INTRAOPERATIVE MONITORING OF THE FACIAL-NERVE IN ACOUSTIC NEUROMA SURGERY BY THE ENLARGED MIDDLE CRANIAL FOSSA APPROACH

Acoustic neuroma ablation by the enlarged middle cranial fossa approach results in a low incidence of postoperative facial nerve dysfunction. In 25 consecutive patients with tumors in the cerebellopontine angle with a diameter up to 2.3 cm, intraoperative monitoring of the facial nerve was evaluated. The facial nerve was monitored electrophysiologically with recordings of the spontaneous EMG and electrically evoked potentials (CMAP) from at least three groups of facial muscles. In 4 cases monitoring was helpful for identification and delineation of the facial nerve. The mechanically elicited potentials during tumor preparation and CMAP after electrical stimulation close to the brain stem were compared with the postoperative facial nerve function and the time course of recovery. If the ongoing EMG recordings resulted in potentials below 0.5 mV and the CMAP at the end of tumor preparation was higher than 0.5 mV, facial nerve function was normal in 9 of 10 cases on the first postoperative day. One patient had mild facial palsy. Poor results could be predicted if numerous discharges with amplitudes higher than 0.5 mV and affected CMAP after stimulation near the brain stem were found at the end of surgery. All 3 patients with these findings had facial paralysis at hospital discharge (an average of 9 days after the operation). Recovery from the palsies was found to be prolonged. Intraoperative monitoring of the facial nerve is now used routinely in surgery of the cerebellopontine angle and the internal auditory canal.