Abstracts

In patients with malformations of cortical development, there is evidence for extensive structural abnormalities beyond the visually identified lesion. In focal cortical dysplasia (FCD), histological abnormalities may be disseminated rather than confined to a single patch (Taylor et al., 1971). To date only a few structural imaging studies based on a limited number of patients have addressed the question of structural changes distant to the FCD lesion. Our purpose was to investigate gray matter (GM) changes in individual FCD patients using voxel-based morphometry. We studied 30 patients (20 females, mean age: 26 [plusmn] yrs8) with known FCD lesions and partial epilepsy and 39 healthy controls (21 females, mean age: 29 yrs [plusmn] 8). 20 (67%) patients underwent surgery and Taylor type FCD was confirmed by histopathology in all of them. FCD was seen on pre-operative MRI in 28 patients and in two others subtle dysplastic changes were found in the tissue removed at surgery. High-resolution MRI was acquired on a 1.5T scanner using a T1-FFE sequence (1mm slice thickness, isotropic voxels). Image processing included: (i) automated correction for intensity non-uniformity; (ii) linear registration to a standardized stereotaxic space; (iii) classification of brain tissue into GM, WM and CSF; (iv) blurring of GM masks with an isotropic Gaussian kernel (12 mm FWHM) to generate 3D-maps of GM concentration. We compared the GM map of each subject (controls and patients) with the average GM map of all controls and obtained a GM z-score map for each individual. To detect significant differences in GM concentration, a voxel was considered abnormal if its value fell outside the mean of the controls [plusmn]4.5 SD (this cutoff was chosen to exceed the maximum GM changes seen in any control subject). Among the 28 patients with an MRI-visible FCD, z-score maps showed an increase in GM that coincided with the lesion in 21 (75%) patients. In 12/21 (57%) of them there were additional areas of GM increase distant from the primary lesion. In 7/28 (25%) patients, no GM increase was found at the FCD location. However, 5 (71%) of these patients had areas of GM increase distant from the primary lesion. In the two patients in whom FCD was diagnosed only at histopathology, areas of increase in GM were found in the region of the corticectomy. Areas of GM decrease were found in 9/30 (30%) patients. Individual analysis of GM using voxel-based morphometry identified the FCD lesion in the majority of our patients. Moreover, this method allowed the detection of subtle FCD that was overlooked by visual MRI evaluation. Extra-lesional GM increase distant from the primary FCD lesion in more than half of our patients demonstrates that FCD is often associated with widespread structural abnormalities. (Supported by Epilepsy Canada
Canadian Institutes for Health Research
The Scottish Rite Charitable Foundation of Canada)