Intraoperative real-time monitoring of brain stem facial evoked response (BFER)

Injury to the facial nerve is of concern in surgery of cerebellopontine angle tumors. The crossed acoustic reflex provides a way to monitor the ipsilateral facial nerve with the auditory stimuli delivered to the contralateral side. Using the method of optimum digital filtering, it is possible to monitor the resulting brain stem facial evoked response (BFER) in real time. This paper presents preliminary experiences in more than 18 such operations monitored using this method. This preliminary study demonstrates a trend for a high (88.8%) correlation between BFER and postoperative facial nerve function. Identical latencies from simultaneous BFER and facial nerve recordings along with findings after facial nerve transection suggest that some portion of the complex BFER waveform derives from facial nerve depolarization.