Mapping the VIIIth cranial nerve by electrical stimulation: Methods for differentiating auditory from vestibular responses

Hypothesis: The goal of this study was to map the VIIIth cranial nerve by e lectrical stimulation. Specifically, the authors sought to 1) characterize auditory and vestibular evoked responses elicited by electrical stimuli del ivered directly to the exposed surface of the VIIIth cranial nerve and 2) c ompare electrically evoked responses elicited in brainstem nuclei with extr acranially recorded far-field potentials. Background: Intraoperative monitoring of auditory brainstem responses is us eful during cerebellopontine angle surgery. Identification of the vestibula r portion of the VIIIth cranial nerve, which traditionally has been perform ed by physical characteristics and some electrophysiologic properties, is i mportant because the vestibular subdivision in humans is indistinct in appr oximately 25% of cases. Positive identification of evoked responses emanati ng from the vestibular nerve would constitute a marked improvement over exi sting intraoperative techniques that use acoustic stimuli only. Methods: Experiments were performed on 12 anesthetized cats. Electrical pul se stimuli were delivered using a bipolar electrode placed directly on the surface of the exposed VIIIth cranial nerve at several sites. Computer-aver aged evoked responses were recorded from far-field electrodes placed on the scalp and from near-field electrodes stereotaxically positioned in or near the inferior colliculus and abducens nucleus. Results: Latencies and morphologies of waves recorded in brainstem nuclei w ere compared with those of waves recorded extracranially. Direct electrical stimulation of the cochlear nerve elicited a four-wave, auditory brainstem response-like extracranial response, strong activity in the inferior colli culus, and weak activity in the abducens nucleus. Direct stimulation of the vestibular nerve produced a two-wave extracranial response, weak inferior colliculus activity, and strong abducens activation. Stimulation at the bor der of the cochlear and vestibular nerves produced intermediate responses t hat possessed both cochlear and vestibular characteristics. Conclusion: Direct electrical stimulation of the cochlear and vestibular su bdivisions elicits evoked responses with distinctly different wave morpholo gies. Obtaining electrically evoked responses intraoperatively is feasible and may be of substantial value in the unambiguous identification of VIIIth cranial nerve subdivisions.