Abstracts

Rationale: The seizure generating regions of cortex in patients with neocortical onset epilepsy are often difficult to localize due to the explosive manner in which they spread. This poses a significant challenge as these patients are typically poor candidates for surgical resection. In this study, we have explored the ability of a functional connectivity measure, specifically the directed transfer function (DTF), to localize the seizure onset zone (SOZ) in patients with neocortical epilepsy. Methods: We obtained electrocorticogram (ECoG) recordings in six pediatric patients with extra-temporal lobe neocortical onset seizures which were refractory to medical treatment. The patients were studied under a protocol approved by the Institutional Review Boards at the University of Minnesota and the University of Chicago. In four of the patients, two or more separate SOZs were identified by the examining neurologist. Three to five second segments during the onset of each seizure were selected for analysis. The DTF method was applied to each ictal segment in order to determine the causal pattern of information flow between the individual electrodes of the ECoG grid(s). The electrode with the greatest amount of source activity as measured by the DTF was compared to the SOZ identified by the neurologists. A total of twenty-three seizures in the selected patients were analyzed in this manner. Results: In all six patients, the DTF calculated sources were highly correlated with the SOZs. For 22 of the 23 seizures, the DTF-identified electrodes were either within or immediately adjacent to the clinical SOZ. Furthermore, of the four patients with multiple seizure foci, three had DTF-identified sources which were correlated with all of the separate clinical foci. In two of the analyzed patients, the majority of the DTF-identified sources (7/10) were adjacent to but not within the clinical SOZ. These patients experienced approximately a 70% reduction in seizure frequency following surgical resection. In the remaining four patients, the majority of the DTF sources were within the SOZs (9/13). Two of the patients experienced a ≥95% reduction in seizure frequency and two were seizure free following resection of the ictal foci. Conclusions: In this study, we have examined the ability of the DTF method to localize neocortical onset seizures in six pediatric patients. We have found that, despite the speed which the seizures spread, the DTF method was able to accurately localize the source of the propagated activity. In the six selected patients, the DTF was found to be in agreement with the clinically identified seizure foci and all experienced a ≥70% decrease in seizure frequency following surgical resection. In cases where the SOZ and DTF-identified sources had significant overlap, the clinical outcome was quite good with these patients experiencing a ≥95% decrease in seizure frequency. These findings support the ability of the DTF method to identify the generators of neocortical onset seizures despite their rapid propagation and may potentially prove useful as an indicator of post-operative seizure outcome.