Abstracts

Rationale: On radiological MRI assessment, mesial temporal lobe epilepsy (TLE) is most commonly associated with ipsilateral hippocampal sclerosis. Quantitative analysis of structural volumes has also revealed global atrophy of other structures, most commonly of the thalamus, striatum and cortex [1]. Thalamocortical and striatocortical mechanisms play important roles in the expression of seizures [2], but the vast majority of MRI research to date has concentrated on global volume loss of subcortical structures without consideration of the specific connectivity profiles of these regions. In the present study, we combined T1-weighted magnetic resonance (MR) imaging and MR diffusion tensor imaging (DTI) to investigate cortical connection probability within the thalamus and striatum in patients with TLE relative to healthy controls using connectivity-based segmentation (CBS). Methods: Thirty-five participants were enrolled into this study, including 14 healthy controls and 21 patients with TLE (9 left, 12 right) and underwent structural and DTI scans during the same scanning session. CBS was performed using the FSL FDT pipeline (www.fmrib.ox.ac.uk/fsl/fdt) for probabilistic tractography from seeds (left / right / thalamus / striatum separately) to classification targets (ipsilateral prefrontal, precentral, postcentral, temporal (TL), parietal and occipital cortices). Seeds and targets were obtained using FreeSurfer software (http://surfer.nmr.mgh.harvard.edu) from T1-weighted MRIs. A ‘hard segmentation' of the intra-thalamic and intra-striatal representations of the cortical targets was performed using the ‘find_the_biggest' function in FSL, and fslstats was used to determine the volume of these cortical representations within the thalamus and striatum. Quantitative analyses of between-group differences in CBS were performed in native (subject-specific) and normalized (MNI) space. Results: In native space, patients with right TLE had significantly reduced volume of thalamic-TL connections in the left (p=0.03) and right (p=0.004) hemisphere relative to controls. Patients with left TLE had a borderline significant reduction of left thalamic-TL connections (p=0.053) compared to controls. When structures were treated as ipsilateral / contralateral to TLE relative to the left/right hemisphere average in controls, significantly reduced ipsilateral thalamic-TL connections were observed for both patient groups (Figure). Findings in MNI space were consistent with native space thalamic-TL data, but only at borderline statistical levels (p<0.05). There were no other statistically significant differences between patients and controls for the striatum or other thalamocortical connections. Conclusions: Here we provide preliminary evidence indicating disrupted thalamic-TL connectivity ipsilateral to the epileptogenic focus in patients with TLE. This disruption may be due to either deafferentation or excitotoxic effects generated by the connected medial temporal lobe. References [1] Keller, S. S., Roberts, N. 2008, Epilepsia, 49(5):741-57. [2] Driefuss, S. et al. 2001, Neurology, 57(9):1636-41.