The effect of vagus nerve stimulation on epileptiform activity recorded from hippocampal depth electrodes

Purpose: To assess the effect of vagus nerve stimulation (VNS) on intericta l epileptiform activity in the human hippocampus. Clinical studies have est ablished the efficacy of vagus nerve stimulation in patients with epilepsy (VNS Study Group, 1995), although the electrophysiologic effects of VNS on the human hippocampus and mesial temporal lobe structures remain unknown. Methods: We report a case study in which a patient with an implanted VNS un derwent intracranial electrode recording before temporal lobectomy for intr actable complex partial seizures. Epileptiform spikes and sharp waves were recorded from a depth electrode placed in the patient's left hippocampus. S pike frequencies and sharp-wave frequencies before and during VNS were comp ared using both a 5- and a 30-Hz stimulus. Different stimulation rates were tested on different days, and all analyses were performed using a Student' s t test. Results: We found no significant differences in spike frequency between bas eline periods and stimulation at 5 and 30 Hz. In contrast. stimulation at 3 0 Hz produced a significant decrease in the occurrence of epileptiform shar p waves compared with the baseline, whereas stimulation at 5 Hz was associa ted with a significant increase in the occurrence of epileptiform sharp wav es. Conclusions: VNS produces a measurable electrophysiologic effect on epilept iform activity in the human hippocampus. Although a clinical response to VN S did not occur in our patient before surgery, 30-Hz VNS suppressed interic tal epileptiform sharp waves that were similar in appearance to those seen during the patient's actual seizures. In contrast, 5-Hz stimulation appeare d to increase the appearance of interictal sharp waves.