Compensatory brain mechanisms at play in siblings of bipolar patients

By Eleanor McDermid, Senior medwireNews Reporter

Patients with bipolar disorder and their unaffected siblings have deficits in a part of the brain responsible for automatic regulation of emotions, report researchers.

However, they found that the siblings may have a compensatory mechanism in the form of increased grey matter in part of the brain involved with voluntary emotional regulation.

In the team’s initial voxel-based morphometry analysis, the only difference to emerge was a significant grey matter reduction in the left orbitofrontal cortex (OFC), specifically in Brodmann area (BA) 11, in 28 patients with bipolar disorder relative to 30 mentally healthy controls.

Ali Saffet Gonul (Ege University School of Medicine, Izmir, Turkey) and colleagues then analysed predefined regions of interest, which showed that 28 siblings of the bipolar patients also had reduced grey matter volumes in this area, albeit to a smaller extent. The difference was independent of age, gender and intracranial volume.

“Smaller OFC volumes can be regarded as an inherited marker of risk for bipolar disorder as patients and their siblings alike were exposed to similar genetic and environmental risks,” the researchers write in Bipolar Disorders. They say that the OFC is involved in automatic emotion regulation, which is disrupted in patients with bipolar disorder.

The region-of-interest analysis also showed that the siblings had significantly larger grey matter volumes in the left dorsolateral prefrontal cortex (DLPFC) than both their bipolar relatives and the controls. The DLPFC “plays a central role in the reappraisal of the emotional stimuli and voluntary control of affective states”, say the researchers, adding that the increased volume in the siblings may confer resistance to bipolar disorder, despite the reduced OFC volume.

“However, because we do not know the volumetric status of the DLPFC before the onset of the illness in patients with bipolar disorder, and because we do not know whether the detected differences remain stable over the clinical course of the disorder, our interpretation remains provisional,” they caution.

In addition, the analysis showed volume reductions in the premotor/motor cortex (BA 6) in bipolar patients versus healthy controls. As this area is closely linked to the DLPFC, the researchers believe the deficit could “represent an impaired voluntary emotion control system paralleling the impaired automatic emotion control system involving the OFC in these patients.”

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