Quantitative parameters of intraoperative electromyography predict facial nerve outcomes for vestibular schwannoma surgery

OBJECTIVE: Facial nerve monitoring is an established method that is routine ly used during cerebellopontine angle tumor surgery. The aim of this study was to determine quantitative electromyographic (EMC) parameters that were predictive of facial nerve outcomes. METHODS: In 137 patients with intra-/extrameatal vestibular schwannomas, th e most proximal (the exit from the brainstem) and distal (the fundus of the internal auditory canal) parts of the facial nerve were stimulated after t otal tumor removal. A quantitative analysis of absolute values and ratios ( proximal/distal) of evoked EMC parameters (amplitude, latency, and duration ) was performed, and parameters were correlated with postoperative (1 and 6 wk and 6 mo) facial nerve function (FNF). RESULTS: Absolute values of EMG amplitudes were statistically correlated wi th FNF (P < 0.05). Amplitude ratios (proximal/distal) demonstrated an even greater predictive power. The risk of exhibiting facial palsy 6 months afte r surgery increased from 1.6% (amplitude ratio of >0.8) to 75% (ratio of <0 .1). For EMG latencies, only the ratios revealed a significant correlation with FNF. The latency ratio-dependent risk of facial palsy after 6 months i ncreased from 2.9% (ratio of <1.05) to 33% (ratio of >1.35). The durations of the muscle responses were not significantly correlated with clinical out comes. CONCLUSION: The predictive power of the amplitudes and latencies of electri cally evoked muscle responses could be improved by calculating proximal/dis tal ratios, The proximal/distal amplitude ratio proved to be the most power ful parameter for intraoperative assessment of postoperative FNF.