Abstracts

RATIONALE: Vagus nerve stimulation (VNS) is an adjunctive therapy for patients with refractory epilepsy and utilizes intermittent electrical pulses delivered to the left cervical vagus nerve. Limited capability exists to determine when VNS battery deterioration becomes clinically significant. Initial generator models employed batteries lasting between 2-5 years. We evaluated 14 patients following VNS re-implantation to examine the clinical course observed during generator end of service (EOS).
METHODS: We evaluated 9 males and 6 females with medically refractory epilepsy with a mean age of 25.1 years. These fifteen patients had epilepsy for a mean of 17.9 years and 14/15 underwent re-implantation with VNS after reaching EOS. Ten patients had partial epilepsy (PE) and 5 patients had symptomatic generalized epilepsy (SGE) failing a mean of 11.3 AEDs. Four patients had epilepsy surgery (1 had successful surgery following VNS) including 2 callosotomies and 2 extratemporal topectomies. Re-implantation was performed after a mean of 2.64 years in 14 patients. Clinical symptoms prior to VNS re-implantation were examined potentially denoting EOS. Symptoms were assessed with a 12-question survey given to the patient or primary caretaker following replacement. VNS stimulus parameters were compared before and after re-implantation.
RESULTS: Seventy-two patients were implanted with VNS. Fifteen patients were suspected to be near EOS and candidates for re-implantation. One was found in follow-up to be at EOS with an inability to interogate the generator. EOS was predicted based upon the duration of use and stimulus parameters. Eight of 9 PE patients and 1/5 SGE patients had complaints of altered VNS function. Five patients with SGE were non-verbal.
An increase in seizures was the most frequent sign and was noted in 8/14 (57.1%). A change in seizure pattern was noted in 6/14 (42.9%), and seizure intensification was observed in 4/14 (28.6%). Less intense, missed, or erratic stimulation was noted by 6/14 (42.9%). Painful stimulation and behavioral worsening each occurred in 2/14 (14.3%). Following re-implantation, improvement of stimulator function was noted by 9/14 (64.3%), with a greater stimulus intensity perceived in 6/14 (42.9%) and improved stimulus regularity and magnet consistency in 5/14 (35.7%). Overall, stimulus current averaged -0.48 ma less when compared to initial parameters. All patients (or their caretakers) felt surgical re-implantation should be performed before EOS became clinically evident.
CONCLUSIONS: We conclude that clinical features may become evident in patients using VNS for refractory epilepsy that signify battery deterioration heralding EOS. Seizure increase or change in seizure pattern was the most frequent sign observed in our series. Re-implantation yielded more intense stimulation after replacement suggesting a graded battery decay and prompting a reduction in re-implant curent intensity. Battery replacement before EOS becomes clinically evident appears desirable from a patient perspective.