Abstracts

RATIONALE: We postulate that electrical stimulation of the vagus nerve can lead to alteration of cerebral electrical activity resulting in improvement of seizure control and such changes may be helpful in determining the optimum setting for the vagus nerve stimulator device. METHODS: Twenty patients, aged 4 to 31 years (mean 14.6 ? 6.8 years) with intractable epilepsy and active epileptiform activity on baseline EEGs were recruited in the study. EEGs were performed at baseline, 1, 3, 6 months and one year after implant. Patients without significant EEG changes or <50% seizure reduction at 6 months follow-up were subjected to VNS programming during EEG recording to various different parameters and switched to parameter settings associated with maximum EEG changes. RESULTS: 16 of the 20 patients (Group 1) showed significant EEG changes with clustering and synchronization of epileptiform activity with progressively increasing intervals of spike free epochs. Four patients (Group 2) did not show significant EEG changes. There was significant difference in percent seizure reduction between the two groups (90.58% in Group 1 versus 31.58% in group 2; p<0.0001) at six months follow-up. With programming of vagus nerve stimulator at six months to settings associated with maximum EEG changes in Group 2 patients all showed improvement in seizure control with mean seizure reduction of 79% within 3 months. At one year follow-up 6 of the 20 patients were seizure free and overall seizure reduction of all patients was 91.4% CONCLUSIONS: Effective vagus nerve stimulation results in synchronization of epileptiform activity on EEG with progressively increase in spike-free intervals. These changes take time to develop and likely results from facilitation or modulation of synapses causing synchronization of epileptiform activity. These are followed by periods of cortical inhibition which progressively increase in duration over time. Once these pathways are facilitated it appears that VNS programming at 6 months can cause immediate changes on EEG which can be helpful to determine optimum stimulator setting for each induividual patient.