Do premorbid and post-onset cognitive functioning differ between schizophrenia and bipolar disorder? A systematic review and meta-analysis

Background. Schizophrenia (SZ) is characterized by a broad global cognitive impairment that precedes the onset of the disease. By contrast, some studies suggest that premorbid deficits are absent, or even reversed, in bipolar disorder (BD). However, studies have shown impairments in cognitive functioning after the illness onset in both disorders. The aim of this study was to systematically review and meta-analyze those studies that compared premorbid and/or post-onset global cognitive function between SZ and BD. Method. We searched Medline (PubMed), EMBASE and PsycINFO for studies where information on cognitive functioning was collected in both SZ and BD within the same study or using the same methods. Results. Compared to healthy comparison groups, SZ patients showed a significant premorbid cognitive impairment [standardized mean difference (SMD) -0.597, 95% confidence interval (CI) -0.707 to -0.487, p< 0.0001] and a large post-onset impairment (SMD -1.369, 95% CI -1.578 to -1.160, p< 0.0001). We found small significant deficits in premorbid intellectual function in the BD group when this was assessed retrospectively (-0.147, 95% CI -0.238 to -0.056, p=0.001) but not prospectively (-0.029, 95% CI -0.199 to + 0.142, p=0.744), and moderate cognitive impairment after onset (SMD -0.623, 95% CI -0.717 to -0.529, p< 0.0001). Conclusions. SZ is characterized by significant deficits in premorbid intellectual function but the evidence regarding premorbid function in BD is equivocal. After illness onset, patients with both disorders seem to suffer a further decline in cognitive function but the magnitude of the impairment remains greater in SZ than in BD.