Electrocochleography During Translabyrinthine Approach for Vestibular Schwannoma Removal.

Abstract

Intraoperative electrocochleography (ECochG) can differentiate hair cell and neural dysfunction caused by a vestibular schwannoma (VS) and help define the site of lesion as peripheral or central to the spiral ganglion. Hearing loss in patients with a VS can be caused by both sensory (cochlear) and neural dysfunction. Round-window ECochG using low and high-frequency tone bursts was performed in 49 subjects with a VS. Responses were analyzed for magnitude, spectrum, and shape of the time waveform. Components examined included the cochlear microphonic, auditory nerve neurophonic, compound action potential (CAP), and summating potential (SP). Variability in the summed response magnitudes across frequency, or "total response" (ECochG-TR), varied from 0.1 to 100 μV. Responses were larger for lower frequencies. Subjective estimates revealed a wide range of neural contributions from the auditory nerve neurophonic to low frequencies and the CAP to high frequencies. Cases with larger CAPs had smaller SPs. The correlation of ECochG-TR, with word recognition score (WRS), was moderate (r = 0.67), as well as the correlation between pure-tone averages and ECochG (r = 0.63). The cochlea remained functional in two cases of sudden sensorineural hearing loss with 0% WRS. Reduced ECochG-TR and neural activity in many cases indicates both sensory and neural deficits. Smaller SPs when CAPs are present indicate a neural contribution to the SP. Good cochlear function with 0% WRS, and cases of sudden sensorineural hearing loss with retained cochlear function, indicate retrocochlear effects, typically proximal to the spiral ganglion cells.