Facial Nerve Functional Outcome with Simple Neuromonitoring Paradigm in 87 Cerebellopontine Angle Tumor Surgeries in a Developing Country

Abstract

Objective

With the advent of intraoperative neuromonitoring (IONM), facial nerve (FN) preservation improved to nearly 85% in cerebellopontine tumor (CPA) surgeries. Various parameters, such as FN proximal and distal absolute electromyography (EMG) amplitude, stimulation threshold, proximal-to-distal amplitude ratio, and free-run EMG train time, have been used to predict outcomes but not consistently. Hence the aim of this study is to investigate the outcome of FN function in the immediate postoperative period versus the final outcome at 1-year postop and assess factors affecting it.

Methods

A total of 87 consecutive patients of CPA tumor surgery were retrospectively analyzed between 2015 and 2023 to allow a minimum 12-month postoperative period. Tumor size, immediate postoperative House–Brackmann (HB) scale, and 1-year postoperative HB scale were analyzed. All patients with HB II and above were advised physical therapy. A statistical analysis was conducted to investigate any relationships with FN outcome.

Results

Eighty-seven patients underwent resection; 62% had vestibular schwannoma followed by nearly 9% each of meningioma, trigeminal schwannoma, and epidermoid. In addition, 80.5% were large tumors and did not have any statistical relation with outcome. FN continuity was preserved in all 100% patients. Furthermore, 32 (36.7%) had immediate postop facial paralysis. The severity of immediate postoperative paralysis had a statistically significant correlation with the final FN outcome (p = 0.006). Compliance with physical therapy improved the HB grade in 100.0% of the cases in a statistically significantly manner (p = 0.001).

Conclusion

Use of IONM improves FN outcome by enabling anatomical continuity. FN function immediate postop can be used to predict long-term outcome. Counseling for continued physical therapy for at least 1-year postoperative period improves outcomes.

 

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Keywords

cerebellopontine angle tumor - neuromonitoring - facial nerve outcome

Around 10% of brain tumors are cerebellopontine angle (CPA) tumors. Of these, vestibular schwannoma (VS) is the commonest.[1] The incidence of facial paralysis following removal of CPA tumors was as high as 40 to 50% prior to use of microsurgical techniques. Intraoperative neuromonitoring (IONM) helped to preserve facial nerve (FN) function in nearly 85% of the cases.[2] Among the various structures that are at risk, FN is at high risk due to its anatomical proximity to these tumors, particularly in tumors greater than 3 cm. The tumor capsule may be adherent to the nerve or the nerve may be thinned out and splayed apart by the tumor and thus get damaged during tumor resection because of poor differentiation from adjacent structures and or difficulty in identification. Free-running and triggered electromyography (EMG) are commonly used to monitor FN function.[2] House–Brackmann (HB) scoring is widely used to document facial function.[3] [4] Various factors such as FN proximal and distal absolute EMG amplitude, stimulation threshold, proximal-to-distal amplitude ratio, and free-run EMG train time have been used to predict outcomes but not consistently.[5] [6] In this study, the robustness of a simple technique of anatomical preservation of FN followed by compliance to physical therapy was analyzed. To the best of our knowledge, this has not been analyzed before.

Methods

Subjects

IONM data and clinical data of patients undergoing CPA tumor surgery were retrospectively analyzed from 2015 to 2023, including the 1-year postoperative period. All patients were assessed for FN outcome immediately post-surgery on postoperative day 1 and at 1 year either in-person or telephonically. Telephonic follow-up consisted of ascertaining eye closure completeness and smile symmetry. The Jaslok Hospital Ethics Committee approved this study.

 

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Surgical Procedure

This study was conducted at a tertiary hospital in Mumbai, India, which also has teaching programs. Short-acting neuromuscular block was used only for induction. Anesthesia was mainly intravenous.[7] If inhalational agents were used, the mean alveolar concentration was less than 0.5. All surgeries were done using a retro-sigmoid approach. An operating microscope was used in all surgeries. The bone opening was done using a manual drill and punch. The intra-dural procedure was done with microsurgical instruments, bipolar cautery and suction, and IONM.

 

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IONM Set Up

The FN monitoring protocol was standard for all patients and consisted of free-running EMG and triggered EMG. EMG was recorded at frontalis, orbicularis oculi, orbicularis oris (upper and lower), and mentalis using subdermal needle electrodes in all patients to give a wide coverage for all branches of the FN. EMG activity was displayed with audio for visual and auditory recognition. Settings for sensitivity for free-run and triggered EMG was 100 µV/division. The time base for free-run EMG was 100 ms/division and for triggered EMG it was 5 ms/division. A monopolar probe was used for triggered EMG. Once exposure was complete, the surgeon performed tumor mapping using the monopolar probe. A stimulation rate of 2 to 3 Hz, a duration of 50 to 300 µs, and an intensity of 3 mA were used in all cases. As the dissection progressed, triggered EMG was conducted whenever there was a need to identify neural structures. The stimulation intensity was gradually reduced as the dissection progressed to avoid false-positive interpretation due to high intensity. The FN stimulation threshold was always noted. In all cases it was 0.5 mA or lesser. Free-run EMG was used to alert for neurotonic discharges lasting more than 1 second and bursts. These could be caused by mechanical stretch during dissection and thermal injury during cautery. False positive due to patient anesthesia being light, movement artefact, and electrode artefact was taken cognizance of prior to alert.[2]

 

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House–Brackmann Grading System

The FN function was scored according to the HB grading system (Grades I–VI: Grade I, normal function; Grade VI, total palsy).[3] Good FN function was defined as HB I and II, whereas HB III and VI were defined as poor outcome. The time points for evaluation of FN function outcome were immediately post-surgery on postoperative day 1 and at 1-year follow up.

 

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Physical Therapy

All patients who had HB II and above in the immediate postoperative period were advised physical therapy. It was started on postoperative day 1 by a physical therapist. Subsequently, patients were asked to do the exercises independently at home. Mirror exercises and facial massages were advised. Mirror exercises help to prevent synkinesis and enhance symmetry. Facial massages create small recruitments and prevent hyperactivity of adjoining muscles. Patients were encouraged to practice the exercises more than once every day and counselled regarding slow gradual improvement to ensure compliance.

 

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Results

Study Population and Tumor Characteristics

The study population consisted of 27 males (31%) and 60 females (69%) with the age range of 20 to 82 years, with the mean age of 47.82 years. Nine patients (10.3%) were operated for tumor recurrence. The tumors were categorized as small (less than 2 cm, 3.4%), medium (2–3 cm, 16%), and large (more than 3 cm, 80.5%). The tumor size ranged from 1.40 to 8.20 cm with the average being 3.78 cm and the median was 3.50 cm. The final pathology result showed VS in 62%, followed by meningioma in 8%, trigeminal schwannoma and epidermoid in 9.1%, and others in 11.4%.

 

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Association between Short-Term and Long-Term Facial FN Outcomes

FN functional continuity was preserved in 100% of the cases.

In all cases, FN threshold was 0.5 mA or less, lowest being 0.2 mA.

Short-term facial outcome was good in 75.8% of cases (HB I: 63.2%; HB II: 12.6%), and poor outcome was obtained in 24.1% (HB III, IV: 21.8; HB V, VI: 2.2%).

Two patients were lost to follow-up at the end of 1 year and one patient expired at 3 months post-surgery due to myocardial infarction.

Long-term facial outcome was poor in 5 (5.7%) cases and good in 94.2% cases. In addition, 100.0% of the cases with immediate post-surgery HB II (mild facial weakness) and 91% of the cases with immediate post-surgery HB III (moderate facial weakness) had full recovery (grade HB I). The Chi-square test showed a statistically significant difference for poor outcome at the end of 1 year for patients who had severe immediate postoperative weakness ([Table 1]). [Table 2] shows the clinical profile of patients with poor outcome at the end of 1year post-surgery.

Table 1 Association between FN outcomes immediately post-surgery and at 1 year post-surgery

Facial weakness immediately post-surgery	N	1-year outcome (N = 29)				p-Value
		HB I		HB II or above
		No.	%	No.	%
HB II (mild)	11	11	100.0	–	–	0.006
HB III (moderate)	11	10	90.9	01	09.1
HB IV (moderate–severe)	06	03	50.0	03	50.0
HB V (poor)	01	–	–	01	100.0

Abbreviations: HB, House–Brackmann; FN, facial nerve.

Note: By Chi-square test; significant ≤ 0.05.

Table 2 Profile of patients with poor FN function at the end of 1 year

Patient	Age in years	Tumor size in cm	Histopathology	MST in mA	TT length in seconds	Extent of resection	Facial function immediately postop	Facial function at 1-year postop
1.	43	8.2	VS	0.2	>1	Near total	HB IV	HB IV
2.	42	2.8	VS	0.5	>1	Near total	HB III	HB III
3.	41	4.0	VS	0.5	>1	Subtotal	HB IV	HB IV
4.	41	4.1	VS recurrent tumor	0.5	>1	Near total	HB IV	HB IV
5.	59	3.4	VS	0.2	>1	Near total	HB V	HB III

Abbreviations: HB, House–Brackmann; MST, minimum stimulation threshold; TT, train time; VS, vestibular schwannoma.

 

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Association between Tumor Size and Severity of Paralysis at 1 year Post-surgery

At 1 year post-surgery, 5.8% of the cases with tumor size less than 3 cm had HB III and above as against 5.7% of the cases with tumor size more than 3 cm. This difference was not statistically significant ([Table 3]).

Table 3 Association between tumor size and severity of paralysis at 1-year post-surgery

Tumor size	N	No. of cases = 87				p-Value
		HB II		HB III–VI
		No.	%	No.	%
Tumor size less than 3 cm	17	12	70.6	1	5.8	0.482
Tumor size greater than 3 cm	70	43	61.4	4	5.7

Note: By Chi-square test; significant ≤ 0.05.

 

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Association between FN Outcome and Physical Therapy

All patients with HB II and above were advised physical therapy. In addition, 95.5% of the cases with mild and moderate immediate postoperative weakness and 57.1% with severe and poor immediate postop weakness who were compliant with physical therapy showed improvement, thus implying a statistically significant relation with compliance to physical therapy ([Table 4]). One patient with tumor size less than 3 cm had poor outcome. This patient was uncooperative for physical therapy. An EMG nerve conduction study (EMG NCS) was done only for this patient from this series. The test was done at 10 months post-surgery. It showed attenuated distal facial motor compound muscle action potential amplitudes from the orbicularis oculi, nasalis, and orbicularis oris suggestive of distal continuity. EMG showed fibrillations and positive sharp waves at rest. No recruitment was noted on maximal volitional effort from single-site needle EMG for frontalis, orbicularis oculi, and orbicularis oris.

Table 4 Association between compliance with physical therapy and outcome at 1-year post-surgery

Immediately postop weakness	N	Advised physical therapy (N = 29)				p-Value
		Compliant and improved		Noncompliant, incomplete recovery
		No.	%	No.	%
Mild and moderate	22	21	95.5	01	04.5	0.001
Severe and poor	07	03	42.9	04	57.1

Note: By Chi-square test; significant ≤ 0.05.

 

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Profile of Patients Operated for Tumor Recurrence

Nine patients were operated for tumor recurrence. All had large tumors, i.e., more than 3 cm. Furthermore, 62.5% of the patients had HB II and above immediate post-surgery weakness, ranging from HB II to HB IV. But at the end of 1 year, eight patients (88.8%) had no facial weakness. Only one patient had HB IV at the end of 1 year post-surgery. This patient was noncompliant for physical therapy.

 

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Discussion

With the advent of IONM, FN preservation after CPA tumor surgeries has improved significantly. Using both free-run and triggered EMG is the accepted paradigm for IONM for FN monitoring. However, because the tumor is in close proximity and often adherent to the FN, there is always a concern for FN injury in spite of IONM.

The incidence of long-term facial dysfunction after VS surgery ranges from 4.8 to 41%, the mean being approximately 19%.[8] In the present study, 80% tumors were large but not associated with poor outcome. On the other hand, interestingly, only one patient with tumor less than 3 cm who was not compliant with physical therapy had poor outcome. No correlation for tumor size has been reported previously in 100 patients of VS.[5]

Several studies have shown that immediate postoperative FN function has a direct correlation with the final FN outcome. In our study, 100.0% of the cases with immediate post-surgery HB II (mild facial weakness) and 91% of the cases with immediate post-surgery HB III (moderate facial weakness) had full recovery (grade HB I). An immediate postoperative HB IV–VI correlated with poor outcome, in accordance with published literature.[6] [7] A higher immediate postoperative HB grade was associated with a poorer outcome at the end of 1 year in this study. This finding has been noted in other studies too.[4] [9]

Various monitoring techniques have been described for monitoring FN function intraoperatively, such as free-running EMG, triggered EMG, A-train time, FN threshold, facial motor-evoked potential, and blink reflex (BR) study. However, there is an overall lack of standardization even in electrode montage. Usually, a montage of one to three muscles has been used in previous studies.[5] [9] In this study, four muscles were used to provide a wider and complete coverage. Because the nerve might be splayed by the tumor into fascicles, sometimes the responses are noted in just one muscle. Hence using more muscles for monitoring reduces the chances of missing the nerve. This four-muscle montage serves well for both free-run EMG and triggered EMG. As a neuromonitoring paradigm, employing more target muscles for motor monitoring provides very high sensitivity.[10] A study comparing two-muscle montage versus four-muscle montage may objectively corroborate the effectiveness.

On exposure, tumor mapping at 3 mA is highly recommended. Direct electrical stimulation (mapping) of FN was the first neurophysiological method, due to the high incidence of FN palsy during VS removal.[11] [12] [13] Nerve tissue can be identified from the surrounding tumor tissue so that a nerve-sparing dissection can be attempted. Direct stimulation of a normal nerve root using less than 2 mA can ensure an electrical activity in the appropriate muscle group.[14] However, the lowest “safe” threshold is not described. The lowest threshold is a measure of nerve excitability. It is not an unequivocally reliable prognostic indicator of FN function.[15] Also, the lowest possible stimulation may vary between IONM equipment. Hence, in this study, no attempt was made to correlate the lowest threshold for FN stimulation, although in all patients, the FN threshold was obtained at 0.5 mA or lower. Since the lowest threshold is not standardized, the latency and amplitude can vary and thus their absolute values and ratios may be rendered obsolete for prognostication. Thus, the mainstay of functional outcome would rely on preventing neurotmesis. Gazia et al[9] noted in a series of 157 patients that at 1 year postoperative assessment, a minimum stimulation threshold of 0.1 mA showed a sensitivity and a specificity of 62 and 73%, respectively. With a CMAP cut-off < 200 µV, for long-term prediction, sensitivity was 73% and specificity was 73%. Turel et al[5] documented that latency and amplitude ratios were not reliable predictors of FN outcome. A sustained neurotonic discharge may be produced due to mechanical injury during dissection or thermal injury during electrocautery. But the correlation of discharge duration, also called A-train based on such software; for FN outcome is not consistent.[16] [17] Although longer train times would be associated with severity of postoperative FN function, it is difficult to quantify the duration of train time because sometimes these may occur as repetitive bursts rather than continuous trains. They may also be produced due to cold saline, patient's anesthesia being light, or a combination of these factors. Liu et al[18] proposed that BR and facial corticobulbar motor evoked potentials (FCoMEP) may be used in combination to complement each other for predicting FN outcome. The study focused on eye closure function as the main measure of FN outcome. Valid BR was recorded in 93.6% of the patients and FCoMEP was recorded in 39% of the patients. BR had better predictive value compared to FCoMEP and needs further evaluation for feasibility.

In this series, one patient with a tumor size less than 3 cm and noncompliance to physical therapy had poor outcome. His EMG NCS at 10 months showed preserved distal continuity of FN, ongoing axonal degeneration, and poor reinnervation. The role of physical therapy in improving outcomes after nerve injury is well known and appears to be a major confounding variable in this case, considering that this was the only patient with poor outcome in the tumor group less than 3 cm. Hence, from this series we may conclude that noncompliance to physical therapy leads to poor outcome irrespective of tumor size. FN is at risk during parotid surgeries and the role of IONM is gaining wider acceptance for FN preservation in parotid surgeries. The study by Molinari et al showed that in the patients with FN preservation, 2-year FN rehabilitation program led to sustained significant improvement.[19]

Limitations

This study can be improved by correlating FN outcomes to extent of resection, cystic/solid nature of the tumor, extent of tumor into internal auditory meatus, neurofibromatosis status, and surgeon's experience. The main limitation of this study is that this protocol relies mainly on FN identification, which appears rather simplistic without the absolute and relative quantifications. Also, a comparison of this protocol with individual techniques like train length, minimum stimulation threshold, BR study, and facial corticobulbar motor-evoked potentials may be particularly useful in large tumors causing brainstem compression. Finally, the outcome assessment relies on self-reporting. A video consultation may be a good alternative whenever feasible. Follow-up may be extended to 2 years because late recovery is known.

 

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Conclusion

Thus, in conclusion, this study uses a simple IONM paradigm using dense muscle montage, emphasis on FN anatomical preservation followed by physical therapy for at least 1 year in the postoperative period. This kind of protocol may be particularly useful to implement in settings with limited technical expertise or advanced software which preclude prognostication based on ratios.

 

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Conflicts of Interest

None declared.

• References

• 1 Ezer H, Banerjee AD, Nanda A. Skull base tumors: part II. Contemp Neurosurg 2010; 32 (20) 1-6
• 2 Aatif MH. A Practical Approach to Neurophysiologic Intraoperative Monitoring. New York, NY: Demos Medical Publishing; 2008
• 3 House JW, Brackmann DE. Facial nerve grading system. Otolaryngol Head Neck Surg 1985; 93 (02) 146-147
• 4 Arriaga MA, Luxford WM, Atkins Jr JS, Kwartler JA. Predicting long-term facial nerve outcome after acoustic neuroma surgery. Otolaryngol Head Neck Surg 1993; 108 (03) 220-224
• 5 Turel MK, Babu KS, Singh G, Chacko AG. The utility of facial nerve amplitude and latency ratios in predicting postoperative facial nerve function after vestibular schwannoma surgery. Neurol India 2014; 62 (02) 178-182
• 6 Isaacson B, Kileny PR, El-Kashlan H, Gadre AK. Intraoperative monitoring and facial nerve outcomes after vestibular schwannoma resection. Otol Neurotol 2003; 24 (05) 812-817
• 7 Axon PR, Ramsden RT. Intraoperative electromyography for predicting facial function in vestibular schwannoma surgery. Laryngoscope 1999; 109 (06) 922-926
• 8 Huppelschoten M. Catch the A-train: Detection of A-trains in EMG during Vestibular Schwannoma Resection Making Use of Neural Networks [thesis]. Enschede, The Netherlands: University of Twente; 2019
• 9 Gazia F, Callejo À, Pérez-Grau M. et al. Pre- and intra-operative prognostic factors of facial nerve function in cerebellopontine angle surgery. Eur Arch Otorhinolaryngol 2023; 280 (03) 1055-1062
• 10 Shah PA. Transcranial motor evoked potential monitoring outcome in the high-risk brain and spine surgeries: correlation of clinical and neurophysiological data - an Indian perspective. Ann Indian Acad Neurol 2013; 16 (04) 609-613
• 11 Hilger JA. Facial nerve stimulator. Trans Am Acad Ophthalmol Otolaryngol 1964; 68: 74-76
• 12 Delgado TE, Bucheit WA, Rosenholtz HR, Chrissian S. Intraoperative monitoring of facila muscle evoked responses obtained by intracranial stimulation of the facila nerve: a more accurate technique for facila nerve dissection. Neurosurgery 1979; 4 (05) 418-421
• 13 Møller AR, Jannetta PJ. Preservation of facial function during removal of acoustic neuromas. Use of monopolar constant-voltage stimulation and EMG. J Neurosurg 1984; 61 (04) 757-760
• 14 Gonzalez AA, Jeyanandarajan D, Hansen C, Zada G, Hsieh PC. Intraoperative neurophysiological monitoring during spine surgery: a review. Neurosurg Focus 2009; 27 (04) E6
• 15 Amrutha JS, Kumar Chittoria R. Guidelines for management of facial palsy. Jr of Glob Med Edu and Res. 2024; 7 (02) 53-64
• 16 Hendriks T, Kunst HPM, Huppelschoten M, Doorduin J, Ter Laan M. TcMEP threshold change is superior to A-train detection when predicting facial nerve outcome in CPA tumour surgery. Acta Neurochir (Wien) 2020; 162 (05) 1197-1203
• 17 Prell J, Rampp S, Romstöck J, Fahlbusch R, Strauss C. Train time as a quantitative electromyographic parameter for facial nerve function in patients undergoing surgery for vestibular schwannoma. J Neurosurg 2007; 106 (05) 826-832
• 18 Liu J, Fan X, Yang L. et al. Predictive value of Blink reflex and facial corticobulbar motor evoked potential in cerebellopontine angle tumor surgery. Clin Neurophysiol 2024; 162: 165-173
• 19 Molinari G, Calvaruso F, Barbazza A. et al. Patterns and timing of recovery from facial nerve palsy after nerve-sparing parotid surgery: the role of neuromuscular retraining. Eur Arch Otorhinolaryngol 2024; 281 (10) 5465-5472

Publication History

Article published online:
10 September 2025