Abstracts

Rationale: The presurgical evaluation of the ictal onset zone in extratemporal epilepsy is often complicated by incomplete visualizations using noninvasive neuroimaging. This is due to the diffuse and unpredictable nature of cortical connectivity, leading to widespread and explosive electrographic seizure onsets that propagate rapidly. MEG visualizations of interictal activity have repeatedly shown to be reliable in the prediction of the epileptogenic onset zone, but do not address potential directions of ictal propagation. This study combines MEG localizations of interictal epileptogenic activity with diffusion tensor tractography, with the purpose of uncovering white matter tracts involved in the propagation of seizure activity. This approach may provide additional information regarding the presence of underlying epileptogenic networks, which may determine the extent of surgical resection necessary for a curative extratemporal epilepsy surgery. Methods: Analyses were performed on a 19 year old female with an ictal semiology consisting of an aura of fear followed by a progressive visual field defect (a ‘curtain being drawn’) over the right homonymous visual field. Brain MRI revealed bilateral posterior periventricular heterotopias, and video EEG analyses showed ictal onsets originating from the left parietoocciptal region, with an active independent interictal focus in the right parietooccipital region. MEG correlates of nterictal spike-wave activity was noted and visualized using Equivalent Current Dipole (ECD) analysis. DTI (Diffusion Tensor Imaging) sequences were obtained at 2x2x2mm and 16 directions of diffusion sampling per slice. Probabilistic tractography on these images were performed using FSL (FDT toolbox), using a single ECD localization as a seed point, and other local and distant ECD localizations as targets. Results: ECD localizations were consistently visualized in the left posterior temporoparietal region and the right midtemporal region, spatially correlating to the patient’s heterotopias. Tractography revealed white matter connectivity between ipsilateral and contralateral ECD points, suggesting the presence of local and distant epileptogenic connectivity. The most prominent tracts visualized using left hemisphere ECD localizations as seed points and right hemisphere ECD targets revealed strong transcallosal connectivity through the visual tracts, which is consistent with the patient’s ictal semiology and video-EEG propagation. Additionally, tracts between ipsilateral ECD localizations were visualized, suggesting the presence of local epileptogenic networks with underlying white matter connectivity. Conclusions: This preliminary study highlights a potentially promising methodology in the analysis and visualization of local and distant epileptogenic networks. This additional knowledge appears to be have a role in assisting the epileptologist and surgeon in more accurate placements of intracranial electrodes, which may in turn lead to more targeted epilepsy surgeries.