Vagus nerve stimulation for epilepsy: is output current correlated with acute response?

Objectives - Vagus nerve stimulation (VNS) is an effective treatment for intractable epilepsy. It is unknown whether acute response is correlated with the amplitude of output current. The purpose of this study was to determine if the output current of VNS is correlated with percent reductions in seizure frequency and response. Materials and methods - Retrospective analysis of a multicenter randomized trial of three unique paradigms of VNS was carried out in patients with intractable partial onset epilepsy. Output current at 1 and 3 months was correlated with percent reduction in seizure frequency and response rates. Results - Sixty-one subjects were enrolled and completed the study. Output current, ranging from 0.25 to 1.5 mA, was not correlated with reductions in seizure frequency, or with >= 50% reduction in seizures. Six of seven initial non-responders did experience >= 50% reductions in seizures after current was increased. Conclusions The output current is not a major determinant of acute response to VNS for epilepsy. Many patients respond to low current (< 1 mA). Some (20%) initial non-responders may respond to an increase in output current.