Abstract
Successful resection of cortical tissue engendering seizure activity is efficacious for the treatment of refractory, focal epilepsy. The pre-operative localization of the seizure focus is therefore critical to yielding positive, post-operative outcomes. In a small proportion of focal epilepsy patients presenting with normal MRI, identification of the seizure focus is significantly more challenging. We examined the capacity of resting state functional MRI (rsfMRI) to identify the seizure focus in a group of four non-lesion, focal (NLF) epilepsy individuals. We predicted that computing patterns of local functional connectivity in and around the epileptogenic zone combined with a specific reference to the corresponding region within the contralateral hemisphere would reliably predict the location of the seizure focus. We first averaged voxel-wise regional homogeneity (ReHo) across regions of interest (ROIs) from a standardized, probabilistic atlas for each NLF subject as well as 16 age- and gender-matched controls. To examine contralateral effects, we computed a ratio of the mean pair-wise correlations of all voxels within a ROI with the corresponding contralateral region (IntraRegional Connectivity – IRC). For each subject, ROIs were ranked (from lowest to highest) on ReHo, IRC, and the mean of the two values. At the group level, we observed a significant decrease in the rank for ROI harboring the seizure focus for the ReHo rankings as well as for the mean rank. At the individual level, the seizure focus ReHo rank was within bottom 10% lowest ranked ROIs for all four NLF epilepsy patients and three out of the four for the IRC rankings. However, when the two ranks were combined (averaging across ReHo and IRC ranks and scalars), the seizure focus ROI was either the lowest or second lowest ranked ROI for three out of the four epilepsy subjects. This suggests that rsfMRI may serve as an adjunct pre-surgical tool, facilitating the identification of the seizure focus in focal epilepsy.
Highlights
Current standards of care for the treatment of pharmacoresistant, focal epilepsy includes the surgical resection of epileptogenic cortex
The IntraRegional Connectivity (IRC) ranking for this subject was 3, indicating that this regions of interest (ROIs) showed the third lowest local fc ranking when mean local fc was directly contrasted with its contralateral counterpart
GROUP-LEVEL CONTRASTS To determine whether local fc in the seizure focus ROI was lower in the epilepsy group, we compared the rank value of the seizure focus ROI between NLF and controls across our three sets of rankings (ReHo, IRC, and mean rank)
Summary
Current standards of care for the treatment of pharmacoresistant, focal epilepsy includes the surgical resection of epileptogenic cortex. The tissue targeted for resection encompasses an extended area around the seizure focus believed to be involved in the propagation of epileptiform discharges, generally referred to as the epileptogenic zone (Rosenow and Lüders, 2001; Laufs, 2012). The precise localization of the seizure focus and the extended epileptogenic zone is critical to yielding positive, postoperative outcomes. Pre-surgical evaluations aimed at identifying the seizure focus are comprised of any number of interdisciplinary approaches, including electrophysiological investigations [e.g., electroencephalography and less frequently sub-dural electrophysiology such as electrocorticography (ECoG) or stereoelectroencephalography], traditional neuropsychological evaluation, modern structural [e.g., structural magnetic resonance imaging (MRI)], metabolic [e.g., [18F]fluoro-2-deoxy-glucose positron emission tomography (FDG-PET)], and functional imaging based approaches (e.g., functional MRI). The typical clinical evaluation identifies sites of pathology from structural-based MR scans and www.frontiersin.org
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