Abstracts

Rationale: VNS is the first widely used neuromodulation technique for epilepsy. Despite its extensive use in drug refractory epilepsy, it is not entirely clear which patients and what type of seizures benefits most. In this retrospective study, we looked into long term seizure data in a group of patients on VNS and analyzed various patient-related characteristics with seizure outcome.

Methods: We retrospectively reviewed 55 patients on VNS with an average follow up range of 1-10 years. Data about seizure frequency, type of seizures, VNS parameters and anti-seizure medicines were collected. Paired t test was used to compare seizure types and frequency at baseline with subsequent years. Paired t test and ANOVA were used to study relation of various predictors on seizure outcome. Median follow up for the group was 4 years.

Results: Out of 55 subjects, 26 (47.9%) were male. Mean age of seizure onset and VNS insertion were 9.48±9.1 and 33.31±10.5 years respectively. Mean seizure frequency at the time of VNS implantation was 4.54/day (Range .07-40; 95% CI 2.35-6.72), which showed significant reduction yearly for 4 years post VNS implantation (2.9/day at 1 year, 2.48 at 2, 1.61 at 3 and 2.7 at 4 years, p< .05). Among seizure types, only focal to bilateral tonic clonic seizures showed significant decrease in frequency annually for 5 years (.2/day at baseline, .08 at 1 year, .05 at 2, .03 at 3, .03 at 4, .01 at 5 years; all p< .05). Though seizure frequency was not different among lesional and non-lesional subjects (2.5 vs 6.3/ day; p=.07) at the time of VNS insertion, non-lesional group showed significant seizure reduction yearly for 2 years (4.41/ day at 1 year, 3.77 at 2 years; p< .05). Subjects without history of prior surgery had higher number of seizures per day at baseline (5.23 vs 1.76; p< .05) and showed significant seizure reduction yearly for 5 years (p< .05). Seizure frequency was significantly different among the types of epilepsy (focal vs generalized vs unclassified vs both focal and generalized; p< .05). At their baseline, the focal epilepsy group showed significant seizure reduction at 1, 3 and 4 years (2.13/day at baseline vs 1.14 at 1, 1 at 3 and 0.87 at 4 years; p< .05). Subjects with focal epileptiform discharges on EEG had a significant reduction in seizure frequency for 2 years after VNS (1.44/day at baseline vs 0.52 at 1 and 0.39 at 2 years; p< .05). Overall, 3 subjects (5.4%) were seizure free at 3 years follow up, all of whom had focal epilepsy.