Structural brain abnormalities found in patients with bipolar disorder may have been wrongly attributed to the condition and in fact be associated with comorbid attention-deficit hyperactivity disorder (ADHD), US researchers believe.
ADHD is a frequent additional diagnosis in adults with bipolar disorder, with a comorbidity prevalence of 5-20%, yet this is often overlooked when interpreting brain imaging data.
In the present study, Lori Altshuler (University of California at Los Angeles, USA) and team attempted to dissect the individual contributions of ADHD and bipolar disorder to brain abnormalities detected on magnetic resonance imaging (MRI).
They recruited 85 individuals, of whom 17 had bipolar disorder only, 19 had ADHD only, 18 had both bipolar disorder and ADHD, and 31 had no psychiatric illness. All patients with bipolar disorder were euthymic at the time of imaging and were not taking lithium.
All participants underwent MRI for assessment of cortical thickness. Analysis of the prefrontal cortex and anterior cingulate cortex indicated that overall cortical thickness was reduced in patients with bipolar disorder both with and without comorbid ADHD.
However, there were localized interactions with diagnosis in two regions of interest. Specifically, the effect of bipolar disorder on cortical thickness was different in patients with and without ADHD in the right orbitofrontal cortex and the left subgenual cingulate.
In the right orbitofrontal cortex, bipolar disorder was associated with significant cortical thinning only in the absence of an ADHD diagnosis; conversely, in the left subgenual cingulate, the presence of ADHD eliminated the cortical thinning associated with bipolar disorder relative to controls.
The effects of bipolar disorder and ADHD in these regions were found to be not additive but rather interdependent, "resulting in a unique phenotypic signature for the comorbid diagnostic group," write Altshuler et al in Bipolar Disorders.
Noting that this was one of the first studies to assess interactions between bipolar disorder and ADHD diagnoses on brain structure, the researchers conclude: "An accurate depiction of the underlying neural phenotype of [bipolar disorder], as opposed to that of a combined bipolar disorder/ADHD diagnosis, is essential for the understanding of the pathophysiology of bipolar disorder and developing targeted treatment approaches."
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