Estimating Long-Term Vagus Nerve Stimulation Effectiveness: Accounting for Antiepileptic Drug Treatment Changes.

OBJECTIVE: To investigate the effectiveness of vagus nerve stimulation (VNS) in combination with pharmacological therapy in a longitudinal retrospective study at a single center.

MATERIALS AND METHODS: Data from 130 consecutive patients implanted with a VNS device between the years 2000 and 2013 was analyzed. Seizure frequency and pharmacological antiepileptic drug (AED) treatments were recorded prior to as well as at one, two, and five years after VNS implantation.

RESULTS: Median age at epilepsy onset was five years and mean years from diagnosis to VNS implantation was 16.5 years. There was a significant seizure reduction overall (all p < 0.001). The responder (≥50% seizure frequency reduction) rate increased from 22.1 to 43.8% between the first and fifth year for the cohort as a whole, with the largest increase between the first and second year (22.1-38.1%) and regardless of AED changes. VNS effectiveness did not differ between patients who altered or remained on the same AEDs. Patients were treated with a median of three AEDs throughout the study and the number of AEDs significantly increased after two (p = 0.007) and five (p = 0.001) years.

CONCLUSIONS: VNS is a well-tolerated palliative neuromodulatory treatment for drug resistant epilepsy with a 43.8% seizure reduction after five years. Our data supports the idea that VNS effectiveness increases with time. Therefore we suggest that VNS should be evaluated for at least two years after implantation. AED changes should try to be kept to a minimum during evaluation in order to determine the effectiveness of VNS.