The latency of auditory nerve-brainstem responses in sensorineural hearing loss

The use of auditory nerve-brainstem responses in differential diagnosis of hearing loss is based on several properties of these responses including response latency. The auditory nerve response latency has been shown to be prolonged in conductive hearing loss. The latency of the brainstem responses is also often prolonged in retrocochlear hearing loss. However, the effect of sensorineural hearing losses on auditory nerve response latency is not clear. Several authors report that response latency is prolonged in sensorineural loss, whereas others claim that it is unchanged. To study this, auditory nerve-brainstem responses to 75 dB HL clicks were recorded in normal-hearing subjects and in those with various degrees of high-frequency sensorineural hearing loss. In the more extreme hearing losses, the auditory nerve response could not be seen in the response trace, so the latency of the earlobe positive wave from the region of the inferior colliculus was considered as mirroring auditory nerve response latency, since the time interval between these two waves has been shown to be constant. The average latency of the more severe hearing loss group (more than 40 dB hearing loss at 4kHz) was found to be only 0.35 ms longer than that of the normal-hearing group. This value is smaller than that seen in most conductive and retrocochlear hearing losses. This result warrants continued use of prolonged auditory nerve response latency (greater than 0.35 ms) as an indicator of conductive hearing loss. Possible explanations for smaller latency prolongation than expected of the auditory nerve response in sensorineural hearing loss are discussed based on the properties of single auditory nerve fibers.