Abstracts

Rationale: In several studies, it is reported that cortical stimulation is effective in seizure reduction. Stimulation is often applied in the seizure onset zone (SOZ). We assume that stimulation remotely, in the effective network, instead of directly in the SOZ, also results in effects in the SOZ. Using clinical electrocorticography (ECoG) data with Electrical Stimulation Mapping (ESM), we investigate in SOZ-electrodes whether we observe a difference in effects of stimulation when stimuli are applied in electrodes connected to the SOZ compared to electrodes not connected to the SOZ. Methods: Three patients (Table 1) with intractable epilepsy in the primary sensory-motor cortex underwent invasive pre-surgical evaluation with subdural electrodes. ESM was applied (50 Hz, 1-5s, 1-15 mA, interstimulus interval: 3-17s) to map the location of eloquent cortex. Single Pulse Electrical Stimulation (SPES) was applied (0.2 Hz, 1ms, 8 mA) to delineate epileptogenic region. Both ESM and SPES were part of the standard clinical protocol. SOZ was defined as the electrodes with the earliest ictal activity.From SPES, we constructed an effective network based on early responses (present within 100 ms post-stimulus). We divided ESM stimulus pairs into two groups based on the presence or absence of a connection to SOZ-electrodes as determined by SPES. We estimated the spectral content in SOZ-electrodes in the alpha (8-12 Hz), beta (13-30 Hz), and low gamma band (30-45 Hz) in baseline data (3s prior to ESM) and during 3s after ESM.In the SOZ electrodes, we calculated power ratios (PR) between pre- and post-stimulation periods in these three bands. PR1 means enhancement. We compared the effects of stimulation of electrode pairs within and outside the network on these PRs. Significance (p < 0.05) was determined using the Mann Whitney U test. Results: In patient 1 (Figure 1), we found a lower PR in the alpha band (p < 0.05), and a higher PR in the low gamma band (p < 0.001) in SOZ-electrodes connected to the ESM-stimulus pair. This suggests suppression in the alpha band, and enhancement in the low gamma band. We did not observe any significant differences in PRs in patient 2 and 3. Conclusions: PRs changed with stimulation in the network in patient 1. A reason that this effect was not observed in patient 2 and 3 could be that ESM inter-stimulus intervals were too short, resulting in effects present from prior stimulus lasting throughout the following stimuli. Considering only starting stimulus pairs, we did observe significant power suppression (4-20 Hz), when there was a connection, whereas no suppression was seen when there was no connection.The observations in patient 1 suggest that stimulation of electrodes connected to SOZ electrodes may cause suppression of the lower EEG bands in SOZ-electrodes and increase in the (low) gamma band.We conclude that ESM in electrodes connected to the SOZ can have a modulating effect on spectral content, suggesting its potential ability to modulate cortical excitability in the SOZ and therefore cause seizure reduction remotely. Funding: Dutch Epilepsy Foundation nr 17-07