The implantation effect: delay in seizure occurrence with implantation of intracranial electrodes

Objective: A transient decrease in seizure frequency has been identified during therapeutic brain stimulation trials with stimulator in patients in the inactive sham group. This study was performed to examine whether the implantation of intracranial electrodes decreases seizure occurrence and explores factors that may be associated. Methods: A retrospective review of 193 patients was performed, all evaluated with both scalp video EEG monitoring and intracranial EEG (iEEG) monitoring. Data about the number of seizures per day during the monitoring period, the number of days until the first seizure, anti-epileptic drugs (AEDs), pain medications, types of implanted electrodes, and anesthetic agents were reviewed. We conducted a repeated measure analysis for counted data using generalized estimating equations with a log-link function and adjustment for number of days and anti-epileptic medication load on the previous day to compare seizure frequencies between scalp and iEEG monitoring. Results: The time to the first seizure was significantly prolonged during iEEG monitoring as compared to scalp monitoring after correction for AED withdrawal (hazard ratio: 0.81, CI 0.69–0.96). During scalp video EEG monitoring, patients experienced an average of 1.09 seizures/day vs 1.27 seizures/day during iEEG monitoring (P=.066). There was no significant difference in seizure frequency in patients that received craniotomy vs burr holes only for intracranial implantation. An increasing number of electrodes implanted increased the delay to seizures (P=.01). Of all anesthetic agents used, desflurane seemed to have an anticonvulsive effect compared to other anesthetics (P=.006). Pain medication did not influence delay to seizures. Significance: Seizures are delayed during iEEG as opposed to scalp monitoring illustrating the “implantation effect” previously observed. Surgical planning should account for longer monitoring periods, particularly when using larger intracranial arrays.