Abstracts

Objective: At the end of this activity the participants should be able to recognize loss of seizure control as a possible precursor to end of service of the vagal nerve stimulator battery.
Vagal nerve stimulation (VNS) has been shown to be a safe and effective adjunctive therapy for medically intractable epilepsy. Over 10,000 patients have been implanted with the NeuroCybernetics Prosthesis (NCP) worldwide. However, accurately determining whether the battery is approaching end of service can be difficult. Older models (NCP model 100 with serial number below 10,000) rely on chart calculations. Newer models (NCP model 100 serial number above 10,000 or model 101) have an elective replacement indicator. We propose there is a clinical pattern that suggests the battery is approaching end of service or is at least insufficient to deliver the required output to maintain seizure control.
METHODS: A retrospective chart review was performed of the first 22 patients who underwent battery replacements at USC Medical Center between July 1998 and November 2001. Monthly seizure counts were compiled from time of implantation to the time of replacement. The average monthly seizure count over the three month period prior to replacement was then compared to the average monthly seizure count over the six month period of best seizure control.
RESULTS: There were 23 battery replacements and one mortality in these 22 patients (14 males and 8 females). Eight patients were excluded: four subjects had insufficient seizure calendars, two patients were VNS non-responders, one patient had two replacements due to high lead impedance, and one patient had significant medication non-compliance. Of the remaining 14 battery replacements, 8 were performed due to clinical deterioration or status epilepticus (n=3). The one mortality was due to an additional case of status epilepticus. Of these 9 adverse events, 5 (56%) had an average monthly increase in seizures between 30-100% in the prior three month period, 3 (34%)had an increase over 100%, and one patient had no increase. Six battery replacements were done prophylactically. None of these patients had a seizure increase greater than 30%.
CONCLUSIONS: VNS is relatively safe as adjunctive therapy for intractable epilepsy. Although, accurately determining if the VNS battery is approaching end of service or generating sufficient output for the device settings selected may be difficult. Of the 14 battery replacements reviewed, 9 adverse events were identified. Adverse events included seizure exacerbation, status epilepticus, or death. Eight out of nine patients (89%) who experienced an adverse event showed a 30% or greater increase in seizure frequency in the prior three months.
Our institution tends to use higher duty cycles and rapid cycling. It may be that patients at these settings are more sensitive to reductions in battery output before complete battery depletion. To ensure patient safety when utilizing VNS therapy, a clinical judgment should supercede device diagnostics. When seizure control is lost and no other provoking factors can be identified, one should procede with VNS battery replacement. Long term prospective studies evaluating clinical outcomes with thorough explanted battery testing is warranted.
(Disclosure: Grant - Cyberonics, Honoraria - Cyberonics)