Brain functional connectivity differs between unipolar and bipolar depression

Patients with bipolar depression differ from their peers with unipolar depression in having stronger functional connectivity in the right posterior cingulate cortex (PCC), US study findings indicate.

Moreover, the degree of connectivity in this region may be associated with depression severity and suicide risk but only in patients with unipolar depression.

"This result provides preliminary evidence that right PCC functional connectivity not only differentiates unipolar versus bipolar II depression but may also mediate differences in neural processes associated with symptom expression in unipolar versus bipolar II individuals," say William Marchand (George E Wahlen Veterans Affairs Medical Center, Salt Lake City, Utah) and colleagues.

They add: "If replicated, our results could eventually lead to the development of objective neuroimaging methods to differentiate the two conditions. Furthermore, these results expand our understanding of the neurobiology of these disorders."

The researchers studied 14 individuals with bipolar II depression and 26 patients with recurrent unipolar depression, aged between 21 and 45 years. All participants underwent functional magnetic resonance imaging (fMRI) and functional connectivity analyses while performing two repetitions of a motor activation task. In addition, they were assessed using a variety of scales, including the Montgomery-Asberg Depression Rating Scale and the Beck Scale for Suicide Ideation.

The two groups did not significantly differ in their task performance. However, bipolar patients had significantly stronger functional connectivity between the PCC and one cluster in the right parietal/insular region, compared with unipolar patients. This cluster included portions of the right inferior parietal lobule, the precentral gyrus and insula, and surrounding regions.

As reported in Journal of Affective Disorders, PCC functional connectivity with several brain regions was associated with depression severity and current suicidal ideation.

Specifically, depression severity was correlated with functional connectivity between the PCC and clusters in the left temporal, bilateral frontal, right inferior frontal, and right middle frontal cortical regions in unipolar patients.

Current suicidal ideation was correlated with functional connectivity between the PCC and clusters in the left precentral, middle and inferior frontal gyri, and the right middle frontal gyrus, again in unipolar patients.

No significant correlations between functional connectivity and depression severity or current suicidal ideation were found for bipolar patients.

"Further studies are warranted to replicate this result as well as further dissociating neural processes that may differentiate the two conditions. Such work might eventually lead to the development of clinically useful diagnostic methodologies," the researchers conclude.

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