Abstracts

Rationale: There has been no direct marker to verify the afferent propagation of nerve impulses in the clinical setting of vagus nerve stimulation (VNS) Therapy. We established a method for recording the vagus nerve evoked potential (VN-EP) during implantation of the VNS Therapy system. We describe its electrophysiological properties and discuss the significance of measuring VN-EP in VNS Therapy in the clinical setting.Methods: Vagus nerve evoked potentials were measured in 12 patients who underwent implantation of a VNS Therapy system for drug-resistant epilepsy at our institution (4 F/8 M; median age: 14.5 y/o). After induction of general anesthesia, scalp EEG electrodes and a ground electrode around the upper neck were set. After placing the VNS lead around the left vagus nerve, VN-EP was recorded while adjusting the stimulation parameters (current: from 0.25mA to 1.5mA, pulse width: from 130 to 500) and adjusting the location of the VNS lead on the nerve. Recordings were triggered by stimulation current detected by needle electrodes placed in the cervical wound and averaged 1000 times. Recordings were made before and after administration of a muscle relaxant.Results: A reproducible positive peak around 3.41 msec was detected in Cz-A1 in all patients, which remained after administration of the muscle relaxant. Meanwhile no significant early peaks were detected in Cz-A2, Cz-C3, or Cz-C4. The latency of the peak was shortened by moving the lead anteriorly. Although high-amplitude EPs around 7.0-10.0 msec were detected in many channels, all signals disappeared with the addition of the muscle relaxant. No EPs were detected when the lead was placed around the ansa cervicalis.Conclusions: The peak detected around 3.41 msec was regarded as an afferent VN-EP that originated around the skull base or brainstem because it had a short latency, did not disappear with the addition of the muscle relaxant, and was maximized in Cz-A1. The later potentials were regarded as the laryngeal myogram. This study is the first to demonstrate direct evidence of upward propagation of vagus nerve impulses in the clinical setting with VNS Therapy. The recording of VN-EP provides basic information on electrophysiological properties of the vagus nerve and serves as confirmatory evidence for appropriate placement of the VNS lead during surgery. Furthermore, if similar recordings can be accomplished in the awake setting, they may serve as a surrogate marker of vagus nerve propagation during VNS Therapy simulation, which may assist in fine tuning simulation parameters.