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Introduction
Cognitive impairment is frequently observed in patients suffering from depression and is associated with poor response to treatment (Potter et al. 2004; Story et al. 2008; Roiser et al. 2012). Impaired cognition has been estimated to occur in around two-thirds of depressed patients (Abas et al. 1990; Butters et al. 2004; Afridi et al. 2011). Impaired ability to think, concentrate or make decisions is a DSM-IV-TR (APA, 2000) diagnostic criterion for major depressive episode. Consistent with this, several systematic reviews have demonstrated cognitive deficits in patients suffering from depression (Burt et al. 1995; Veiel, 1997; Zakzanis et al. 1998; Stefanopoulou et al. 2009; Snyder 2013), including first-episode patients (Lee et al. 2012).
Impairments in cognition have been found to persist beyond acute episodes of depression, and between one-third and one-half of remitted depressed patients are thought to be affected by cognitive deficits (Abas et al. 1990; Bhalla et al. 2006; Reppermund et al. 2009). Furthermore, one study revealed that 94% of patients who had cognitive impairment while depressed continued to experience deficits in cognition when remitted from depression (Bhalla et al. 2006).
To our knowledge, to date, only two groups have reviewed cognitive function in patients remitted from depression (Hasselbalch et al. 2011; Bora et al. 2013). The review by Hasselbalch et al. (2011) included 500 remitted patients (and 472 controls) and revealed impaired cognitive performance in nine of the 11 included studies. Their review also assessed the association between cognitive function and other clinical features such as residual depressive symptoms and current medication status. However, drawbacks of this review relate to the large number of different cognitive tests that were used across studies and the lack of implementation of standardized effect sizes to reflect magnitude of impairment. Meanwhile, the review by Bora et al. (2013) included 895 remitted patients (and 997 controls) from 27 studies and, using standardized effect sizes, revealed cognitive deficits in a composite measure of global cognition, in individual cognitive domain composites and in a subset of specific tasks. The review also separately assessed cognitive function in early-onset and late-onset patients and included a meta-regression to uncover the influence of other clinical and demographic factors on cognitive performance. Again, a minor drawback of...