Abstracts

Rationale: Several studies demonstrate the efficacy of vagal nerve stimulation (VNS) therapy in populations of patients with refractory epilepsy, yet outcome for individual cases remains unpredictable. Since it is the left vagus nerve that is commonly targeted for VNS therapy,we explored whether there was an asymmetry to the response to VNS dependent on the laterality of the epileptogenic zone (EZ) in focal epilepsy. That is, whether VNS therapy was more effective in left versus right-sided epilepsies, or vice-versa. Methods: Retrospective review of patients who had undergone VNS implantation at our center in the 12-year period 2004-2015 was undertaken. Data collected included age, sex, seizure type, seizure frequency per month prior/after VNS and the lateralization (left or right) of the EZ, the latter identified based on the summary of the clinical findings, neuroimaging and interictal and ictal video-electroencephalographic data. Patients in whom there was insufficient data to make a robust electroclinical diagnosis of a focal (left or right lateralized) or generalized epilepsy, or those with poor follow-up data were excluded. A total of 69 patients met criteria for involvement in the study. Response to VNS therapy as measured by percentage reduction in seizure frequency (numbers of seizures per week or month) was assessed six months after implantation in four categories (I: < 25%, II: 26-50%, III: 51-75%, IV:76-100%). Ordinal logistic regression analysis to explore the relation between EZ lateralization and response type was performed in SPSS. Results: Patient ages ranged from 6-82 years with a mean age of 30. Generalized epilepsy syndromes comprised 44% of all diagnoses. Forty-one patients (59.4 %) had predominantly generalized EEG findings, 16 (23.2%) had predominantly right sided and 12 (17.4%) had predominantly left sided EEG findings. Six months post VNS implantation, 23.2% patients achieved >75% seizure reduction, 29% patients had 51-75% reduction, and another 29% had seizure reduction by 26-50%. Only 18.8% had less than 25% seizure reduction. Lateralization of the presumed EZ did not have any significant effect on seizure reduction frequency (p = 0.74), with EEG lateralization explaining only 7.8% of the variation in seizure reduction frequency. Conclusions: Lateralization of the presumed epileptogenic zone (left hemispheric versus right hemispheric) appears uncorrelated to response to vagal nerve stimulation therapy in our patient group. It is possible that one or more of a larger patient group, different measures of outcome (e.g. frequency of disabling seizures, or numbers of seizure-free days), a longer follow-up period, or interacting explanatory variables (e.g. patient age and EZ lateralization) may yield statistically significant conclusions. These ideas will be pursued in further work aimed at determining the predictors of favorable response to VNS in refractory epilepsy. Funding: No funding was received for the study.