Abstracts

Magnetic Resonance Diffusion Tensor Imaging (DTI) provides information on the micro-structural state of white matter based on the diffusion of water molecules. DTI tractography uses the directionality of water movement to virtually dissect the fiber tracts, otherwise not discernible with conventional imaging methods. The fornix and cingulum are two of the most prominent fiber bundles of the limbic system. They have been visualized with tractography in healthy individuals, but not in patients with Temporal Lobe Epilepsy (TLE). The objective of this study was to determine whether evidence of axonal degeneration could be detected in vivo within the fornix and cingulum in a series of epilepsy patients with unilateral Mesial Temporal Sclerosis (MTS). We performed DTI derived tractography of the fornix and cingulum and subsequent quantitative analysis of water diffusion behaviour in a series of eight patients with medically intractable TLE and clinical imaging evidence of unilateral MTS and nine healthy controls. We found bilateral and symmetrical reduction in fractional anisotropy (FA) in the fornix of patients with TLE as compared with controls (FA = 0.48 [plusmn] 0.02 and FA = 0.53 [plusmn] 0.02, respectively, p [lt] 0.0001 for both sides), along with an increase in water mobility perpendicular to the axis of the fibers and a small, yet significant, reduction in diffusion parallel to the fibers. The mean FA for the cingulum in the control group was 0.50 [plusmn] 0.03, while the patients had a mean FA value of 0.44 [plusmn] 0.02 in the cingulum ipsilateral to MTS (p [lt] 0.001) and a mean FA value of 0.46 [plusmn] 0.02 in the contralateral side (p = 0.005). The rest of the findings in the cingulum were similar to our observations in the fornix with the exception that while the fornix had no change in bulk diffusivity (regardless of directionality), this measurement was significantly increased in the cingulum. Reduced diffusion anisotropy in association with an increase in water mobility perpendicular to the axis of fibers is consistent with axonal degeneration within the fornix and cingulum. Our findings of strikingly symmetrical bilateral abnormalities of axonal integrity in the fornix and cingulum in a series of patients with unilateral MTS, strongly suggest that TLE with unilateral MTS is in fact associated with extensive bilateral limbic system pathology. (Supported by The Savoy Foundation, AHFMR, CIHR, Promep, the Canada Foundation in Innovation, Alberta Science and Research Authority, and the University Hospital Foundation. Fiber tracking software provided by Drs. Susumu Mori and Hangyi Jiang.)