Role of Facial Nerve Motor-Evoked Potential Ratio in Predicting Facial Nerve Function in Vestibular Schwannoma Surgery Both Immediate and at 1 Year

Objective: To determine whether transcranial electrical stimulation-induced facial motor-evoked potential (FMEP) monitoring of the facial nerve (FN) during vestibular schwannoma (VS) tumor resection can predict both immediate and 1 year postoperative FN functional outcome. Design: Prospective consecutive non-comparative observational case series. Setting: Tertiary referral center. Main Outcome Measures: Facial function, immediate post operation and at 1 year using House-Brackmann (HB) grading scale. Methods: The study included 367 consecutive patients (men 178; women 189; age 13-81 years) monitored during primary sporadic VS microsurgery between November 2002 and April 2015. Neurofibromatosis type II, revision surgery, previous radiotherapy treatment, preoperative facial nerve weakness, and non-VS cases were excluded retrospectively during analysis of data. Data of facial function were missing from eight patients at 1 year and were excluded. The correlation between the final-to-baseline FMEP ratio and immediate and 1 year facial nerve function was examined. Results: Using logistic regression model, the cut-off points of FMEP ratio were 0.62 (PPV 0.96) and 0.59 (PPV 0.98) which predicted satisfactory FN function (HB grades 1 or 2) immediately postoperative and at 1 year after surgery, respectively. Conclusion: Transcranial electrical stimulation FMEP is a valuable tool for monitoring facial nerve function during resection of vestibular schwannoma. Maintaining a FMEP event-to-baseline ratio of 60% or greater is predictive of satisfactory long-term FN function.