Alterations in microstructure and local fiber orientation of the white matter are associated with outcome following mild traumatic brain injury.

Mild traumatic brain injury (mTBI) can result in long-lasting consequences. We investigated white matter (WM) alterations at 6-12 months following mTBI using diffusion tensor imaging and assessed if the alterations associate with outcome. Eighty-five patients with mTBI underwent diffusion-weighted MRI on average eight months post-injury and patients' outcome was assessed at the time of imaging using extended Glasgow outcome scale. Additionally, thirty age-matched patients with extracranial orthopedic injuries were used as control subjects. Voxel-wise analysis of the data was performed using tract-based spatial statistics approach and differences in microstructural metrics between groups were investigated. Furthermore, the susceptibility of the abnormalities to specific fiber orientations was investigated by analyzing the first eigenvector of the diffusion tensor in the voxels with significant differences. We found significantly lower fractional anisotropy and higher mean and radial diffusivity in patients with mTBI compared to control subjects while no significant differences were observed in axial diffusivity between the groups. The differences were present bilaterally in several WM regions and correlated with outcome. Moreover, multiple clusters were found in the principal fiber orientations of the significant voxels in anisotropy, and similar orientation patterns were found for the diffusivity metrics. These directional clusters correlated with patients' functional outcome. Our study showed that mTBI is associated with WM changes at the chronic stage and these alterations occur in several WM regions. In addition, several significant clusters of WM alterations in specific fiber orientations were found and these clusters were associated with outcome.