Intraoperative electrophysiologic identification of the nervus intermedius

Objective: Although enormous attention has been directed to the localization and preservation of the facial nerve in acoustic neuroma surgery, the nervus intermedius has largely been ignored. In this article, we describe a method for intraoperative electrophysiologic identification of the nervus intermedius. Study Design: Retrospective case review. Setting: University hospital (tertiary care center). Patients: Thirty-three patients who underwent intraoperative facial nerve monitoring for various cerebellopontine angle procedures. Recording electrodes were placed in the orbicularis oculi and orbicularis oris muscles. A constant-voltage stimulator was used to stimulate both the facial nerve and the nervus intermedius. Interventions: None. Main Outcome Measure: Electrophysiologic response after stimulation of the nervus intermedius. Results: Stimulation of the nervus intermedius produced a long-latency, low-amplitude response recorded only on the orbicularis oris channel. The response had a mean threshold of 0.4 V, a mean latency of 11.1 ms, and a mean amplitude of 11.1 mu V, all significantly different from responses to stimulation of the facial nerve. Conclusion: Knowledge of electrophysiologic features of nervus intermedius stimulation can help protect the facial nerve during cerebellopontine angle surgery. The surgeon must recognize that stimulation of the nervus intermedius can cause electromyographic activity in the facial nerve monitoring channels, but the main trunk of the facial nerve may lie in an entirely different location in the cerebellopontine angle.