Abstracts

Rationale: There is conflicting evidence as to whether executive function (EF) improves or worsens in patients with refractory TLE after anterior temporal lobectomy (ATL) and how these changes relate to changes in white matter integrity (Stretton and Thompson, 2012). In this study, we examined pre- to post-ATL changes on a measure of EF in TLE, and studied the relationship between these changes and diffusion tensor imaging (DTI) - based estimates of white-matter integrity. We studied the uncinate fasciculus (UNC) since this tract connects the temporal lobes to the inferior frontal cortex that is implicated in EF tasks, and is routinely severed during ATL. We hypothesized that the TLE patients would show post-surgical changes in DTI-based metrics consistent with models of Wallerian degeneration adjacent to the surgical lesion, and that greater degeneration (e.g., lower FA, higher MD) would be associated with greater changes in EF.Methods: We obtained DTI from 24 patients with refractory TLE before and 1-year after standard ATL. We used Amira, a novel bio-imaging software, to segment the UNC, whose boundaries were estimated using an automated, Matlab-based implementation of a probabilistic tracing technique (Wakana et al. 2004, Hagler et al., 2009). We generated DTI parameters and tract volume estimates of the frontal and the temporal segments of UNC separately. For 20 patients, data was also available for pre and post-surgical performance on a visuomotor switching task (Trail Making Test; TMT). We used changes in the contralateral (i.e., unoperated) UNC as a within-subject control tract.Results: We found that the majority of patients (>80%) improved in task-switching following ATL; 21% of the patients showed improvements that exceeded the reliable change index (RCI) for TMT-B, while the remaining 79% showed improvement or decline that did not exceed the RCI. We also found that significantly lower FA and higher MD was only observed in the temporal UNC ipsilateral to the surgery side. Smaller, less significant changes were also observed in the ipsilateral frontal UNC. Diffusion changes were more observable for FA (p<0.00001) than MD (p< 0.001), and the decreases in FA were mostly driven by increases in transverse diffusion, with minimal changes in longitudinal diffusivity. There were no correlations observed between change in TMT performance and change in DTI metrics following ATL. DTI metrics for those showing EF improvements did not significantly differ from DTI metrics of those who did not show significant improvements.Conclusions: In our cohort, improvements in EF were common following epilepsy surgery, although there was significant variability among patients. These improvements are observed despite evidence of degeneration along key frontotemporal white matter tracts. Our findings indicate that EF in TLE does not depend on the integrity of the ipsilateral UNC, and that more complex and indirect structure-function associations may exist to help compensate for the Wallerian degeneration observed along the frontal UNC post ATL.