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Preservation of hearing in surgical removal of acoustic neuromas of the internal auditory canal and cerebellar pontine angle

The surgical results in 69 patients with unilateral tumors of the cerebellopontine angle or internal auditory canal in whom total tumor removal was accomplished, and in whom an attempt was made to preserve hearing, are presented. The success rate of preservation of hearing and facial nerve function was correlated with the size of the tumor. Useful hearing, as defined by speech reception threshold no poorer than 70 dB and a discrimination score of at least 15%, was preserved in 73% of cases in which the tumor extension to the posterior fossa was no greater than 0.5 cm. In contrast, useful hearing was preserved in 22% of cases in which posterior fossa extension was greater than 2.5 cm. No significant correlation was found between preoperative evoked responses and success in preservation of hearing. The techniques and value of intraoperative monitoring of electrocochleogram (ECoG) and brain stem evoked response are discussed. A theory of pathogenesis of intraoperative hearing loss, based on correlation of changes in evoked responses and simultaneous surgical events, is presented.