Abstracts

Rationale: Cortical-cortical evoked potentials (CCEPs) offer the possibility of understanding connectivity with in seizure networks to improve diagnosis and more accurately identify candidates for seizure surgery. The process of identifying the epileptogenic zone includes characterizing sites of EARLY seizure spread (within three seconds). We sought to determine if CCEPs can distinguish sites of EARLY seizure spread from sites of LATE spread (over 3 seconds). Further, we wanted to determine if post- stimulation oscillatory changes at ictal spread sites could be used to understand connectivity within the seizure network. Methods: 37 patients undergoing stereoelectroencephalography were tested using the CCEP paradigm employing 1 Hz stimulation at the ictal onset site. All electrode sites in the implantation montage were classified according to the speed of ictal spread. For each patient, an EARLY spread site was matched to a LATE spread site equidistant from the onset zone. Root-mean-square was used to quantify evoked responses, which were compared between EARLY and LATE spread sites within each patient. Post-stimulation gamma band power was extracted using the Hilbert transform and compared between EARLY and LATE sites, and post-stimulation gamma band coherence was quantified between the ictal onset zone and the spread locations. Post-implantation electrode localization was performed by fusing 3-D reconstructed volume acquisition MRI sequences (coronal MPRAGE) with post-operative thin-sliced CT scan, to allow measurement of Euclidean distance for EARLY and LATE sites and match them for distance from the seizure onset location. Results: Sites of EARLY spread exhibited significantly greater evoked responses after stimulation across all patients (t(36)= 2.973, p = 0.004). Stimulation elicited enhanced gamma band activity at EARLY but not LATE spread sites (t(143) = 3.393, p < 0.001); this gamma band oscillation was highly coherent between the onset zone and EARLY spread sites. To confirm that the sites we chose were successfully matched for distance from the seizure onset location, we measured the Euclidean distance for EARLY and LATE sites, by determining the x, y, z coordinates for each location during the co-registration of post-operative CT images with the pre-operative volume acquisition MRI. Conclusions: Previous data has suggested that stimulation of a seizure onset site elicits exaggerated CCEP responses, compared to sites outside the epileptic network. Our study has addressed the question whether CCEPs can be used to directly predict seizure onset spread. Our findings are the first attempt to use CCEP results to distinguish EARLY vs LATE seizure spread locations. This is manifest by an exaggerated response to stimulation (EARLY) compared to a smaller amplitude response (LATE). Post- stimulation changes in gamma band activity also distinguish EARLY versus LATE sites.