Abstracts

Rationale: Microstructural alterations seen in the epileptic cortex have been associated as a cause and also as a result of multiple seizure activity. In the present study, we attempted to evaluate the diffusion changes of water at different cortical thickness fractions of the epileptic cortex and compared them with electrographically normal cortex and also with corresponding cortical regions of controls. Methods: We selected 18 children (mean age: 7.8 years ± 4.46) who had normal MRI findings and underwent two stage epilepsy surgery for control of their seizures. 18 normal age-matched volunteers (mean age: 7.8 ± 4.6) were used as controls. Structural and diffusion MRI images were acquired using a 3-Tesla GE MR Scanner. Delineation of the grey-white and grey-pial intersection surfaces were performed using the structural (T1) image (Freesurfer). Mean diffusivity values were calculated and the diffusion images were spatially aligned with the structural image using rigorous non-linear registration (FNIRT, FSL). Using the delineated surfaces as reference, the whole cortex was fractionated along the cortical thickness into Grey-White zone, inner-, middle-, outer-fractions and Grey-Pial zone. Cortical regions representing seizure onset and ipsilateral electrographically normal (IPSI) cortex were differentiated using the electrocorticography and mean diffusivity (MD) values for these regions were noted for all cortical fractions in the patients as well in the controls. Statistical analysis involved repeated measures with group, electrographic type and cortical fractions as factors. Results: Three-way interaction between the group, electrographic type and cortical fractions was significant (p=0.024). In seizure onset regions, the MD of all five cortical fractions were significantly (p<0.01) higher compared to the IPSI regions within the patient group (Figure). A specific and significantly (p=0.001) marked increase in MD was noted in the outer-fraction (and middle-fraction: less marked but significant; p=0.047) of the onset regions compared to the corresponding regions of the controls as well as IPSI region (p=0.001) (Figure). In IPSI regions, the MD values were significantly (p<0.022) lower in the inner-, middle-fraction and Grey-Pial zone (and apparently lower in the grey-white zone and outer-fraction) as compared to the controls. Conclusions: Our analysis suggests specific patterns of diffusion changes in the outer cortical fractions of the seizure onset zone compared to IPSI and control regions. This abnormal increase in MD of the outer and middle cortex might be associated with microstructural abnormalities commonly seen in layer II through IV of the epileptogenic cortex or to alterations caused by repeated seizure activity. Furthermore, the MD values were lower in electrographically normal regions compared to seizure onset and control regions; this decrease in diffusivity might be a common feature of the epileptic brain.