White Matter Connectivity Alterations in Patients with MRI negative Temporal Lobe Epilepsy Using Hippocampus Specific Tractography

Temporal lobe epilepsy (TLE) patients with normal-appearing MRI scans on neuroradiological evaluation are very common (30% of all TLE), but pre-surgical evaluation is challenging for this TLE cohort (MRI-negative TLE). As a result, surgery does not often achieve a seizure-free outcome. The purpose of this study is to analyze diffusion magnetic resonance imaging (dMRI) by constructing tractography models that were seeded from the hippocampus to superficial white matter underlying the neocortex and subcortical grey matter structures. We compared mean fractional anisotropy (FA) values in 96 regions of interest between patients with unilateral TLE (10 left lateralized TLE, LTLE and 16 right lateralized TLE, RTLE) and 18 healthy controls. We found that both LTLE and RTLE showed significantly decreased FA values in hippocampal pathways to putamen, pallidum, parahippocampal and entorhinal cortex. In particular, LTLE patients mainly displayed unilateral FA decreases in hippocampal connections ipsilateral to the epileptic focus, whereas RTLE showed bilateral FA decreases. No connectivity changes were found between the hippocampus and the neocortical regions. Our analysis provides novel evidence of alterations in connectivity between the hippocampus and its proximal grey matter structures in patients with MRI-negative TLE.