Wallerian degeneration in the optic radiation after temporal lobectomy demonstrated in vivo with diffusion tensor imaging

Purpose: Diffusion tensor imaging allows the quantitative assessment of the microstructural organization of tracts in vivo (MR tractography). We used the new technique of MR tractography to demonstrate the effects of temporal lobectomy on the optic radiation. Methods: Spatially normalised maps encoding magnitude of the bias (anisotro py) of diffusion of three patients with temporal lobe resections were compa red with spatially normalised diffusion maps of 22 control subjects. All th ree patients were operated on for the treatment of medically intractable te mporal lobe epilepsy and had a normal neurologic examination before surgery . One patient had an amygdalocorticectomy. Two patients had standard en blo c resections, one of whom developed a homonymous hemianopia after surgery. Results: In the patient with hemianopia, a significant reduction of diffusi on anisotropy (greater than mean +/- 2 SD) consistent with wallerian degene ration was demonstrated in the optic radiation on the side of the temporal lobectomy, extending from the temporal to the occipital lobe. In the other patient with standard en bloc resection but clinically no hemianopia, the o ptic radiation was only marginally affected. In the third patient (amygdalo corticectomy), the diffusion anisotropy was within the normal range in the expected position of the optic radiation. Conclusions: Our findings show that MR tractography may be a useful tool to demonstrate wallerian degeneration in the optic radiation after temporal l obectomy in patients with hemi anopia. This is the first time that this new method has been applied in postoperative imaging; it enables us to visuali se the morphologic correlate of dysfunctional pathways after epilepsy surge ry in vivo. The potential for using MR tractography to study other aspects of epilepsy is discussed.