Abstracts

RATIONALE: Vagal nerve stimulation reduces epileptic seizures, and a device for electrical stimulation of the left vagus nerve (Neuro-Cybernetic Prosthesis, NCP; Cyberonics, Inc.) has been approved by the U.S. FDA for treatment of epilepsy. However, little is known about the mechanisms by which vagal nerve stimulation controls seizures. In some patients the antiepileptic effect is very dramatic, while in others it is not effective. The antiepileptic effect may depend on the type of nerve fibers stimulated by the device. METHODS: We recorded compound action potentials from the human vagus nerve exposed at surgery for implantation of a NCP for control of epilepsy. RESULTS: Stimulation of the vagus nerve elicited compound action potentials with A, B, and C waveforms identified according to their conduction velocities. Waveforms were recorded with a Nihon Koden Neuropack and stimulated with the Neuropack or with the NCP. The stimulation intensities and duration used clinically activate all three-fiber types. In many subjects even the lowest stimulus duration and intensity available from the NCP is capable of eliciting obvious A fiber and C fiber responses. There was no evidence of decrement in response with repetitive stimulation, but we were limited to 5 Hz stimuli. CONCLUSIONS: A, B and C fibers are stimulated in the left vagus nerve with the Neurocybernetic Prosthesis. No decrement of the response is seen with 5 Hz repetitive stimulation. Stimulus-response curves for these three fiber types can be generated; we are currently investigating whether seizure control is associated with selective fiber type activation.