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Midbrain structure volume, estimated myelin and functional connectivity in idiopathic generalised epilepsy

Background: Previous neuroimaging studies reported functional and structural impairments of the upper basal ganglia (specifically, the striatum) in idiopathic generalised epilepsy (IGE). However, these studies often overlook lower basal ganglia structures located in and adjacent to the midbrain due to poor contrast on clinically acquired volumetric T1-weighted scans. Here, we acquired 3D isotropic T1-weighted, T2-weighted and resting state functional MR images to investigate differences in volume, estimated myelin content and functional connectivity in three midbrain and midbrain-adjacent structures in patients with IGE: the substantia nigra (SN), subthalamic nuclei (SubTN) and red nuclei (RN). Methods: A total of 33 patients with IGE (23 refractory, 10 non-refractory) and 39 age and sex matched healthy controls underwent MR imaging. The SN, SubTN and RN were automatically segmented from T2-weighted images. Estimated median myelin content for each structure was determined using a previously described T1-weighted/T2-weighted ratio method. Functional connectivity analysis between midbrain structures and the rest of the brain was performed using seed-based resting state fMRI analysis and compared between participant groups using the CONN toolbox running in SPM12 (pFDR<0.05, cluster corrected). Results: An increased volume of the right RN was found in patients with IGE relative to healthy controls. Structural volumes of the right SubTN differed between patients with non- refractory and refractory IGE. No myelin alterations of midbrain structures were found in patients with IGE or between treatment outcome groups. Functional connectivity alterations were found for all midbrain regions in patients with IGE, including significantly decreased functional connectivity between the left SN and the thalamus compared to controls, and significantly increased functional connectivity observed between the right SubTN and the left superior frontal gyrus in patients with IGE relative to controls. Connectivity alterations specific to patients with non-refractory IGE were also found. Conclusion: We report volumetric and functional connectivity alterations of midbrain and lower basal ganglia structures in patients with IGE. We postulate that an increased structural volume of the RN in patients is due to increased iron deposition that impacts on T2-weighted contrast. These findings are consistent with previous experiential work demonstrating pathophysiological abnormalities of the lower basal ganglia in animal models of generalised epilepsy.