[Peroperative monitoring of early auditory evoked potentials]

Brainstem auditory evoked potentials (BAEP) were monitored during 37 neurosurgical operations (acoustic neurinomas with preoperative useful hearing, microvascular decompression of cranial nerves for hemispasm or trigeminal neuralgia, cerebellopontine angle tumors other than acoustic neurinomas, brainstem tumors and posterior circulation surgery). Intraoperative BAEPs were unchanged in 13 patients. Transient BAEP alterations (delay of I-V interval, transient obliteration of BAEP for as long as 8 minutes and 20 minutes) were seen in 13 other patients; irreversible BAEP alterations (loss of evoked response in 6 patients, delay of I-V interval) were seen in 11 patients. BAEP stability or alterations have been correlated with the ongoing surgical maneuver, the neurological outcome and the postoperative auditory function. BAEPs were found to be good predictors of post-operative auditory function but poorer predictors of neurological outcome. Some alterations are strictly associated with surgical retraction or with eighth nerve manipulation either immediately after the surgical maneuver or several minutes later. No detectable cause of BAEP changes was found in a few cases. The value of this monitoring is discussed. It may also help elucidate the mechanisms of hearing loss in acoustic neurinoma surgery.