Cognitive functioning in patients with schizophrenia and bipolar disorder: A quantitative review
Introduction
Accumulating evidence has cast doubt on the Kraepelinian notion (Kraepelin, 1913) that bipolar disorder, in contrast to schizophrenia, is not characterized by cognitive decline (Altshuler et al., 2004, Krabbendam et al., 2000, Martinez-Aran et al., 2004, Van Gorp et al., 1998). A review of 42 studies concluded that cognitive deficits are common in bipolar disorder, predominantly, though not exclusively, during episodes of mania and depression (Quraishi and Frangou, 2002). During recovery, deficits in the areas of verbal memory, sustained attention, and, less consistently, executive functions and visual memory persist (Quraishi and Frangou, 2002). Whether the pattern of cognitive deficits in bipolar disorder is comparable to the deficits observed in schizophrenia is still a matter of debate. Some authors have claimed that remitted patients with bipolar disorder perform notably better than stable patients with schizophrenia (Goldberg, 1999, Quraishi and Frangou, 2002) and that the cognitive profile in bipolar disorder is characterized by selective rather than generalized deficits (e.g., in the area of visual processing) (Goldberg, 1999, Green et al., 1994). Others have emphasized the similarities, stating that in both disorders the cognitive profile is characterized by a relatively generalized pattern of deficits, even though quantitative differences may exist (Hoff et al., 1990, Krabbendam et al., 2000, Martinez-Aran et al., 2004).
Any overlap in cognitive impairment in bipolar disorder and schizophrenia would be in line with many other findings. Although classification systems such as DSM-IV and ICD-10 attempt to distinguish between schizophrenia and bipolar disorder, it is a common observation in clinical practice that many patients do not fit properly into the dichotomy. The frequency of schizoaffective disorder testifies to that (Marneros, 2003, Tien and Eaton, 1992), as does the finding that the positive symptoms of psychosis and the mood symptoms of mania and depression frequently co-occur in the same patient (Brockington et al., 1980, Brockington et al., 1979, Siris, 2000). In addition, risk factors and epidemiological characteristics tend to overlap between schizophrenia and bipolar disorder. Developmental factors such as low educational achievement and delayed motor and language milestones increase the risk for both schizophrenia and affective disorders, although effect sizes tend to be greater for schizophrenia (Jones and Tarrant, 1999, Van Os et al., 1997). Similarly, social factors such as ethnicity and adverse life events affect the risk for affective disorders in particular, but also have their impact on the risk for schizophrenia (Van Os et al., 1998). Familial co-aggregation of schizophrenia and bipolar disorder suggest that the overlap may in part be based on a common genetic aetiology (Cardno et al., 2002, Maier et al., 2002). Recent studies have indeed implied shared susceptibility genes (Bramon and Sham, 2001, Wildenauer et al., 1999).
The apparent overlap between the two disorders in expression and aetiology has for more than a century fuelled spirited discussion as to whether schizophrenia and bipolar disorder are truly distinct diseases (Crow, 1998, Jablensky, 1999, Murray et al., 2004). To further our understanding of the relationship between the two disorders, it is crucial to identify the similarities and dissimilarities. The aim of this study was (i) to review quantitatively the studies on cognitive performance in schizophrenia and bipolar disorder and (ii) to examine whether the magnitude of any difference in performance is influenced by the clinical state of the study population.
Section snippets
Study selection
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*”. Furthermore, the reference list of recent reviews on the neuropsychology of bipolar disorder (Martinez-Aran et al., 2000, Quraishi and Frangou, 2002) was screened for any additional studies. The following criteria
All studies
The literature search yielded 31 studies that evaluated neuropsychological performance in patients with schizophrenia and patients with bipolar disorder using standardized neuropsychological instruments. These studies are listed in Table 1, along with the sample sizes, the main sample characteristics, and the task parameters yielded by the studies. The studies by Seidman and colleagues (Kremen et al., 2003, Seidman et al., 2002, Seidman et al., 2003) were based on the same sample, as were the
Discussion
Quantitative analysis of 31 studies that compared neuropsychological performance in patients with schizophrenia and patients with bipolar disorder indicated significantly worse performance in the patients with schizophrenia in nine out of eleven cognitive domains. The only areas in which performance of the two patient groups were not statistically significant were delayed visual memory and fine motor skills. However, the substantial heterogeneity of the effect sizes derived from the individual
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