Abstracts

Patients receiving VNS therapy for refractory epilepsy often appear on follow-up to be more alert, attentive, and energetic, independent of the effects of VNS on seizure frequency. This effect has not been fully demonstrated, and potential mechanisms of action have not been explored. We hypothesized that the apparent improvement in attention and energy results from a reduction in IEDs, which may interfere with cognitive processes., A heterogeneous group of 12 subjects with medically refractory, localization related epilepsy was evaluated at baseline, and at 6 months after VNS implantation surgery. Subjects had different epilepsy syndromes, baseline seizure and IED rates, and baseline IQs. During a 6 hour EEG recording session, an extensive battery of neuropsychological tests was administered to test for global cognitive function in several domains, with particular emphasis on attention. EEG spikes were detected with Persyst software, while custom software was used for EEG noise reduction and analysis of power spectrum, coherence, and statistics. Quality of life was examined with the QOLIE-31 questionnaire. Seizure frequency was recorded by subject and caregiver report. The paired Student[apos]s t test or Wilcoxon signed rank test was used for statistical analysis (p[lt]0.05 considered significant)., The group had nonsignificant reductions in seizure frequency (mean 10.0 to 2.4/month, p=0.07) and IED rate (64.1 to 7.7 spikes/hr, p=0.08). There were no significant changes in relative EEG power acutely (VNS-on vs -off) or chronically (baseline vs VNS-off), although there was a trend of decreased delta and increased alpha power after VNS. Coherence did not change acutely with stimulation, but 9 of 12 subjects had significant chronic changes. Quality of life trended toward improvement (QOLIE-31 raw score 52.5 to 61.9, p=0.17).
No changes were seen in any of the cognitive domains examined in standard neuropsychological testing, but the longer lasting of the two computerized tests of sustained attention / vigilance trended toward improvement (Steer clear driving: 7.02 vs. 4.95 cows/min hit, p=0.11). The longer duration sustained attention driving test performance was correlated with baseline IED rate (r2=0.27, p=0.05), and improved performance correlated with decreased IED rate after VNS (r2=0.31, p=0.04)., Although nonsignificant, 6 months of VNS in a small number of subjects resulted in a trend of reduced seizure frequency and IED rate, but had no measurable effect on cognition. The trend of reduced IED rates significantly correlated with improved ability to sustain attention in a longer duration task. Perhaps a larger study on a more homogeneous group of subjects selected for high IED rates and low to moderate baseline cognitive function would show larger changes in cognition., (Supported by Cyberonics, Inc.)