Auditory brain stem responses in cerebellopontine angle tumors.

The characteristic features of auditory brain stem responses (ABSR) are described in seven cases of cerebellopontine angle tumors: five acoustic neuromas, one congenital cholesteatoma, and one meningioma. An attempt is made to correlate the N1 response with the degree of auditory nerve involvement, to gain information about the presence of brain stem displacement, and to evaluate the cranial extension of the lesion by recording responses to contralateral stimuli. The empirical nature of deductions based on crossed stimulation is emphasized. In tumors originating from the VIIIth nerve, the ABSR pattern is characterized by a well-defined N1 complex, indistinct or absent additional responses, and prolonged brain stem conduction time (BSCT) on the affected side whenever its assessment is feasible. Recording from the affected side on stimulation of the contralateral side may show normal responses from stations central to the auditory nerve in cases of small-sized tumors. Medium or large tumors may be accompanied by abnormal N2 and/or N3 responses suggesting involvement of these stations.