Patients with depression have areas of reduced cortical thickness, with clear distinctions between those with bipolar disorder and major depressive disorder (MDD), report US researchers.
"Cortical thickness may prove to be a useful tool to distinguish [bipolar disorder] from MDD, and may contribute to the pathophysiology of manic episodes," write lead study author Martin Lan (Columbia University College of Physicians and Surgeons, New York) and team in Bipolar Disorders.
They note that distinguishing the two conditions in the absence of previous manic episodes is difficult, but also crucial because they require different therapeutic approaches.
Using magnetic resonance imaging, the team identified three areas that had significantly different cortical thickness between 18 patients with bipolar disorder and 56 with MDD. The most highly significant of these was in the right caudal middle frontal region, with cortical thickness at this area being an average of 7.5% less in the bipolar than the MDD patients.
This finding is consistent with several lines of evidence showing a role for the dorsolateral prefrontal cortex in bipolar disorder, say the researchers. Imaging and postmortem studies have shown altered structure, metabolism, and activation in this region in bipolar disorder patients.
Cortical thickness in this area was also significantly less in bipolar disorder patients than in 54 mentally healthy controls, by 9.1% on average.
The other two areas to significantly differ between bipolar and MDD patients were located in the left inferior parietal and the right precuneus regions, with average cortical thickness being 8.2% and 7.5%, respectively, less in the bipolar patients.
Cortical thickness in all three areas correlated with current age, but not with age at first episode, Hamilton Depression Rating Scale score, or total brain volume.
Another three areas that significantly differed between bipolar patients and healthy controls were located in the left superior parietal, right posterior cingulate, and right supra marginal regions.
Lan et al say that a "prospective, hypothesis-driven study" in a larger number of patients is needed to determine if these cortical thickness markers are predictive of disease course or outcomes.
And they add: "Establishing whether patients show differences in cortical thickness at their first symptoms of [bipolar disorder] could determine whether this finding can be used for diagnostic purposes."
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