Abstracts

Rationale: Structural imaging studies have demonstrated that medial temporal lobe epilepsy (MTLE) is associated with limbic atrophy involving not only the hippocampus but also peri-hippocampal and extra-temporal structures. While MTLE is related to static structural limbic compromise, it is unknown whether the limbic system undergoes a dynamic functional reconfiguration during seizures. In this study, we aimed to investigate state specific (i.e. ictal versus interictal) configuration of functional limbic networks in patients with MTLE. Methods: We studied the clinical information and single positron computed tomography (SPECT) images obtained with intravenous infusion of the radioactive tracer Technetium Tc 99m Hexamethylpropyleneamine Oxime (Tc-99m HMPAO) during ictal and interctal state in 20 patients with unilateral MTLE (12 left and 8 right MTLE). The SPECT data was normalized for whole brain radiotracer uptake, and spatially normalized using linear and nonlinear parameters using SPM8. Pair-wise voxel-based analyses were used to define global changes in tracer between states. Regional uptake was calculated for anatomical areas, and state specific adjacency matrices were constructed based on regional correlation of uptake across subjects. Graph theoretical measures were then applied to investigate global and regional state specific network reconfigurations. Results: A significant increase in tracer uptake was observed in the medial temporal region, cerebellum, thalamus, insula and putamen, in the ictal state compared with the interictal state (Figure 1). A mild increase in whole network clustering coefficient was observed in the ictal state. Regional clustering coefficient and local efficiency were increased over the ipsilateral and contralateral hippocampus, contralateral anterior cingulate, and ipsilateral fusiform gyrus(Figure 2). Compared to whole brain analyses, graph measures unveiled a significant change in network parameters of the fusiform gyrus during the ictal state. Conclusions: These results suggest that MTLE is associated with a functional state specific limbic system organization. Increased ictal tracer uptake suggests an increase metabolic demand of limbic regions associated with epileptogenesis, and the reorganization of the functional network during a seizure suggests higher connectivity and synchrony of limbic areas, notably leading to an increase local participation of the fusiform region.