Repeated Latency Prolongation of the Orbicularis Oris Muscle During Intraoperative Transcranial Facial Motor Evoked Potential Monitoring: A Case Report.

Intraoperative transcranial facial motor evoked potential (Tc-fMEP) monitoring is widely used to preserve facial nerve function during vestibular schwannoma surgery. Although amplitude reduction is commonly recognized as an alarm sign, latency changes have rarely been emphasized. We describe the case of a man in his 50s with neurofibromatosis type 2 who developed progressive growth of a left vestibular schwannoma. The tumor caused cerebellar compression and gait unsteadiness, and surgical resection was performed with Tc-fMEP monitoring. During the first surgery, only a partial resection was achieved due to intraoperative cerebellar swelling. Tc-fMEP amplitudes recorded from the orbicularis oculi and orbicularis oris muscles remained stable; however, latency prolongation of the orbicularis oris response appeared near the end of the procedure. Two weeks later, the patient developed mild facial paresis associated with intratumoral hemorrhage and tumor enlargement, necessitating reoperation. During the second surgery, latency prolongation of the orbicularis oris Tc-fMEP was again observed from the early stages, reproducing the previous finding, without further intraoperative changes. Facial nerve function improved during the six-month follow-up after the second surgery. In this rare case, we demonstrate reproducible Tc-fMEP latency prolongation suggestive of impaired neural conduction during vestibular schwannoma surgery. Latency prolongation of facial nerve motor evoked potentials, together with amplitude reduction, may represent a potential indicator of facial nerve functional impairment.