Handheld cochlear nerve action potential mapping for real-time functional navigation in vestibular schwannoma surgery

OBJECTIVE Hearing preservation in vestibular schwannoma (VS) surgery remains challenging due to frequent cochlear nerve displacement and the limited reliability of brainstem auditory evoked potentials (BAEPs) once waveforms deteriorate. This study evaluated the feasibility and clinical utility of bipolar handheld cochlear nerve action potential (CNAP) mapping for intraoperative nerve identification and compared its prognostic performance with BAEP monitoring. METHODS In this retrospective single-center cohort, 193 patients with histologically confirmed VS underwent intraoperative auditory mapping between September 2021 and November 2024. Two color-coded bipolar handheld probes were used: one for facial nerve stimulation and the other for near-field CNAP recording along the presumed cochlear nerve course. CNAP responses were considered favorable if waveforms remained stable or decreased by < 50% in amplitude. Functional hearing preservation (American Academy of Otolaryngology–Head and Neck Surgery class A–C) served as the primary endpoint. Logistic regression and receiver operating characteristic (ROC) analyses evaluated predictive performance. RESULTS CNAP mapping identified the cochlear nerve in 83.4% of cases, significantly higher than intraoperative BAEP elicitation (46.1%; p < 0.001). Favorable CNAP responses were strongly associated with hearing preservation (91.4% vs 8.1%, p < 0.001; adjusted OR 37.0, 95% CI 9.28–217.8). The combined CNAP + BAEP model showed excellent discrimination (area under the ROC curve = 0.91) and good calibration (p = 0.24, Hosmer-Lemeshow test). CONCLUSIONS CNAP mapping enabled real-time functional localization of the cochlear nerve and provided complementary intraoperative functional information. While postoperative hearing remains dependent on the structural integrity of the cochlear nerve, particularly in large tumors, CNAP mapping provides complementary functional information strongly associated with hearing preservation.