Brainstem auditory evoked response in the diagnosis of inner ear injury in the horse.

Brainstem auditory evoked response (BAER) testing was done to evaluate inner ear/VIIIth cranial nerve (CN8) function in the horse. The BAER test consisted of stimulating the auditory system with clicks and recording far-field responses of the brainstem auditory components via cutaneous electrodes and a signal averaging system. The normal response was shown to be a series of waves occurring within the first 10 msec after the stimulus click. Functional loss of the auditory receptor organ (cochlea) or CN8 results in loss of the entire response on the side of the injury. Because of the anatomic relationships of the peripheral auditory and vestibular systems, trauma to one will injure the other. Therefore, auditory testing (BAER tests) may be used to advantage in the diagnosis of peripheral vestibular disease. The BAER test was used in a horse that had signs suggestive of vestibular dysfunction or a brain lesion. The test helped to demonstrate a unilateral inner ear/CN8 lesion and to discount the probability of a more central lesion.