Abstracts

Rationale: Single-pulse electrical stimulation (SPES) has been applied to identify normal functional and seizure networks as well as to evaluate cortical excitability. Responses to SPES have been classified into 1) early responses called cortico-cortical evoked potential (CCEP, 10-50 ms from SPES) and 2) delayed responses (DRs, >100 ms from SPES). Recently, high-frequency activities (HFAs) after SPES have also been considered as indices of cortical excitability. However, responses on stimulus sites have never been analyzed because of technical problems. Herein, we investigated the responses to SPES upon stimulus sites using a special switching device to compare cortical excitability between epileptic foci and control cortices.

Methods: We recruited 4 patients with medically intractable focal epilepsy who underwent subdural grid electrodes (SDGs) or stereo-electroencephalography (SEEG) for presurgical evaluation (IRB#C1192/C1212). We applied SPES to a total of 7 seizure onset zones (SOZs) and 5 control cortices (CTL). SPES (0.2 or 0.5 Hz, 60 trials) was delivered to the SOZ and CTL at various intensities. We performed a time-frequency analysis time-locked to the stimulation, using short-time Fourier Transform (window width 100 ms, baseline from 0.3 to 0.1 sec before SPES, analysis window 0-1.5 sec). Due to artifacts of the switching device, we could only evaluate HFAs 55 ms after SPES. As another index of cortical excitability, we compared the occurrence rate of DRs after each SPES (1 Hz) in the SOZs between stimuli on SOZ itself and the seizure propagation zone (PZ).

Results: All electrodes showed a positive correlation between stimulation intensity and the degree of HFAs decrease (i.e., neural inhibition), except for 1 SOZ. HFA decreased more on SOZ than CTL. The difference of HFA decrease between SOZ and CTL became larger at higher stimulus intensity. DRs were observed by 4 out of 7 SOZs stimuli in 3 patients even at low intensity. DRs by SOZ stimuli tended to occur around the end of HFA decrease. SPESs on 5 out of 7 PZs induced DRs in SOZs. The occurrence rate of DRs by stimulating SOZs (12.7±4.6%) was lower than that by stimulating the PZs (19.6±5.7%).

Conclusions: The present study indicated that exogenous input such as SPES could modulate the cortical excitability at the site of stimulation, especially on SOZ which may account for the vulnerable property of E/I balance at the epileptic focus. DRs at SOZs were less induced by SPES on SOZ itself than on PZ, indicating that SPES on the seizure focus can produce more intense local inhibition at epileptic focus than network-driven stimulation.

Funding: Please list any funding that was received in support of this abstract.: There is no funding for this abstract.