Predictive Value of Intra-Operative Neuromonitoring for Hearing Preservation After Open Craniotomy Vestibular Schwannoma Resection: A Systemic Review

Introduction: The Intra-operative neuromonitoring (IONM) is a critical technique to shift the management paradigm of vestibular schwannoma (VS) from tumor resection to cranial nerve function preservation. Intra-operative monitoring of VIIth & VIIIth cranial nerves has been standardized in Congress of Neurological Surgeons (CNS) guideline since 2015. In comparison to the definitive benefit of facial nerve monitoring strategy on post-op functional outcome, the optimal monitoring strategy for hearing preservation remains unclear. Methods: Articles containing “(((vestibular schwannoma) AND (hearing preservation)) AND (surgery)) AND (monitor)” 1985 to 2025 were included. Abstracts were manually reviewed. Exclusion criteria were non-English, no full article available, non-human studies, studies not targeting intra-operative, VIIIth nerve monitoring details, or post-op hearing preservation, not trans-cranial surgery or approaches served as a potential conflict factor for hearing damage (trans-labyrinth, trans-canal, trans-meatal), or not research studies (technique notes, comments, video demonstration, or case report). Results: Among 253 articles, 27 studies, 10 review, and 2 guidelines were included, representing 1422 Vestibular Schwannoma cases, which received HP surgical resection through single or combined middle fossa, suboccipital, retro-sigmoid, and/or retro-labyrinth approaches assisted by IONM. HP criteria included American Academy of Otolaryngology-Head and Neck Surgery hearing class (AAO-HNS) A-B (serviceable), A-C (intelligibility), Hannover Classification 1–4, Gardner and Robertson scale, GR I-II (serviceable, 0–50 dB), modified Sanna A-B. Out of 1309 cases with available data, post-op hearing was preserved in 534 (40.8%) while lost in 775 (59.2%). 3 main IONM technique reported were cochlear nerve action potential (CNAP)/ Direct eighth nerve monitoring (DENM) (229 cases), ABR/BAEP (538 cases), and EChoCG (209 cases). By the end of the tumor resection, PPV/NPV of post-op HP was 67.0%/91.2% for CNAP/DENM, 57.1%/84.0% for ABR/BAEP, and 48.6%/94.2% for EChoCG respectively. 2 studies covering 119 cases further investigated the cutoff threshold and value changes in ABR waves and EChoCG action potential throughout tumor resection. Of note, IONM expressed signals loss even at the beginning of the surgery in 23/98 CNAP/DENM and 40/132 Auditory brainstem response (ABR)/Brainstem auditory evoked potential (BAEP) monitoring, with NPV of post-op HP of 78.3% and 87.5% respectively. Combination of IONM methods as well as improved monitoring technique such as new electrodes design and/or location are also under investigation. ( Table 1 –3). Conclusion: IONM has been recognized to improve post-op HP in VS resection and has been recommended as the standard intra-op assistance. CNAP/DENM showed better Positive Predictive Value (PPV) while Electrocochleography (EChoCG) seemed to offer higher Negative Predictive Value (NPV). Specific parameters of different tests demonstrated uneven significance. Nevertheless, none of the present strategies could offer satisfied predictive value correlating the clinical outcomes. Large sample number studies covering sufficient electrophysiological details will be desired. Standardization of key definition and criteria of data collection and assessment in future studies is also pivotal to promise comparable, valuable results. Publication History Article published online: 27 February 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany