserve these nerves during surgery.
  2. The minimal stimulus intensity (in milliamperes), electromyographic (EMG) response amplitude (in microvolts), and proximal-to-distal amplitude ratio was recorded during stimulation applied to these nerves during and after tumor removal.


Inclusion criteria

All patients having CPA tumors admitted in Govind Ballabh Pant Hospital, New Delhi, India.

Methodology

All patients were prepared for surgery as per protocol. All patients involved in this study had normal FN function before surgery. The tumor size was measured in magnetic resonance imaging scan and classified in one of four categories:
  1. Intracanalicular
  2. Extracanalicular < 1.5 cm
  3. Extracanalicular 1.5 to 3.0 cm
  4. Extracanalicular > 3 cm


Operating procedure

Anesthesia technique

A standard anesthesia technique was followed in all patients. Electrode placement is done in a standard way to monitor fifth and seventh CNs. Following electrode placement, muscle relaxant was stopped and anesthesia was maintained using propofol. The requirement of anesthesia was titrated with bispectral index monitoring. Prior to stimulation of CN for the EMG response, the plane of anesthesia was determined. The infusion of both propofol and atracurium was stopped 15 minutes before the proposed stimulation. The decrease in muscle relaxant was confirmed by a trained monitor.

Electrode placement

Paired monopolar needle electrodes were placed intramuscularly in perioral (orbicularis oris muscle) and periorbital (orbicularis oculi muscle) areas to monitor FN. Similarly, paired monopolar needle electrodes were placed in masseter muscle to monitor the trigeminal nerve. All these electrodes were placed on the side of tumor that was operated. A standard reference electrode was placed over the forehead and a ground electrode was placed over the vertex [Figure 1][Figure 2][Figure 3].
Figure 1 Paired monopolar needle.

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Figure 2 Bipolar direct nerve stimulating electrodes probe.

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Figure 3 Placement of needle electrodes on the left side for intraoperative electrophysiological monitoring of facial and trigeminal nerves.

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Intraoperative monitoring procedure

The instrumentation used for monitoring is Nicolet Endeavor CR (Viasys Health Care, U.S.). This instrument is equipped with appropriate EMG protocols for both spontaneous and live electromyography and for eliciting and recording compound muscle action potential. The bipolar probe was used to deliver the stimuli to the operative field [Figure 4][Figure 5][Figure 6][Figure 7].
Figure 4 Nicolet Endeavor CR intra-operative monitoring system (Front view).

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Figure 5 Nicolet Endeavor CR intraoperative neuromonitoring system.

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Figure 6 Nicolet Endeavor CR intraoperative neuromonitoring system.

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Figure 7 Nicolet Endeavor CR intra-operative monitoring system (Back side).

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Intraoperative CN monitoring of FNs and trigeminal nerves involved sampling of EMG activity from perioral, periorbital, and masseter muscles by means of paired monopolar needle electrodes placed intramuscularly. Direct electrical stimulation of the trigeminal nerve and FN was performed using bipolar probe, to assist in mapping the course of the nerves and to confirm the anatomical and functional integrity of both CNs. The electrical stimuli delivered in the operative field consisted of stimulus range between 0.1 and 2.3 mA [Figure 8] and [Figure 9].
Figure 8 Intra-operative stimulation with bipolar probe over the tumor capsule to identify stretched nerve fibers over the capsule and tumor dissection continued (Over view).

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Figure 9 Intra-operative stimulation with bipolar probe over the tumor capsule to identify stretched nerve fibers over the capsule and tumor dissection continued (Focused view).

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The stimulation threshold was elicited by first applying a stimulus of 0.05 mA. If no response was obtained, the stimulus intensity was increased in 0.05 mA increments until a response amplitude was obtained. Once the lowest stimulation threshold was identified and recorded, the EMG response amplitude was recorded at the stimulation threshold. When measuring the response amplitude, care must be taken to ensure that the stimulus probe only contacts the FN or trigeminal nerve and that the cerebrospinal fluid around the nerve has been suctioned.

There can be some variation in the response amplitude based on where and how good the contact is between the probe and the nerve. So, we stimulated the nerve three times, but we only recorded the greatest response amplitude of the three for use in the present study. The channel with the greatest response amplitude was recorded for the present study. At the end of the procedure, after tumor resection, a threshold is established at the proximal site of stimulation. The trigeminal nerve and FN were stimulated at the threshold at a point distal to the location of the tumor and proximally as close to the root entry zone as possible. The peak-to-peak amplitude of the proximal and distal compound muscle action potential at thresholds are established. If the proximal and distal amplitude were identical, the resulting ratio was 1 or 100%. If the proximal amplitude was diminished because of FN injury associated with tumor dissection, the difference was expressed as a percentage. All patients received an intraoperative dose of steroids [Figures 10][Figures 11][Figures 12].
Figure 10 Bipolar probe stimulation over the facial nerve and the same at the end of tumor excision to confirm the anatomical and functional integrity of the nerve (Focused view).

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Figure 11 Bipolar probe stimulation over the facial nerve and the same at the end of tumor excision to confirm the anatomical and functional integrity of the nerve (Over view).

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Figure 12 Electromyographic response due to contraction of orbicularis oris after direct electrical stimulation of facial nerve during cerebellopontine angle surgery.

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Main outcome measures

Trigeminal nerve and FN function [House-Brackmann (HB) grading scale] were assessed clinically on the following days postoperatively:
  1. Immediate postoperative day (0 days).
  2. First postoperative day.
  3. Seventh postoperative day.
  4. 1 month after surgery.
  5. 3 months after surgery.
  6. 6 months after surgery.


Statistical Analysis

The monitoring parameters were expressed as mean with standard error of the mean. Comparison of intensity thresholds, response amplitude, and proximal-to-distal amplitude ratios between postoperative facial function grades at different postoperative days was performed. For statistical analysis of these parameters, Pearson’s formula, multivariate T test (two-tailed test), and (SPSS software, IBM Corporation, U.S.) were used.


  Results Top


This prospective study of role of intraoperative trigeminal nerve and FN monitoring in surgery of CPA mass lesions included 30 patients (n = 30) admitted in the Neurosurgery Ward of Govind Ballabh Pant Hospital, New Delhi, between March 2011 and April 2012. Out of 30 patients, 27 (90%) were of VSs, one was of meningioma (3.33%), and another two (6.66%) were of epidermoid; 11 (36.66%) were male and 19 (63.33%) were female patients. Eleven (37%) were in age group 20 to 30 years, seven (23%) in 31 to 40 years, seven (23%) in 41 to 50 years, four (14%) in 51 to 60 years, and one (3%) more than 60 years. Nineteen (63.33%) tumors were located on the right side and 11 (36.66%) on the left side.

Presenting symptom and size of tumor

Hearing loss was the main presenting symptom in 17 (56%) patients, tinnitus in eight (27%), facial pain in three (10%), and gait instability in two (7%) patients. For schwannomas, 21 (78%) were in category III, four (15%) in category II, two (7%) in category IV, and none (0%) in category I [Table 1].
Table 1 Size of the tumor (schwannoma)

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Distribution of HB grade at 0 postoperative day

In the immediate postoperative day, nine (30%) patients had HB grade I, 10 (33%) had HB grade II, five (17%) had HB grade III, three (10%) had HB grade IV, two (7%) had HB grade V, and one (3%) had HB grade VI [Figure 13].
Figure 13 Distribution of House-Brackmann grade at 0 postoperative day.

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Distribution of HB grade 6 months after surgery

In the follow-up period of 6 months after surgery, 20 (67%) patients had HB grade I, six (20%) had HB grade II, one (3%) had HB grade III, two (7%) had HB grade V, one (3%) had HB grade VI, and none (0%) had HB grade IV [Figure 14].
Figure 14 Distribution of House-Brackmann grade 6 months after surgery.

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Tumor size and facial function (HB grade) in the immediate postoperative course

Patients with tumor size up to 1.5 cm (n = 4) had HB grade I in two (50%) and grade II in two (50%) patients in the immediate postoperative period. Similarly, patients with tumor size 1.5 to 3 cm (n = 22) had HB grade I in seven (31.8%), grade II in eight (36.36%), grade III in five (22.72%), grade IV in two (9.09%), and grades V to VI in zero (0%). Patients with tumor size >3 cm (n = 4) had HB grade IV in one (25%), grade V in two (50%), and grade VI in one (25%) patient [Table 2].
Table 2 Tumor size and facial function (HB grade) in the immediate postoperative course

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Tumor size and facial function (HB grade) 6 months after surgery

All patients with tumor size up to 1.5 cm (n = 4) had HB grade I (100%) 6 months after surgery. Similarly, patients with tumor size 1.5 to 3 cm (n = 22) had HB grade I in 16 (72.72%) and HB grade II in six (27.27%) patients. Patients with tumor size >3 cm (n = 4) had HB grade III in one (25%), grade V in two (50%), and grade VI in one (25%) patient [Table 3].
Table 3 Tumor size and facial function (HB grade) 6 months after surgery

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Mean tumor size and trigeminal nerve mean stimulus threshold

Mean tumor size was 2.81 cm and the mean stimulus threshold for trigeminal nerve was 0.164 mA. The coefficient of correlation was 0.8422 [Table 4] and [Figure 15].
Table 4 Mean tumor size and trigeminal nerve mean stimulus threshold

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Figure 15 Mean tumor size and trigeminal nerve mean stimulus threshold.

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Tumor size and FN mean stimulus threshold

Mean tumor size was 2.66 cm and the mean stimulus threshold for FN was 0.225 mA. The coefficient of correlation was 0.839 [Table 5].
Table 5 Tumor size and facial nerve mean stimulus threshold

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Tumor size and mean response amplitude for trigeminal nerve

Mean tumor size was 2.66 cm and mean response amplitude for trigeminal nerve was 181.5 μV. The coefficient of correlation was −0.70 [Figure 16].
Figure 16 Tumor size and mean response amplitude for trigeminal nerve.

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Tumor size and mean response amplitude for FN

Mean tumor size was 2.81 cm and mean response amplitude for FN was 205.4 μV. The coefficient of correlation was −0.80.

Mean stimulus threshold and HB grade at 0 postoperative day

Mean stimulus threshold for HB grades I to III at 0 postoperative day was 0.2 mA and for HB grades IV to VI was 0.32 mA [Figure 17].
Figure 17 Mean stimulus threshold and House-Brackmann grade at 0 postoperative day.

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Mean stimulus threshold and HB grade 6 months after surgery

Mean stimulus threshold for HB grades I and II 6 months after surgery was 0.2 mA and for HB grades III to VI was 0.37 mA [Figure 18].
Figure 18 Mean stimulus threshold and House-Brackmann grade at 6 months after surgery.

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Mean response amplitude and HB grade at 0 postoperative day

Mean response amplitude for HB grades I to III at 0 postoperative day was 223.4 μV and for HB grades IV to VI was 133.3 μV [Figure 19].
Figure 19 Mean response amplitude and House-Brackmann grade at 0 postoperative day.

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Mean response amplitude and HB grade at 6 months after surgery

Mean response amplitude HB grades I and II at 6 months after surgery was 222.7 μV and for HB grades III to VI was 92.5 μV [Figure 20].
Figure 20 Mean response amplitude and House-Brackmann grade at 6 months after surgery.

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Mean ratio and HB grade at 0 postoperative day

Mean proximal-to-distal amplitude ratio for HB grades I to III at 0 postoperative day was 0.9 and for HB grades IV to VI was 0.62 [Figure 21].
Figure 21 Mean ratio and House-Brackmann grade at 0 postoperative day.

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Mean ratio and HB grade at 6 months after surgery

Mean proximal-to-distal amplitude ratio for HB grades I and II 6 months after surgery was 0.89 and for HB grades III to VI was 0.49 [Figure 22].
Figure 22 Mean ratio and House-Brackmann grade at 6 months after surgery.

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Bispectral index

At the time of electrophysiological monitoring of CNs, it was maintained between 70% and 75%.


  Discussion Top


Tumor size was recorded as the largest tumor diameter; 78% of patients had tumor size ranging from 1.5 to 3 cm in diameter. The benefit of EMG monitoring is greatest in large tumors in which the FN is most likely to be intimately related to the tumor and therefore at greater risk. For tumors >4 cm, Sugita and Kobayashi[13] found anatomic preservation of the FN in 67% with EMG monitoring versus 33% of that without. Sterkers et al.[14] found improvement with monitoring across all tumor sizes, but these effects were greatest for tumors >2 cm.

Patients with tumor size <3 cm had HB grade I or II in 73.03% (19/26) in the immediate postoperative period. Patients with tumor size >3 cm had HB grades III to VI (4/4) and none in grade I or II in the immediate postoperative period.

All patients with tumor size <3cm in the postoperative follow-up after 6 months had HB grade I or II (100%) (26/26). All four patients with tumor size >3 cm had HB grades III to VI in the follow-up postoperative period 6 months after surgery. Anatomical and functional integrity of trigeminal nerve and FN were identified in all the cases. In our study, three patients had trigeminal nerve palsy before surgery that also persisted after the surgery in the postoperative course.

Mean tumor size was 2.81 cm, mean stimulus threshold was 0.164 mA, and mean response amplitude was 181.5 μV for trigeminal nerve. Mean tumor size was 2.66 cm, mean stimulus threshold was 0.225 mA, and mean response amplitude was 205.4 μV for FN. In the immediate postoperative day, 19 (63%) patients had HB grades I and II and 11 (37%) patients had HB grades III to VI. In the follow-up period of 6 months after surgery, 26 (87%) patients had HB grades I and II and four (13%) patients had HB grades III to VI.

The FN outcomes in this series compare favorably with those of previous reports. Normal FN outcomes have been reported at rates from 45% to 93% for tumors of all sizes.[9],[15],[16] For our entire series of patients who presented with normal FN function, a minimal decrease in FN function was achieved in 87% (HB grade II or better) [Table 6].
Table 6 Mean stimulus threshold, response amplitude, and proximal-to-distal amplitude ratio based on immediate postoperative facial function

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Mean stimulus threshold was higher (0.32 mA) in HB grades IV to VI than in HB grades I to III (0.20) in immediate postoperative facial function. Similarly, the response amplitude was less in the HB grades IV to VI (133.3 μV) compared to HB grades I to III (223.4 μV). The proximal-to-distal amplitude ratio was higher (0.9) in HB grades I to III than HB grades IV to VI (0.62) [Table 7].Mean stimulus threshold was higher (0.37 mA) in HB grades IV to VI than in HB grades I and II (0.20) in the follow-up period of 6 months after surgery. Similarly, the response amplitude was less in the HB grades IV of VI (92.5 μV) compared to HB grades I and II (222.7 μV). The proximal-to-distal amplitude ratio was higher (0.89) in HB grades I and II than HB grades IV to VI (0.49).
Table 7 Mean stimulus threshold, response amplitude, and proximal-to-distal amplitude ratio for patients after 6 months of surgery

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In our series, out of patients with small tumors (<3 cm), stimulus threshold <0.2 mA, response amplitude >200 μV, and proximal-to-distal ratio >0.66, that is, two-third had good FN outcome (defined as HB grades I and II) at 6 months after surgery. Similarly, out of patients with tumor size >3 cm, stimulus threshold >0.2 mA, response amplitude <100 μV, and proximal-to-distal ratio <0.66, that is, two-third had poor FN outcome (HB grades I and II) at 6 months after surgery.

Fenton et al.[17] showed that a stimulus threshold less than 0.05 mA and a “smaller” tumor size correlated with an HB grade of III or less in 88% of patients (59 of 67 patients) initially after surgery. Neff et al.[18] found that an intraoperative EMG stimulus threshold of 0.05 mA or less and response amplitude of 240 µV or more can help predict a HB grade I or II FN function with a 98% probability. Goldbrunner et al.[19] demonstrated that response amplitude recorded from stimulating the FN both proximal and distal to the tumor dissection could be used to prognosticate FN function at 6 months after surgery. They showed that a proximal amplitude to the distal amplitude ratio of 0.1 or less correlated with a poor FN function in 75% of patients at 6 months. Response amplitude has also been evaluated as a sole indicator of FN function.


  Conclusions Top


This study highlights the use of intraoperative monitoring of trigeminal nerve and FN to render prognostic information in CPA tumor surgery. Intraoperative neurophysiological monitoring showed strong support in the value of identification of FN and trigeminal nerve, prevention of injury, and value of prognosis. Stimulus thresholds, response amplitude, and proximal-to-distal amplitude ratio can assist the surgeon during tumor dissection. In addition, the tumor size was also found to correlate with postoperative facial function. The FN, in particular, has shown higher rates of preservation (87% of patients in HB grade I or II at 6 months of follow-up after surgery) with the use of intraoperative EMGs. These parameters in conjunction with immediate postoperative FN examination may have the ability to readily identify patients who are at risk for developing permanent FN weakness.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Kartush JM, Lundy LB. Facial nerve outcome in acoustic neuroma surgery. Otolaryngol Clin North Am 1992;25:623-47.  Back to cited text no. 1
    
2.
Sampath P, Holliday MJ, Brem H, Niparko JK, Long DM. Facial nerve injury in acoustic neuroma (vestibular schwannoma) surgery: etiology and prevention. J Neurosurg 1997;87:60-6.  Back to cited text no. 2
    
3.
Moskowitz N, Long DM. Acoustic neuromas. Historical review of a century of operative series. Neurosurg Q 1991;1:2-18.  Back to cited text no. 3
    
4.
Nakamura M, Roser F, Dormiani M, Matthies C, Vorkapic P, Samii M. Facial and cochlear nerve function after surgery of cerebellopontine angle meningiomas. Neurosurgery 2005;57:77-90.  Back to cited text no. 4
    
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Samii M, Migliori MM, Tatagiba M, Babu R. Surgical treatment of trigeminal schwannomas. J Neurosurg 1995;82:711-8.  Back to cited text no. 5
    
6.
Akagami R, Dong CC, Westerberg BD. Localized transcranial electrical motor evoked potentials for monitoring cranial nerves in cranial base surgery. Neurosurgery 2005;57:78-85.  Back to cited text no. 6
    
7.
Delgado TE, Bucheit WA, Rosenholtz HR, Chrissian S. Intraoperative 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.  Back to cited text no. 7
    
8.
Grant GA, Rostomily RR, Kim DK, Mayberg MR, Farrell D, Avellino A et al. Delayed facial palsy after resection of vestibular schwannoma. J Neurosurg 2002;97:93-6.  Back to cited text no. 8
    
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Lalwani AK, Butt FY, Jackler RK, Pitts LH, Yingling CD. Facial nerve outcome after acoustic neuroma surgery: a study from the era of cranial nerve monitoring. Otolaryngol Head Neck Surg 1994;111:561-70.  Back to cited text no. 9
    
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Dickins JR, Graham SS. A comparison of facial nerve monitoring systems in cerebellopontine angle surgery. Am J Otol 1991;12:1-6.  Back to cited text no. 10
    
11.
Givré A, Olivecrona H. Surgical experiences with acoustic tumors. J Neurosurg 1949;6:396-407.  Back to cited text no. 11
    
12.
Romstöck J, Strauss C, Fahlbusch R. Continuous electromyography monitoring of motor cranial nerves during cerebellopontine angle surgery. J Neurosurg 2000;93:586-93.  Back to cited text no. 12
    
13.
Sugita K, Kobayashi S. Technical and instrumental improvements in the surgical treatment of acoustic neurinomas. J Neurosurg 1982;57:747-52.  Back to cited text no. 13
    
14.
Sterkers JM, Morrison GA, Sterkers O, El-Dine MM. Preservation of facial, cochlear, and other nerve functions in acoustic neuroma treatment. Otolaryngol Head Neck Surg 1994;110:146-55.  Back to cited text no. 14
    
15.
Hammerschlag PE, Cohen NL. Intraoperative monitoring of facial nerve function in cerebellopontine angle surgery. Otolaryngol Head Neck Surg 1990;103:681-4.  Back to cited text no. 15
    
16.
Kwartler JA, Luxford WM, Atkins J, Shelton C. Facial nerve monitoring in acoustic tumor surgery. Otolaryngol Head Neck Surg 1991;104:814-7.  Back to cited text no. 16
    
17.
Fenton JE, Chin RY, Fagan PA, Sterkers O, Sterkers JM. Facial nerve outcome in non-vestibular schwannoma tumour surgery. Acta Otorhinolaryngol Belg 2004;58:103-7.  Back to cited text no. 17
    
18.
Neff BA, Ting J, Dickinson SL, Welling DB. Facial nerve monitoring parameters as a predictor of postoperative facial nerve outcomes after vestibular schwannoma resection. Otol Neurotol 2005;26:728-32.  Back to cited text no. 18
    
19.
Goldbrunner RH, Schlake HP, Milewski C, Tonn JC, Helms J, Roosen K. Quantitative parameters of intraoperative electromyography predict facial nerve outcomes for vestibular schwannoma surgery. Neurosurgery 2000;46:1140-6.  Back to cited text no. 19
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18], [Figure 19], [Figure 20], [Figure 21], [Figure 22]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]



 

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Introduction
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