Intraoperative monitoring of the electrocochleogram and the preservation of hearing during acoustic neuroma excision.

We have monitored the electrocochleogram (ECochG) of 24 patients, using a transtympanic electrode, during acoustic neuroma excision. All patients had unilateral tumours with good preoperative hearing and complete excision was achieved in each case. Of the 24 patients, seven retained some hearing, however, a further two patients had normal ECochG waveforms at the end of operation but were nevertheless deaf. Thus, there is not an invariable correlation between immediate preservation of the ECochG and hearing. As expected, tumour size was important in hearing preservation. Five of seven patients with tumours less than 1.5 cm in diameter retained some hearing after operation, whereas 15 of 17 patients with tumours greater than 1.5 cm in diameter were deaf.