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 electrical stimulation. Specifically, the authors sought to 1) characterize auditory and vestibular evoked responses elicited by electrical stimuli delivered directly to the exposed surface of the VIIIth cranial nerve and 2) compare electrically evoked responses elicited in brainstem nuclei with extracranially recorded far-field potentials. Background: Intraoperative monitoring of auditory brainstem responses is useful during cerebellopontine angle surgery. Identification of the vestibular portion of the VIIIth cranial nerve, which traditionally has been performed by physical characteristics and some electrophysiologic properties, is important because the vestibular subdivision in humans is indistinct in approximately 25% of cases. Positive identification of evoked responses emanating from the vestibular nerve would constitute a marked improvement over existing intraoperative techniques that use acoustic stimuli only. Methods: Experiments were performed on 12 anesthetized cats. Electrical pulse stimuli were delivered using a bipolar electrode placed directly on the surface of the exposed VIIIth cranial nerve at several sites. Computer-averaged 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 were 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 colliculus, 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 border of the cochlear and vestibular nerves produced intermediate responses that possessed both cochlear and vestibular characteristics. Conclusion: Direct electrical stimulation of the cochlear and vestibular subdivisions elicits evoked responses with distinctly different wave morphologies. Obtaining electrically evoked responses intraoperatively is feasible and may be of substantial value in the unambiguous identification of VIIIth cranial nerve subdivisions.