Intraoperative transcranial facial motor evoked potential in vestibular schwannoma reflects short-term post operative facial nerve function.

Preservation of facial nerve function is extremely important in vestibular schwannoma surgery. Intraoperative transcranial facial motor evoked potential (Tc-fMEP) monitoring was used, and its accuracy and the correlations of Tc-fMEP results with postoperative facial nerve function at various time points were investigated. Factors associated with postoperative deterioration of facial nerve function were also examined. Forty-five consecutive cases of vestibular schwannoma that underwent surgery at our hospital from January 2013 to July 2022 were retrospectively reviewed. The correlation between intraoperative Tc-fMEP results and postoperative facial nerve function was investigated in each period from immediately after surgery to one year later. The warning criterion for Tc-fMEP was a decrease of 50% or more compared to the baseline amplitude. The relationships of age, sex, side, tumor size, and tumor nature with postoperative facial nerve function were also examined. Intraoperative Tc-fMEP monitoring was successfully performed in all 45 cases. Intraoperative Tc-fMEP results were significantly correlated with facial nerve function one week (P < 0.01) and one month after surgery (P < 0.01). The negative predictive value was 89% one month after surgery (P < 0.01). One year after surgery, the facial nerve function preservation rate (House and Brackmann: grade I-II) was 88%. In addition, facial nerve function one year after surgery was significantly worse with cystic tumors than with solid tumors (P = 0.04). Intraoperative Tc-fMEP monitoring may reflect facial nerve function one week to one month after surgery. In addition, in our study cystic tumors had significantly higher deterioration in postoperative facial nerve function than for solid tumors.