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Background: More and more epidemiological, genetic and neuroimaging studies show similarities between bipolar disorder (BD) and schizophrenia (SZ). Cognitive functions are known to be highly impaired in SZ and are increasingly studied in BD. When both populations are compared, the conclusions appear to be contradictory. The purpose of this review is to help define the profile of cognitive deficits in BD and in SZ. Methods: A systematic review of the literature of neuropsychological studies comparing BD and SZ was made, beginning in January 1990 and ending in January 2005. Thirty-eight studies met the required quality criteria and were included in this review. Results: Bipolar patients exhibit extensive cognitive abnormalities with a pattern of deficits that is not unique to this disease. However, when compared to schizophrenic patients, bipolar patients demonstrate a lesser degree of deficits, particularly concerning premorbid and current intelligence quotient and perhaps attention, verbal memory and executive functions. When looking into effect sizes, there seem to be different profiles even in studies finding no significant differences. Conclusions: The neuropsychological differences reported between both groups could be due to the presence of psychotic features, to environmental factors (stressful events, duration of the disease and number of hospitalisations) and could also be related to differences during the neurodevelopmental phase. Further studies should confirm whether these results are truly related to different neurobiological backgrounds.

Background: Neurocognitive impairment has consistently been considered a central and stable feature in schizophrenia. As this possibility has been far less studied in bipolar disorder, we aimed to prospectively investigate the stability and specificity of cognitive performance in bipolar disorder compared to schizophrenia. Methods: Fifteen DSM-IV bipolar type I patients and 15 schizophrenic patients were assessed twice with a comprehensive neuropsychological battery and the Positive and Negative Syndrome Scale over a 3-year follow-up. The cognitive performance of the groups was compared at baseline and 3 years later as a mean with that of 26 healthy volunteers. Endpoint and baseline assessments were also compared for each patient group in order to evaluate the stability of cognitive impairment. Results: At both time points, bipolar and schizophrenic patients showed significant deficits on most of the cognitive tasks compared to healthy subjects. Overall, the cross-sectional cognitive profile was similar for both patient groups. Moreover, after controlling for age and length of illness, the two groups’ cognitive function did not differ over time in any test. With the exception of the Stroop color-word interference task, performance at baseline for each test but neither length of illness nor diagnostic category predicted the endpoint performance. Conclusion: This preliminary study suggests that cognitive impairment is also mainly stable over time in bipolar I disorder and thus not specific to schizophrenia.