New perspectives in intraoperative facial nerve monitoring with antidromic potentials.

Electromyographic (EMG) activity recorded from the facial muscles in response to electrical and mechanical stimulation is the most popular method for continuous monitoring of the facial nerve during cerebellopontine-angle surgery. EMG recording is, however, extremely sensitive to the administration of neuromuscular blockers. An alternative technique for the continuous monitoring of the facial nerve [i.e., monopolar recording of facial nerve antidromic potentials (FNAPs)], is described. Ten subjects undergoing retrosigmoid vestibular neurectomy for Meniere's disease and 11 subjects operated on with acoustic neuroma surgery via a retrosigmoid approach (tumor size range, 12-28 mm) participated in the investigation. Bipolar electrical stimulation of the marginalis mandibulae was performed to elicit FNAPs. Stimulus intensity ranged from 0 to 10 mA with a delivery rate of 7/s. Antidromic potentials were recorded with a silver-wire monopolar electrode positioned intracranially on the proximal portion (root entry zone) of the acoustic-facial bundle. Bipolar recordings with two silver electrodes were also performed from the trigeminal and facial nerves in the cerebellopontine angle to define the specific origin of the action potentials. FNAP amplitude increased as a function of stimulus intensity. The average latency was 3.35 ms (range, 3.0-3.7 ms). Action potentials recorded intracranially during electrical stimulation of the marginal nerve originated specifically from the facial nerve. Changes in latency and amplitude of FNAPs were analyzed as a function of the main surgical steps in patients operated on for acoustic neuroma. FNAP monitoring provided quantitative real-time information about damaging maneuvers performed on the nerve and postoperative facial function.