Schizophrenia and Bipolar Disorder In Terms of Cognitive Impairments; Similarities and Differences - A Systematic Review

There is growing evidence about cognitive impairments in bipolar disorder (BB) as well as schizophrenia (SZ) in the last years. The identification of probable shared endophenotypes, as cognitive impairment, in these two diagnoses increased the discussions that emphasize the inadequacy of categorical classification. The studies published in the last ten years that investigated the level of cognitive impairments in adult patients with SZ and BB was reviewed systematically. Forty-nine studies met the inclusion criteria and were classified as follows: original comparative research (42), meta-analysis (2), review (3), and comment/discussion (2). The results of studies that implied the cognitive impairment in SZ and BB were qualitatively similar, but more severe and common in patients with SZ, and differed most significantly in IQ and verbal learning. These differences are not specific enough to make differential diagnosis. Despite the difficulties caused by the variety of methods and selection of patient population in interpreting the results, the existence of different subgroups of patients with different level of cognitive impairment in both diagnoses draws the attention. The cognitive impairment is more significant with negative symptoms or deficit syndrome in SZ patients and psychotic symptoms in BB patients. The clear existence of unaffected first degree relatives with cognitive impairment in SZ and probable in BB shows the genetic liability of cognitive impairment. (Archives of Neuropsychiatry 2010; 47: 150-7)