Continuous facial corticobulbar motor evoked potential monitoring during vestibular schwannoma surgery: predictive value for postoperative facial nerve function

Objective To evaluate the feasibility of continuous facial corticobulbar motor evoked potential (FcMEP) monitoring during vestibular schwannoma surgery and its association with severe early postoperative facial weakness. Methods We retrospectively reviewed 51 consecutive patients who underwent unilateral vestibular schwannoma resection through a retrosigmoid approach at a single tertiary center between July 2019 and July 2022. FcMEP deterioration was defined in the operative protocol as a substantial increase in the stimulation threshold (more than 50% from baseline) and/or a marked reduction or loss of the response. Severe early facial weakness was defined as House–Brackmann (HB) grade V–VI. Diagnostic performance is reported with 95% confidence intervals. Results FcMEPs were interpretable in 50/51 patients (98.0%). Fourteen patients developed severe early postoperative facial weakness. Thirteen of these patients had FcMEP deterioration; the remaining patient had a nonmonitorable examination. Among the 50 monitorable patients, there were 13 true-positive, 37 true-negative, no false-positive, and no false-negative classifications. Sensitivity and positive predictive value were each 100% (95% CI, 75.3–100%); specificity and negative predictive value were each 100% (95% CI, 90.5–100%); and accuracy was 100% (95% CI, 92.9–100%). When the nonmonitorable examination was counted as a test failure, sensitivity was 92.9% (13/14; 95% CI, 66.1–99.8%). Nine of the 14 affected patients improved to HB II during follow-up, whereas five had residual HB IV dysfunction. The available data did not support reliable discrimination between reversible and persistent deficits. Conclusions In this retrospective series, FcMEP deterioration was closely associated with severe early postoperative facial weakness when a response could be recorded. This finding requires caution: only a small number of outcome events occurred, the confidence intervals were wide, one affected patient had no monitorable response, and FcMEP findings influenced decisions during surgery. FcMEP monitoring may therefore be a useful addition to multimodal monitoring, but our data neither establish independent predictive value nor support routine use without prospective validation.