Diffusion tensor imaging of cryptogenic and acquired partial epilepsies

Current optimal MRI identifies a relevant structural abnormality in up to 80% of patients with refractory partial seizures. Identification of a structural lesion is fundamental to pre-surgical evaluation. We used diffusion tensor imaging (DTI) and statistical parametric mapping (SPM) to examine objectively the diffusion properties, and hence structural organization, of cerebral tissue in 10 patients with partial seizures and acquired lesions and 30 patients with partial seizures and normal MRI. Fractional anisotropy and mean diffusivity maps were calculated and, using SPM, individual patients were compared with a group of 30 control subjects. Diffusion tensor imaging and voxel-by-voxel statistical comparisons identified significant increases in diffusivity and significant reductions of anisotropy in all patients with acquired non-progressive cerebral lesions and partial seizures. In all of these patients, the areas of increased diffusivity, and in nine patients the areas of decreased anisotropy, concurred with abnormalities identified on visual inspection of conventional MRI. In addition, there were 10 areas which were normal on conventional imaging which exhibited abnormal anisotropy or diffusivity. Individual analyses of the 30 patients with partial seizures and normal optimal MRI identified a significant increase in diffusivity in eight of the subjects. In six of these, the areas of increased diffusivity concurred with the localization of epileptiform EEG abnormality. Analysis of anisotropy in the MRI-negative patients revealed significant differences in two patients, one of which concurred with electroclinical seizure localization. Group analysis of nine patients with normal conventional MRI and electroclinical seizure onset localizing to the left temporal region revealed a significant increase in diffusivity and a significant reduction in anisotropy within the white matter of the left temporal lobe. DTI analysed using SPM was sensitive in patients with acquired cerebral damage. Significant differences in the diffusion indices in individual MRI negative patients and the group effect in patients with left temporal lobe epilepsy suggest that minor structural disorganization exists in occult epileptogenic cerebral lesions. These techniques are promising, non-invasive imaging methods for identifying the cause of partial seizures, and can contribute to pre-surgical evaluation.