Abstracts

In contrast to well known VNS group responder rates, less is known if individual patient response (IPR) is maintained during long-term follow-up. IPR was examined at 3, 6 and 12 month visits in a constant 12-month patient cohort of 1525 patients on add-on VNS therapy collected from the Cyberonics[trade] company registry. Response was defined as 50% or more average seizure reduction per month versus pre-VNS baseline. The group response to VNS therapy in 1525 patients increased from 711 (47%) at 3 months, to 806 (53%) and 845 (55%) at 12 months of VNS therapy. When IPR was compared at two consecutive 6 month intervals at 6 and 12 months visits, IPR was maintained in 628 responders (41%) and in 502 nonresponders (33%). However, in 178 patients (12%), a response shift from responder to nonresponder was noted, while 217 (14%) improved from nonresponder to responder. Limitations of this uncontrolled study include variable co-medication and a potential selection bias of the registry. The mechanism for the decline of VNS response needs further study, but may include development of tolerance to the effect of VNS in a small subgroup of patients. Individual patient response to VNS is maintained or improved over 12 months in 88% of patients. However, a decline of response is seen in a small subgroup of 12% of patients. The results also indicate that maintained or improved group response is not sufficient to exclude decline of individual response in a small subgroup of patients. (Supported by Cyberonics Europe, S.A.)