Abstract
Anterior temporal lobectomy (ATL) is commonly adopted to control medically intractable temporal lobe epilepsy (TLE). Depending on the side of resection, the degree to which Wallerian degeneration and adaptive plasticity occur after ATL has important implications for understanding cognitive and clinical outcome. We obtained diffusion tensor imaging from 24 TLE patients (12 left) before and after surgery, and 12 matched controls at comparable time intervals. Voxel-based analyses were performed on fractional anisotropy (FA) before and after surgery. Areas with postoperative FA increase were further investigated to distinguish between genuine plasticity and processes related to the degeneration of crossing fibers. Before surgery, both patient groups showed bilateral reduced FA in numerous tracts, but left TLE patients showed more extensive effects, including language tracts in the contralateral hemisphere (superior longitudinal fasciculus and uncinate). After surgery, FA decreased ipsilaterally in both ATL groups, affecting the fornix, uncinate, stria terminalis, and corpus callosum. FA increased ipsilaterally along the superior corona radiata in both left and right ATL groups, exceeding normal FA values. In these clusters, the mode of anisotropy increased as well, confirming fiber degeneration in an area with crossing fibers. In left ATL patients, pre-existing low FA values in right superior longitudinal and uncinate fasciculi normalized after surgery, while MO values did not change. Preoperative verbal fluency correlated with FA values in all areas that later increased FA in left TLE patients, but postoperative verbal fluency correlated only with FA of the right superior longitudinal fasciculus. Our results demonstrate that genuine reorganization occurs in non-dominant language tracts after dominant hemisphere resection, a process that may help implement the inter-hemispheric shift of language activation found in fMRI studies. The results indicate that left TLE patients, despite showing more initial white matter damage, have the potential for greater adaptive changes postoperatively than right TLE patients.
Highlights
Temporal lobe epilepsy (TLE) is the most frequent type of epilepsy, with its intractable form commonly leading to surgical treatment [1]
In an attempt to identify factors that contribute to WM abnormalities and the potential for reorganization, specific distinctions have been made between clinical features, i.e. left vs. right TLE patients, with vs. without mesial temporal sclerosis (MTS), and early vs. late seizure onset
Given that fractional anisotropy (FA) increases could be a marker of neuroplastic processes in white matter, we focused our investigation on clusters that in showed FA increase, with the aim of distinguishing genuine neuroplasticity from changes that could be attributed to processes related to the degeneration of crossing fibers
Summary
Temporal lobe epilepsy (TLE) is the most frequent type of epilepsy, with its intractable form commonly leading to surgical treatment [1]. White matter disruptions have been reported in TLE patients before undergoing any surgery. These disruptions involve tracts both proximal to the epileptogenic area, such as, the uncinate, the parahippocampal, and the inferior fronto-longitudinal fasciculi [2,3,4,5,6,7,8,9,10], as well as distal tracts, such as, the arcuate, the fornix, the cingulate, the corpus callosum, etc. In an attempt to identify factors that contribute to WM abnormalities and the potential for reorganization, specific distinctions have been made between clinical features, i.e. left vs right TLE patients, with vs without mesial temporal sclerosis (MTS), and early vs late seizure onset. The uncinate fasciculus has a normal rightward asymmetry with higher FA and more fibers on the right [19,20], creating the possibility of higher WM risk in those tracts bearing the burden of seizure dispersion (i.e., the uncinate [21])
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