Research reportA meta-analysis of cognitive deficits in euthymic patients with bipolar disorder
Introduction
Although classically conceptualised as a disorder of mood, a consensus is emerging that patients with bipolar disorder show cognitive deficits both during the acute phase of illness and during remission (Bearden et al., 2001, Quraishi and Frangou, 2002, Savitz et al., 2005). The significance of cognitive deficits in remitted patients is currently not known, but evidence suggests they are negatively related to functional outcome (Atre-Vaidya et al., 1998, Martinez-Aran et al., 2002) and may worsen with increased number of episodes (Denicoff et al., 1999). A profile of deficits has not yet been established as many of the studies in euthymic patients have had low statistical power or have not reported effect sizes. Establishing the magnitude of size effects makes the profile of deficits clearer.
The aim of the present review is to meta-analyse the neuropsychological findings of published studies of euthymic bipolar patients which will aid in both identifying deficits and limiting type II errors from studies with small samples.
Section snippets
Search strategy and study selection criteria
The title and abstract fields of the electronic databases Medline, Embase, Web of Knowledge, and PyschInfo were searched using the following search terms: bipolar disorder or manic depress⁎ to identify the relevant illness group, and cognit⁎, attention, learning, memory, executive, or neuropsych⁎ to identify relevant studies (⁎ indicates a wildcard). The search was limited to studies in English available between 1980 and August week 1 2005. Inclusion criteria comprised: (1) included an adult
Results
Table 2 and Fig. 1 report the magnitude of the effect sizes computed by the meta-analyses. In all instances (except overall IQ) patients with bipolar disorder performed significantly more poorly than controls (p < 0.004; Table 2). Bipolar patients did not differ significantly from controls in years of formal education received.
The greatest impairment was found in measures of executive function and verbal learning, whereas attention, psychomotor speed and immediate memory were much less affected.
Discussion
These meta-analyses provide strong evidence of cognitive impairments, especially in executive function and verbal learning in bipolar disorder. Large effect sizes were noted for two aspects of executive function (category fluency and mental manipulation) and one of verbal learning. Moderate effect sizes were reported for indices of short and long delay verbal memory, response inhibition, sustained attention, psychomotor speed, abstraction and set-shifting. Small effect sizes were noted for
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