Background: White matter abnormalities have been frequently associated with bipolar disorder. Recent diffusion tensor imaging studies found the white matter integrity changes in neural circuits that mediate emotional regulation. In the current study, differences and similarities in microstructural white matter alterations between bipolar I and bipolar II disorders were investigated.
Methods: Eleven patients with bipolar I disorder, 11 patients with bipolar II disorder and 26 healthy controls underwent diffusion tensor imaging with diffusion gradients applied along 15 directions. Fractional anisotropy (FA) and mean apparent diffusion coefficient (ADC) maps were compared between groups using voxel-based whole brain analyses.
Results: The bipolar I group had a lower FA in the right prefrontal, left temporal, extra-striatal and the bilateral cingulate regions, and a higher FA in the pons than controls. On the other hand, a decreased FA in the bipolar II group was found only in the right prefrontal region. There was no differences in the FA between the bipolar I and bipolar II groups. The ADC was increased in the frontal, temporal and limbic regions in the both patients groups, compared to controls. The bipolar I group had a higher ADC in the right striatal and bilateral prefrontal regions than the bipolar II group.
Conclusions: The results suggest that regional microstructural white matter alterations may contribute to the pathophysiology of bipolar disorders. Additionally, the different patterns of FA and ADC changes between bipolar I and bipolar II groups suggest distinct pathophysiological mechanisms between the two subtypes.