gms | German Medical Science

Objective: Mesial temporal lobe epilepsy (mTLE) due to hippocampal sclerosis (HS) is a major cause of focal epilepsy and often treated by hippocampectomy. The hippocampal formation as part of the medial limbic circuit (Papez-Circuit) is a substantial structure for cognitive function and memory. The intention of our study was to determine alterations of hippocampal association fibers of the medial limbic circuit by global fiber tracking and its diagnostic value for presurgical assessment.

Method: Twenty patients with structural mTLE based on unilateral (right n=9, left n=8) or bilateral (n=3) HS were included in the study and matched with n=20 corresponding control specimens concerning age, gender and handedness. High-angular diffusion weighted images were acquired with 61 directions on a 3Tesla MRI scanner and whole brain global fiber tracking was performed (Reisert et al. 2011). Fiber tracts of interest of the medial limbic circuit, i.e. fornix, cingulum and the hippocampal part of the cingulum, were defined on color encoded DTI images superimposed on a T1w3D sequence. Diffusivity parameters as the mean diffusivity, radial diffusivity, axial diffusivity and fractional anisotropy were determined using a MATLAB-based “inhouse” software (FiberViewerTool). Numbers of streamlines were extracted and statistically analyzed using a paired Wilcoxon Rank-Sum test. Results were correlated to clinical and neuropsychological findings of presurgical epileptological workup.

Results: The analysis of streamline counts revealed a highly significant decrease within the ipsilateral hippocampal associated tract of the cingulate gyrus compared to the contralateral tract (p=0.0098). Comparison of the ipsilateral limbic tract and the individually matched healthy controls also showed a highly significant decrease of streamline counts in the whole cingulate gyrus (p=0.000021) as well as in the hippocampal associated tract of the cingulate gyrus (p=0.00054).

Conclusions: The analysis of fiber alterations by delineation of substructures of the limbic system using whole brain tractography methods reveals highly significant changes within the ipsilateral tracts in HS. Global fiber tracking proves to be a promising method for further investigations of changes in connectivity of focal epileptogenic lesions. Considering clinical and neuropsychological findings these results offer an additional diagnostic method for presurgical assessment in the future.