Introduction: Facial weakness is a major complication of vestibular schwannoma surgery.(1) Lower, post-resection facial nerve stimulation thresholds (ST) are predictive of better post-operative facial function.(2,3,4) A reliable, baseline facial nerve ST to guide extent of tumor resection would be useful.
Methods: Prospectively and with IRB approval, baseline STs were obtained in 33 patients with normal facial function undergoing microvascular decompression for hemifacial spasm. Using constant voltage stimulation, currents of 0.2 mA, 0.1 mA, 0.05 mA, and 0.025 mA were applied to the proximal facial nerve (i.e. along its attached segment to the pons). In response to stimulation, compound muscle action potentials (CMAP) and latency times were recorded for the orbicularis oculi and mentalis muscles, respectively. The CMAP amplitude, defined as the magnitude of the maximum positive peak of the waveform generated, was used to evaluate muscle response.
Results: CMAP responses were generated at all of the tested stimulation currents in the orbicularis oculi and mentalis muscles indicating effective nerve conduction. Decreasing CMAP amplitude, increasing onset latency, and shorter response duration were observed with decreasing stimulation currents. The onset latency in all cases was >4.5 ms, helping to exclude potential non-specific current spread and peripheral facial nerve activation. The mean onset latency for the population across all stimulation protocols was 6.2 ms for the orbicularis oculi and 5.2 ms for the mentalis muscle.
Conclusions: The presence of CMAP amplitudes in response to 0.025 mA indicates that facial nerve conduction can take place at such low stimulation values. Our results provide reference values of parameters that are useful in determining facial nerve functionality during tumor resection, particularly vestibular schwannoma, within the CPA. A proximal facial nerve ST of <0.05 mA should be considered as a reference baseline for an expected functional (i.e. House-Brackmann grade I) facial nerve postoperatively.
Patient Care: Surgery, particularly involving vestibular schwannoma, within the cerebellopontine angle is challenging and can lead to permanent facial nerve dysfunction. While no clinical trial has been conducted to determine the benefits of neuro-monitoring during CPA surgery, several studies have established a clear improvement in facial nerve function with the use of intraoperative monitoring. The National Institutes of Health Consensus Statement on Acoustic Neuroma recommend routine use of intraoperative facial nerve monitoring during relevant surgical procedures. However, stimulation techniques and parameters have not been standardized. Our presentation of parameters obtained from intact facial nerves not encased in tumor tissue can help provide a point of comparison that can be used to more accurate identify facial nerve compromise.
Learning Objectives: By the conclusion of this session, participants should be able to:
(1) Understand why electrophysiological monitoring of an anatomically normal facial nerve is useful for establishing references for comparisons during intraoperative neuro-monitoring
(2) Describe CMAP values and latencies expected from stimulation of a facial nerve with preserved function.
(3) Identify <0.05 mA as effective stimulation to evoke facial nerve responses
References: 1. Harris DL, Carr AT. Prevalence of concern about physical appearance in the general population. British journal of plastic surgery 2001;54:223-226.
2. Marin P, Pouliot D, Fradet G. Facial nerve outcome with a peroperative stimulation threshold under 0.05 mA. The Laryngoscope 2011;121:2295-2298.
3. Silverstein H, Willcox TO, Jr., Rosenberg SI, Seidman MD. Prediction of facial nerve function following acoustic neuroma resection using intraoperative facial nerve stimulation. The Laryngoscope 1994;104:539-544.
4. Sughrue ME, Kaur R, Kane AJ, et al. The value of intraoperative facial nerve electromyography in predicting facial nerve function after vestibular schwannoma surgery. Journal of clinical neuroscience : official journal of the Neurosurgical Society of Australasia 2010;17:849-852.