Cochlear microphonic potentials in patients with vestibular schwannomas.

We evaluated the cochlear function of 44 patients with unilateral vestibular schwannomas by measuring cochlear microphonic potentials (CM) in electrocochleography. We used short tone bursts with 0.5-, 1-, 2-, and 4-kHz frequencies as acoustic stimuli to evoke CMs, and determined CM detection thresholds. The CM detection thresholds in vestibular schwannomas ranged from normal to no response. Cochlear dysfunction was seen in 35 (79.5%) of the 44 cases; these patients had higher CM detection thresholds than normal subjects. Although a significant correlation was found between mean hearing level (0.5-4 kHz) and mean CM detection threshold (0.5-4 kHz), 20 patients had lower CM detection thresholds than were found using audiometry. This suggests that there is an additional retrocochlear component responsible for this hearing loss.