medwireNews: Researchers have found further evidence of significant cognitive impairment in patients with bipolar disorder, even during periods of euthymia.
However, their large meta-analysis of individual patient data across 31 studies showed effect sizes that were smaller than previously reported.
This reduction in effect sizes was partly due to improved controlling of confounding factors such as age, IQ, and gender, which are known to affect neuropsychologic test performance.
"Cognitive deficits are also not simply explained as side-effects of drug therapy," note Corin Bourne (University of Oxford, UK) and colleagues.
They found that patient performances on most of the 11 neuropsychologic tests assessed were unaffected by drug treatment.
"The only possible exception is on measures of verbal memory with antipsychotics having an impairing effect on VLT (Verbal Learning Task) Total1-5 and drug-free status being associated with improved performance on VLT Total1-5 and LongDelay (relative to any drug)," the team reports.
Data were available for 1267 euthymic bipolar patients (54.7% female) and 1609 mentally healthy individuals on 11 measures from four common neuropsychologic tests - California or Rey VLT, Trail Making Test (TMT), Digit Span and/or Wisconsin Card Sorting Task (WCST).
Significant cognitive deficits were found for all 11 test measures after accounting for confounding factors, with effect sizes ranging from -0.63 on TMT B to 0.60 on the reverse Digit Span test. Executive function and memory were particularly affected, but the effect sizes were small to medium in magnitude across all the cognitive domains studied.
"Our results could therefore… be interpreted as being consistent with the notion of cognitive impairment in bipolar disorder being a relatively non-specific effect on multiple functional brain networks," the team writes in Acta Psychiatrica Scandinavica.
Heterogeneity of some tests was high, ranging from a moderate 39% to 84%, with TMT B showing the highest heterogeneity. The magnitude of the effect size was associated with increased heterogeneity and also with residual depression, but these two factors did not fully account for the effect of bipolar disorder on cognition.
Performances on some of the neuropsychologic measures worsened with increasing illness severity. For example, increasing number of manic episodes was associated with a worse performance on certain VLT measures, while TMTA performance was worsened by potential illness progression effects such as number of total episodes and number of hospitalizations.
But the researchers say that "longitudinal data from earlier in the illness course are needed to show that the relationship is causal and clinically important."
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