Microstructural white matter changes in euthymic bipolar patients: a whole‐brain diffusion tensor imaging study

Objectives: Brain structures of a distributed ventral‐limbic and dorsal brain network have been associated with altered mood states and emotion regulation in affective disorders. So far, diffusion tensor imaging studies in bipolar patients have focused on frontal/prefrontal brain regions and found alterations in white matter integrity in manic, depressed, and euthymic bipolar patients, observed as changes in fractional anisotropy and mean diffusivity. To extend previous findings, we investigated whole‐brain modifications in white matter integrity in euthymic bipolar patients with minimal manic and depressive symptoms. Methods: Twenty‐two patients with a DSM‐IV‐TR diagnosis of bipolar I and II disorder in remission, with no lifetime or present comorbidities of substance abuse, and 21 sex‐ and age‐matched healthy controls underwent diffusion tensor imaging with diffusion gradients applied along 41 directions. Fractional anisotropy and mean diffusivity group differences were explored using two voxel‐based, whole‐brain analyses that differ in their normalization approaches. Results: Fractional anisotropy was significantly increased in bipolar patients relative to healthy controls in medial frontal, precentral, inferior parietal, and occipital white matter. No group differences in mean diffusivity were found. Conclusions: The result of increased fractional anisotropy in euthymic bipolar patients in the present study suggests increased directional coherence of white matter fibers in bipolar patients during remission.