Intraoperative Monitoring Methods for the Lower Cranial Nerves

Abstract

Intraoperative monitoring methods of the lower cranial nerves (from the VII th nerve to the XII th nerve) were reviewed. All of the lower cranial nerves except the VIII th cranial nerve could be monitored intraoperatively using evoked muscle responses. It was essential to use low amplitude monopolar stimulation (less than 0.6mA) for obtaining adequate intraoperative evoked facial muscle response (EFMR). Postoperative facial function was correlated to the FEMR amplitude obtained with stimulation at most proximal sites of the facial nerve in the cerebello-pontine (CP) cistern after removal of the CP angle tumors. When the amplitude was more than 99 microvolts, the patient's facial functions was preserved. Auditory brainstem responses (ABR) were used for intraoperative cochlear nerve monitoring during microvascular decompression surgery for the VII th cranial nerve. As criteria for warning the surgeon, we adopted such changes as the disappearance or marked decrease in amplitude of the wave V. Cochlear nerve compound action potentials (CNAP) were also used for cochlear intraoperative monitoring. Cochlear nerves within the cisternal portion of the VIII th cranial nerve could be identified using CNAPs. CNAP had another advantage when cochlear function remained but could not be monitored by ABR in a case of CP angle tumor. The IX th and the X th cranial nerve functions could be monitored using evoked pharyngeal muscle response (EPMR). More studies are required to determine the precise relationship between EPMR and the IX th and the X th cranial nerve functions. Intraoperative lower cranial nerve monitorings has become am essential method to achieve greater safety and more successful outcomes in posterior fossa surgery.