NEW PERSPECTIVES IN INTRAOPERATIVE FACIAL-NERVE MONITORING WITH ANTIDROMIC POTENTIALS

Electromyographic (EMG) activity recorded from the facial muscles in r esponse to electrical and mechanical stimulation is the most popular m ethod for continuous monitoring of the facial nerve during cerebellopo ntine-angle surgery. EMG recording is, however, extremely sensitive to the administration of neuromuscular blockers. An alternative techniqu e for the continuous monitoring of the facial nerve [i.e., monopolar r ecording of facial nerve antidromic potentials (FNAPs)], is described. Ten subjects undergoing retrosigmoid vestibular neurectomy for Menier e'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 f rom 0 to 10 mA with a delivery rate of 7/s. Antidromic potentials were recorded with a silver-wire monopolar electrode positioned intracrani ally on the proximal portion (root entry zone) of the acoustic-facial bundle. Bipolar recordings with two silver electrodes were also perfor med from the trigeminal and facial nerves in the cerebellopontine angl e to define the specific origin of the action potentials. FNAP amplitu de increased as a function of stimulus intensity. The average latency was 3.35 ms (range, 3.0-3.7 ms). Action potentials recorded intracrani ally during electrical stimulation of the marginal nerve originated sp ecifically from the facial nerve. Changes in latency and amplitude of FNAPs were analyzed as a function of the main surgical steps in patien ts operated on for acoustic neuroma. FNAP monitoring provided quantita tive real-time information about damaging maneuvers performed on the n erve and postoperative facial function.