Abstracts

Rationale: Intracranial electroencephalography, often performed before surgery in patients with drug resistant focal epilepsy to localize the seizure onset zone (SOZ), samples only a limited amount of brain tissue and thus requires strong hypotheses regarding the possible epileptogenic zone. Simultaneous EEG/functional magnetic resonance imaging (EEG/fMRI) of interictal epileptic discharges is a non-invasive diagnostic tool resulting in hemodynamic responses that could contribute to generating such hypotheses. This study aims to assess how well these responses, despite their origin in interictal discharges, predict the region of seizure onset. If they do, better implantation targets can be defined and implantation may even be avoided in some patients. Methods: Among 65 consecutive patients who underwent EEG/fMRI study and stereo-EEG (SEEG) implantation, 37 with significant hemodynamic responses and post-implantation imaging were studied. The statistical maps of hemodynamic responses were generated and co-registered to post-implantation anatomical imaging including electrode locations. The hemodynamic responses were compared to SEEG electrode contacts involved during seizure onset to determine their concordance. Then, a discriminant analysis was performed to distinguish the hemodynamic responses having a high probability of being the SOZ, based on electrophysiological and hemodynamic features available in the EEG/fMRI analysis. Results: A primary cluster was defined as the cluster with the maximum hemodynamic response. At patient level analysis, 31 of the 37 patients had at least one primary cluster close to an electrode, and 24 of the 31 (77%) had at least one primary cluster concordant with the SOZ. Each patient could have multiple interictal discharge types and therefore multiple studies. At study level analysis, among 59 studies with a hemodynamic response from the 37 patients, 44 had a primary cluster explored with an electrode and 30 (68%) were concordant with the SOZ. The discriminant analysis revealed that the SOZ can be predicted with high confidence (larger than 90%) if the primary cluster is highly significant, if there is no other cluster, or if the next highest cluster is much less significant. Conclusions: This study demonstrates that in a high proportion of patients, hemodynamic responses generated by interictal discharges delineate the subset of the often widespread irritative zone that generates seizures. These responses are therefore valuable targets for electrode implantation. In one third of cases in this study, the hemodynamic response fulfilled the conditions of our discriminant analysis and therefore could define the SOZ with a probability larger than 90%. In these cases, interictal EEG/fMRI study may even reduce the need for implantation, whose purpose is to find the SOZ. Funding: This work was supported by grant FDN 143208 of the Canadian Institutes of Health Research. H.M.K. was supported by the Preston Robb fellowship of the Montreal Neurological Institute.