Interhemispheric resting state functional connectivity abnormalities in unipolar depression and bipolar depression.

OBJECTIVES: Depression in the context of bipolar disorder (BD) is often misdiagnosed as unipolar depression (UD), leading to mistreatment and poor clinical outcomes. However, little is known about the similarities and differences in interhemispheric functional connectivity between BD and UD.
METHODS: Patients with bipolar II disorder (n = 36) and UD (n = 32) during a depressive episode as well as 40 healthy controls underwent resting-state functional magnetic resonance imaging. The functional connectivity between any pair of symmetric interhemispheric voxels (i.e., functional homotopy) was measured by voxel-mirrored homotopic connectivity (VMHC).
RESULTS: The three groups showed significant VMHC differences in the posterior cingulate cortex (PCC), fusiform and lingual gyrus, anterior lobe of the cerebellum (CeAL), and posterior lobe of the cerebellum (CePL). In the BD group, the VMHC decreases in the fusiform/lingual gyrus, CeAL, and CePL were shown relative to controls. In the UD group, the VMHC decreases in the PCC, fusiform/lingual gyrus, and CePL were shown relative to controls. No regions of increased VMHC were detected in either patient group. There was no significant difference in the VMHC values in any brain region between the BD group and the UD group.
CONCLUSIONS: Depressive episodes in BD and UD have similar impairments of interhemispheric coordination, which might imply an overlap in the neuropathology of depression.