Intraoperative Facial Nerve Monitoring Revealed the Origin of Rapidly Progressing Schwannoma in the Cerebellopontine Angle: A Case of Large Intermediate Nerve Schwannoma.

Schwannoma arising from the intermediate nerve is very rare, with only a few cases reported in the literature. Here, we report a rare case of a rapidly growing schwannoma that originated from the intermediate nerve in a 17-year-old woman. We performed tumor resection using the translabyrinthine approach with continuous intraoperative monitoring, in which the facial nerve root was used to evoke muscle action potential. The monitoring revealed the tumor origin and enabled complete surgical resection with the preservation of facial nerve motor function. The facial nerve root evoked muscle action potential-based continuous monitoring revealed the origin of the rare intermediate nerve schwannoma. Appropriate surgery using this monitoring system could spare facial nerve function even in this rare case and retain the patient's quality of life.