Research shows that patients with bipolar I disorder have reduced white matter integrity, including in the corpus callosum, which connects the brain’s hemispheres.
Furthermore, reduced white matter integrity of the corpus callosum was most striking in the subgroup of patients with psychosis.
In an editorial accompanying the study in JAMA Psychiatry, Kathryn Cullen and Kelvin Lim, both from the University of Minnesota in Minneapolis, USA, say: “This finding makes intuitive sense: the patient group with more severe psychopathology showed a more severe biological abnormality.”
They note that the overall findings support a longstanding theory that bipolar disorder patients have reduced interhemispheric connectivity.
Researchers Josselin Houenou (Hôpital Henri Mondor–Albert Chenevier, Créteil, France) and colleagues looked at fractional anisotropy for 22 known deep white matter bundles in 118 patients with bipolar I disorder and 86 healthy controls who underwent magnetic resonance imaging.
Relative to the controls, patients had significantly reduced fractional anisotropy, indicating decreased white matter integrity, in the body and splenium of the corpus callosum. They also had significant reductions in the anterior segment of the left arcuate and the long fibres of the left cingulum.
There were no differences between patients and controls in the right hemisphere, which contrasts with some previous studies, but the researchers say that, as they calculated fractional anisotropy for each whole tract, they could have missed focal changes.
One of the differences between patients and controls – reduced integrity in the body of the corpus callosum – was even more pronounced among the 57 patients with a history of psychotic features during a mood episode, who had significantly lower fractional anisotropy than the patients without psychosis.
The associations were independent of age and gender and remained significant after accounting for patients with current manic or mixed symptoms, those taking mood stabilisers and those with a history of alcohol abuse. There was no association between current medication use and fractional anisotropy measures.
The researchers say their findings suggest that psychotic features denote a “relevant subtype” of bipolar disorder with “specific pathophysiological features.”
Editorialists Cullen and Lim say: “The next step in the field is to further understand the functional relevance of these findings – how they fit in with the overall pathophysiology of [bipolar disorder] and other conditions, and whether in light of an individual’s specific deficits, interventions could be tailored to enhance the capacity of neural systems to best compensate for these abnormalities.”
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