Abstracts

Rationale: A common clinical observation in patients being evaluated for epilepsy surgery is that seizures may be atypical or less frequent after implanting intracranial electrodes. This “implant effect” has been variously attributed to anesthesia, the craniotomy or the mechanical effect of the electrodes. The purpose of this study was to quantify the sub-acute and long-term electrocorticographic changes at the epileptogenic focus after implanting depth and subdural electrodes.Methods: Sub-acute and long-term electrocorticographic (ECoG) changes were assessed in subjects with epilepsy who were chronically implanted with depth and/or subdural electrodes. All subjects were participating in a two-year, double-blind, randomized, sham-stimulation controlled trial of a responsive neurostimulator as an adjunctive treatment for medically intractable partial onset seizures (RNS® System, NeuroPace). ECoG records stored by the neurostimulator based on time of day (referred to as “scheduled” ECoG records) were analyzed in subjects who had at least 100 scheduled ECoG records stored over at least one year. Overall power, normalized power within the delta (0-4 Hz), theta (4-8 Hz), alpha (8-12 Hz), beta (12-25 Hz), low gamma (25-50 Hz), and high gamma (50-125 Hz) frequency bands, and spike rate were calculated for each record. Month-to-month differences were assessed for each ECoG channel, and group statistics were calculated by averaging the within-channel results.Results: The analysis included 96,162 scheduled ECoG records from 126 patients with an average of one ECoG record per day per person. There were significant month-to-month changes in overall power, normalized power within frequency bands, and spike rate that were most pronounced in the first 3 to 5 months after implant. There was a significant change in over half (55%) of all the ECoG channels from the first to the second month (p < 0.05, two-sample t-test), including 68% of the channels recorded from strip lead electrodes and 47% of the channels recorded from depth lead electrodes. After 5 months, the overall power became more stable, with significant month-to-month changes seen on average in less than 25% of the channels recorded from strip lead electrodes and less than 20% from depth lead electrodes. Similar patterns of changes were observed within frequency bands as well as for spike rate.Conclusions: These results indicate that ECoG data collected in the first weeks and months after implanting depth or subdural electrodes are not stable and may not be representative of the chronic state.