Cognitive functioning in patients with schizophrenia and bipolar disorder: A quantitative review

OBJECTIVE Evidence suggests that cognitive functioning in bipolar disorder may be impaired even in euthymic states, but it is unclear if the pattern of deficits is similar to the deficits found in schizophrenia. The aim of this study was to review quantitatively the studies on cognitive performance in schizophrenia and bipolar disorder. METHODS Articles for consideration were identified through a literature search in MEDLINE and PsycLIT in the period between 1985 and October 2004, using the keywords "schizophrenia" combined with "bipolar disorder", or "manic-depress*" or "manic" combined with "cogniti*" or "neuropsycholog*". Thirty-one studies were included that: i) evaluated cognitive performance using standardized and reliable neuropsychological testing procedures; ii) compared adult patients with schizophrenia and with bipolar disorder; iii) reported test scores of both patient groups, or exact p-values, t-values, or F-values; and iv) were published as an original article in a peer-reviewed English language journal. RESULTS Meta-analyses of all studies indicated that patients with bipolar disorder generally perform better than patients with schizophrenia, but the distribution of effect sizes showed substantial heterogeneity. Results based on a more homogeneous subset of studies that matched patient groups on clinical and demographic characteristics pointed in the same direction, with effect sizes in the moderate range. CONCLUSIONS Patients with bipolar disorder show better cognitive performance than patients with schizophrenia, even when matched for clinical and demographic characteristics.