Facial nerve monitoring parameters as a predictor of postoperative facial nerve outcomes after vestibular schwannoma resection

Objective: To evaluate whether the intraoperative stimulus threshold and response amplitude measurements from facial electromyography can predict facial nerve function at 1 year after vestibular schwannoma resection. Study Design: Prospective study. Setting: Tertiary academic center. Patients: Seventy-four consecutive vestibular schwannoma patients. Intervention: The minimal stimulus intensity (in milliam-peres) and electromyographic response amplitude (in microvolts) were recorded during stimulation applied to the proximal facial nerve after vestibular schwannoma removal. Main Outcome Measure: Facial nerve outcomes at I year were evaluated using the House-Brackmann scale. Analysis was then performed to evaluate whether these electrophysiologic recordings and tumor size could predict facial nerve functional outcomes. Results: Of the 74 patients, 66 of 74 (89%) had House-Brackmann Grade I or 11 facial nerve function and 8 of 74 (11%) had House-Brackmann Grade Ill-VI function at I year after surgery. If standards were set for intruoperative minimal stimulus intensity of 0.05 mA or less and response amplitude of 240 mu V or greater, the authors were able to predict a House-Brackmann Grade I or II outcome in 56 of 66 (85%) patients at I year after surgery. With these same electrophysiologic parameters, only 1 of 8 (12%) House-Brackmann Grade III-VI patients also met this standard and thus gave a false-positive result. Logistic regression analysis of the data showed that both a stimulus threshold of 0.05 mA or less and a response amplitude of 240 mu V or greater predicted a House-Brackmann Grade I or II outcome with a 98% probability. However, stimulus threshold or response amplitude alone had a much lower probability of the same result. In addition, although tumor size was found to independently predict facial nerve outcomes at I year, it did not improve the ability to predict facial nerve function over a model using stimulus intensity and amplitude alone. Conclusion: Individually, minimal stimulus intensity or response amplitude was less successful in predicting long-term postoperative facial nerve function. However, if both parameters are considered together, the study demonstrates that they are good prognostic indicators for facial nerve function at I year after surgery.