A practical framework for interpreting intraoperative electrophysiological signals in acoustic neuroma surgery

To develop a practical, stepwise framework for interpreting intraoperative neurophysiological signals to predict facial nerve outcomes after acoustic neuroma surgery.

This methodological report synthesizes evidence from three IONM modalities: facial motor evoked potentials (FMEPs), direct electrical stimulation (DES), and free-running electromyography (EMG). Key predictive parameters, threshold values, sensitivity, specificity, and technical limitations were extracted from published meta-analyses, systematic reviews, and prospective cohort studies. These parameters were then organized chronologically into a stepwise interpretation framework according to their typical intraoperative use.

An end-of-surgery FMEP amplitude reduction >50% predicts poor long-term (≥1 year) facial function with 93% specificity. A DES threshold ≤0.05 mA at the brainstem stump predicts good long-term function (73% sensitivity, 74% specificity). A cumulative A-train duration >10 s on free-running EMG signals high risk for severe immediate palsy (66.7% sensitivity, 88.0% specificity). The 2026 CNS guidelines confirm that electrophysiological parameters predict long-term outcomes better than clinical information alone.

This report provides a clinically usable, stepwise framework that integrates FMEP, DES, and free-running EMG thresholds for intraoperative prediction of facial nerve function.

Despite routine use of intraoperative neurophysiological monitoring, no standardized framework has existed to translate real-time signals into postoperative facial nerve predictions. This framework fills that gap by offering surgeons and monitoring teams a clear, evidence-based, and chronologically ordered decision aid, potentially improving early recognition of poor outcomes and facilitating timely referral for facial reanimation.