Considerations in the differentiation between brainstem lesions and sensori-neural hearing loss using auditory nerve-brainstem evoked responses.

When auditory nerve-brainstem evoked responses are used in neurological diagnosis, an important response parameter to be evaluated is brainstem transmission time - the time interval between the auditory nerve response (wave 1) and the response from the rostal brainstem (wave P4). Occasionally the only response wave present is wave P4 and its latency is prolonged. This can be compatible with a conductive or a sensorineural hearing loss (SNHL), or a neurological (brain stem) lesion such as a retrocochlear hearing loss (RCHL). A conductive hearing loss can be excluded by otoscopic, audiometric and tympanometric examination. In order to differentiate between SNHL and RCHL, several techniques have been suggested in order to enhance the visibility of wave 1, but these are not always applicable or successful. In this study, P4 latency was plotted as a function of the audiometric hearing loss at 4 kHz for 26 ears with SNHL only. From this, the correlation coefficient (r = 0.71) was calculated along with the linear regression equation and the standard error of the prediction. This gave an estimation of the range of P4 latency to be expected for a given SNHL for comparison with the actual latency obtained in patients with neurological complaints. An actual latency greater than the expected latency range would be considered corroboratory of a RCHL. Several clinical examples are presented. These considerations also point out the need to bear in mind the otological context of these evoked responses even when used in neurological diagnosis.