Intraoperative Mapping of Vestibular Nerve Subdivisions by Electrical Stimulation

Vestibular evoked myogenic potentials (VEMPs) are neck-muscle EMGs which are evoked by high-level acoustic stimuli. Recent studies suggest that they are related to saccular hair cell activity and, thus, to inferior vestibular nerve fiber depolarization. The aim of the present study was to investigate if VEMPs, induced by direct electrical stimulation of the human vestibular nerve, are suitable for the intraoperative discrimination between the inferior and superior part of the vestibular nerve. The responses were compared to acoustically evoked VEMPs to evidence the vestibulocollic reflex arch and their saccular origin, respectively. Eight subjects were stimulated at the inferior branch of the vestibular nerve during cerebello-pontine angle surgery. Subdermal needle electrodes were placed in the middle of the sternocleidomastoid muscle (SCM) of both sides. The EMG signals of the SCM were recorded upon bipolar electrical stimulation (0.4–1.0 mA; 0.2 ms duration; 4.7Hz). All subjects showed normal VEMPs upon acoustic stimulation with a latency (±SD) of 12.8±1.4 ms for P13, and 22.7±2.0 ms for the N23 preoperatively. Upon direct electrical stimulation of the inferior vestibular nerve (IVN), a linear correlation of the response amplitudes of VEMPs and the stimulus was found between 0.4 mA –1.0 mA. The mean latency of the positive peak was 9.1±2.2 ms and 13.2±2.3 ms for the negative one. No contralateral SCM response was found. Electrical stimulation of the superior vestibular nerve did not result in any ipsi- or contralateral EMG response of the SCM. The method can be utilized to map VIIIth nerve subdivisions and to intraoperatively monitor IVth nerve integrity in a real-time mode.