180 - Intraoperative Monitoring of Cranial Nerves in Neurotologic Surgery

Abstract

The use of intraoperative electromyography (EMG) for preserving cranial motor nerve (CMN) function involves both mapping and monitoring techniques. Recently the use of corticobulbar motor evoked potentials (CoMEPs) has been developed as an adjunct monitoring method that continuously assesses CMN function. In vestibular schwannoma surgery, CoMEPs provides continuous information about the overall integrity of the facial nerve throughout surgery, specifically before the nerve is exposed and accessible for electrical stimulation. Each technique accomplishes a different target in monitoring; therefore the combination of EMG and CoMEPs increases monitoring advantages and reliability. The best warning sign for deteriorating facial nerve function may not be determined with a single monitoring technique but instead by combining two or more criteria using both EMG and CoMEPs. Further addition of CoMEPs to the conventional monitoring techniques for lower cranial nerve monitoring can help to establish more sensitive criteria for predicting postoperative outcome. It is clear that facial nerve monitoring has had a marked impact on the preservation of function in vestibular schwannoma surgery. Preservation of hearing has been more difficult to achieve because of the more intimate relationship of vestibular schwannoma with the cochleovestibular nerve, but it is now often accomplished, especially in smaller tumors, with the aid of 8th-nerve monitoring. Equally clear is that much room for improvement remains. It is hoped that future research will provide improved techniques in a variety of areas, including the establishment of prognostic criteria based on CoMEPs and its relation to EMG and the relationship between intraoperative recordings and long-term clinical outcome.