Electrocochleographic study in patients with vestibular schwannomas and U-shaped audiograms.

To determine the nature of sensorineural hearing loss in the middle-frequency range (U-shaped audiogram), we compared the differences in electrocochleographic findings for 15 ears with vestibular schwannomas and 10 ears without tumours. Short-tone bursts of 0.5, 1, 2, and 4 kHz were used to evoke cochlear microphonics (CM). Ears with tumours had normal or lower CM detection thresholds than ears without tumours. Input-output curves for 1-kHz frequency were normal in 10 ears with tumours and in 1 ear without tumours. These indicate that tumour ears have no or mild cochlear dysfunction. In addition, CM detection thresholds of ears with tumours were lower than audiometric thresholds, particularly at the 1- and 2-kHz region. These findings suggest that the loss seen by audiometry in ears with vestibular schwannomas was from a retrocochlear component.