Congress of Neurological Surgeons Systematic Review and Evidence-Based Guidelines Update for the Role of Intraoperative Cranial Nerve Monitoring in the Management of Patients With Vestibular Schwannomas

BACKGROUND: Intraoperative neuromonitoring (IONM) has become vital in the management of vestibular schwannoma (VS) with the paradigm shift from tumor eradication to functional preservation. Several facial nerve (FN) monitoring strategies have been explored over the past few decades ranging from free-running electromyography, direct nerve stimulation, continuous nerve stimulation, facial motor evoked potentials, blink reflex, and others. Hearing preservation surgery is guided primarily by far-field auditory brainstem response and real-time cochlear nerve action potentials. Given the heterogeneity in tumor and patient factors, it remains very difficult to accurately predict cranial nerve outcomes, regardless of the monitoring strategy. OBJECTIVE: To critically appraise literature regarding IONM during VS surgery and update the previous evidence-based clinical practice guideline. METHODS: This is a systematic review of the literature, incorporating articles from March 2015 to May 2022. Literature published before 2015 that would have been included in the previous Congress of Neurological Surgeons guideline was not searched again in this update. RESULTS: FN monitoring provides better functional outcomes compared with anatomic dissection alone and may guide the extent of tumor resection. While facial motor evoked potentials and free-running electromyography can provide continuous noninvasive FN monitoring, there are insufficient data to determine which more strongly correlates with facial function outcomes. Both electrophysiological data and tumor size correlate with facial function outcomes. The ideal hearing monitoring strategy remains unclear as there are insufficient data comparing cochlear nerve action potentials with far-field auditory brainstem response. All studies were graded as Class III evidence. CONCLUSION: IONM should be used in all VS cases. While the optimal FN and hearing monitoring strategy remains elusive, available data support the use of a combination of strategies, including preoperative tumor size, to maximize sensitivity and specificity. There remains a significant need for high-quality comparative studies to determine which intraoperative monitoring scheme can provide intraoperative guidance and predict postoperative outcome.