Determining the Extent of Diffusion Tensor Abnormalities in Focal Cortical Dysplasia

Focal cortical dysplasia (FCD) is a common finding in epilepsy surgery in pediatric patients. Children with intractable epilepsy would have extensive tests to identify the cause of epilepsy; this includes MR brain, video EEG and magnetoencephalography (MEG). The white matter next to FCD is frequently found to be abnormal on pathology. Diffusion tensor imaging (DTI) can be used to study the abnormal white matter and the area that often extends beyond the area that is visible.

Focal cortical dysplasia (FCD) is a highly epileptogenic form of malformation of cortical development that may require surgical resection for epilepsy control. With abnormal development and organization of neurons within the cortex, the white matter projecting from the abnormal cortex is likely to be abnormal as well. The abnormality in the white matter involves not only the subcortical white matter, but also the long tracts in the deep white matter associated with the dysplastic cortex. Histologically, the subcortical white matter adjacent to the dysplastic cortex has been found to be abnormal. Studies using diffusion tensor imaging (DTI) to investigate the white matter adjacent to the MR visible abnormality have demonstrated reduced fractional anisotropy. However, electrographic abnormality in FCD often extends beyond the visible MR abnormality and surgical outcome of epilepsy surgery in FCD is dependent on excising the MR visible abnormality as well as electrographically abnormal area beyond the MR visible abnormality. The cortical and white matter abnormalities are therefore assumed to extend beyond the MR visible lesion. The short-term goal of this study is to determine whether quantitative measures of the abnormal white matter using DTI are able to provide surrogate markers for the extent of FCD. Whilst surgical outcome data is not available for the purpose of this study, these children will be followed up and in the longer term, the extent of FCD as determined by DTI will be compared with clinical outcome post surgery. This study will help determine the potential value of this technique in identifying areas of FCD that appear normal on structural MR. In the long term, this technique can be extended to study children with intractable epilepsy with (i) MR occult lesion and (ii) developmental tumor with MR occult FCD adjacent to the tumor.