Abstracts

Vagus nerve stimulation (VNS) is one currently used adjunctive neurostimulation therapy for refractive epilepsy. The purpose of this study was to determine the effects of neurostimulation on cortical lateral interactions manifest in the steady-state VEP. A further aim was to compare these effects with those found previously with adjunctive gabapentin treatment in which VEP indices of cortical lateral interactions were reversibly changed. For two chronic VNS patients (VNS therapy [gt] 7 years, both with duty cycles of 30 sec on/48 sec off, stable AED monotherapy regimen with CBZ or TPM, seizure-free [gt] 1 year) steady-state VEPs were recorded with the stimulator turned on (STIM-ON) and off (STIM-OFF) in the same recording session. Two electrodes placed on the neck indicated activity of the VNS stimulator. In the STIM-ON condition, trials (duration: 30 sec) were initiated when the stimulator cycled off as evidenced in the neck recordings or in the EEG. For analysis, the 30 sec trials were divided into 10 sec epochs. We excluded all epochs in which there were artifacts and in which the device cycled on. Twelve age-matched normal subjects served as controls (mean age: 40 years). Stimuli consisted of conventional checkerboards and the radial windmill-dartboard pattern. For each subject, VEP waveforms were Fourier analyzed. First- and second-harmonic response components were used to calculate indices reflecting facilitatory and inhibitory cortical interactions, along with confidence limits on these indices. For the chronic VNS patients, baseline measures of facilitatory and inhibitory response indices were within the range encountered in the normal control group. For these patients, there were no within subject differences in the responses recorded in the STIM-ON versus STIM-OFF conditions. Furthermore, there were no differences in the response variance in these two stimulation conditions. Responses to conventional checkerboard stimuli were also within the normal range and were unchanged in STIM-ON and STIM-OFF conditions. In patients with refractory epilepsy, visual processing as measured by the steady-state VEP, can be used as a tool to assess lateral cortical interactions. Steady-state VEPs can be reliably recorded during VNS neurostimulation and VEP indices of cortical lateral interactions do not appear to be affected by chronic long-term neurostimulation therapy. This is in contrast to our previously reported findings of reversible facilitatory effects in patients with adjunctive gabapentin treatment. (Supported by NIH EY7977.)