Scalp-recorded evoked potentials as a marker for afferent nerve impulse in clinical vagus nerve stimulation.

BACKGROUND Vagus nerve stimulation (VNS) is a palliative treatment for drug resistant epilepsy for which the efficacy and safety are well established. Accumulating evidence suggests that ascending vagal signals modulate abnormal cortical excitability via various pathways. However, there is no direct evidence for an ascending conduction of neural impulses in a clinical case of VNS. OBJECTIVE We recorded and analyzed the short-latency components of the vagus nerve (VN) evoked potential (EP) from the viewpoint of determining whether or not it is a marker for the ascending neural conduction. METHODS EPs within 20 ms were prospectively recorded simultaneously from a surgical wound in the neck and at multiple scalp sites during implantation surgery in 25 patients with drug-resistant epilepsy. Electrical stimulation was delivered using the clinical VNS Therapy system. A recording was made before and after a muscle relaxant was administered, when changing the rostrocaudal position of stimulation, or when stimulating the ansa cervicalis instead of the VN. RESULTS The short-latency components consisted of four peaks. The early component around 3 ms, which was most prominent in A1-Cz, remained unchanged after muscle relaxation while the later peaks disappeared. Rostral transition of the stimulation resulted in an earlier shift of the early component. The estimated conduction velocity was 27.4 ± 10.2 m/s. Stimulation of the ansa cervicalis induced no EP. CONCLUSIONS The early component was regarded as directly resulting from ascending neural conduction of A fibers of the VN, probably originating around the jugular foramen. Recording of VN-EP might document the cause of treatment failure in some patients.