The problems of differentiating cochlear dysfunction from VIII nerve lesions have received major emphasis during the past several years. In fact, this area has been emphasized to the extent that some audiologists have limited their concept of differential audiology primarily to those tests that assist in localizing the defect within the sensorineural mechanism. The neurologist's interest in sensorineural hearing loss is with regard to the possibility of a cerebellopontine angle tumor. Although many referrals for audiological evaluation are made for this reason, we must emphasize that even the more sophisticated special auditory tests cannot determine the specific pathology underlying the disorder. An MRI may indicate the presence of an abnormality somewhere in the nervous system, but it does not necessarily define the nature of the pathology. The audiological tests, however, highlight patterns of auditory behavior that are generally associated with cochlear or neural involvement.

Routine pure tone and speech testing can yield valuable information on the site of lesion during the initial phase of the differential audiologic study. For example, a pure tone configuration, which is often seen in patients with a presumptive diagnosis of Meniere's disease, is a unilateral hearing loss most pronounced in the low frequency range. In sharp contrast, patients with VIII nerve lesions frequently present a unilateral hearing impairment most evident in the high frequencies and poor speech discrimination. Although such generalizations may describe a substantial number of cases falling into these two categories, numerous exceptions are encountered with either cochlear or neural pathology. Measures, such as tone decay, acoustic reflex measures, acoustic reflex decay, and speech discrimination at high intensity levels must be used to distinguish between VIII nerve, extra-axial and intra-axial brainstem dysfunction.