Cognition and White Matter Alterations in Schizophrenia, Bipolar Disorder

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Direct comparison of white matter tracts between individuals diagnosed with either schizophrenia or bipolar disorder has not been performed to date.
Direct comparison of white matter tracts between individuals diagnosed with either schizophrenia or bipolar disorder has not been performed to date.

According to new findings, published in Journal of Affective Disorders by investigators affiliated with Goethe University in Germany, Cardiff University in the United Kingdom, and Federal University of Ceará in Brazil, white matter, microstructural changes in the fornix are associated with cognitive ability in patients diagnosed with schizophrenia but not in those individuals diagnosed with bipolar disorder.

Schizophrenia and bipolar disorder are characterized by significant genetic overlap, as well as significant overlap in the presentation of clinical symptoms. Brain white matter abnormalities are known to exist in patients diagnosed with psychosis, but until now, the direct comparison of white matter changes between individuals with schizophrenia or bipolar disorder has not been performed.

Investigators used diffusion tensor imaging (DTI) to examine white matter tracts in 32 euthymic bipolar disorder type I patients (15 female), 26 schizophrenia patients (13 female), and 30 matched typical, healthy control participants (16 female). The mean age of onset of bipolar disorder in the sample was 32.9 (SD=10.95), and 24.31 (SD=4.88) for patients with schizophrenia. All participants were taking medications at the time of study enrollment, but patients did not receive benzodiazepine drugs for at least 1 month prior to any brain imaging procedures.

Investigators chose to examine 4 white matter tracts: the bilateral fornix, the bilateral cingulum, the corpus callosum, and the bilateral anterior thalamic radiation. Disruption in fornix integrity in schizophrenia patients has previously been shown. The volume of the fornix in individuals with either schizophrenia or bipolar disorder, however, was found previously not to be significantly different from that of control participants.

With respect to the cingulum, decreased fractional anisotropy (FA) values were observed in individuals with schizophrenia and bipolar disorder. White matter tract abnormalities in the corpus callosum were also reported in both groups of individuals. Finally, microstructural changes in the anterior thalamic radiation were previously observed in patients with schizophrenia, and these abnormalities were linked to cognitive deficits.

In the current study, data show widespread white matter tract abnormalities in patients with schizophrenia compared with control participants [ie, differences were observed in all 4 regions and all indices of microstructral integrity, including fractional anisotropy (FA), number of fiber tracts (NofT), tract length (Le), and tract volume (vol)]. Differences in tract integrity were much smaller when these indices were compared between patients with bipolar disorder and controls, and alterations were noted only in the bilateral fornix.

Additionally, the alterations of the fornix have a functional relevance for cognitive performance (ie, reduced executive functioning and psychomotor speed) in patients with schizophrenia, but not bipolar disorder. These data “suggest that cognitive symptoms are closely associated with white matter changes in the fornix, at a greater (and significant) extent in schizophrenia than in bipolar disorder,” the authors wrote in their publication.

Reference

Knöchel C, Schmied C, Linden DE, et al. White matter abnormalities in the fornix are linked to cognitive performance in SZ but not BD disorder: An exploratory analysis with DTI deterministic tractography. J Affect Disord. 2016;201:64-78.

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