Neuropsychological testing of cognitive impairment in euthymic bipolar disorder: an individual patient data meta-analysis

Objective: An association between bipolar disorder and cognitive impairment has repeatedly been described, even for euthymic patients. Findings are inconsistent both across primary studies and previous meta-analyses. This study reanalysed 31 primary data sets as a single large sample (N = 2876) to provide a more definitive view. Method: Individual patient and control data were obtained from original authors for 11 measures from four common neuropsychological tests: California or Rey Verbal Learning Task (VLT), Trail Making Test (TMT), Digit Span and/or Wisconsin Card Sorting Task. Results: Impairments were found for all 11 test-measures in the bipolar group after controlling for age, IQ and gender (Ps <= 0.001, E.S. = 0.26-0.63). Residual mood symptoms confound this result but cannot account for the effect sizes found. Impairments also seem unrelated to drug treatment. Some test-measures were weakly correlated with illness severity measures suggesting that some impairments may track illness progression. Conclusion: This reanalysis supports VLT, Digit Span and TMT as robust measures of cognitive impairments in bipolar disorder patients. The heterogeneity of some test results explains previous differences in meta-analyses. Better controlling for confounds suggests deficits may be smaller than previously reported but should be tracked longitudinally across illness progression and treatment.