ReviewA meta-analysis of depression severity and cognitive function
Introduction
Cognitive impairment has been widely reported in patients with major depression (e.g. (Austin et al., 2001, Goodwin, 1997). For example, Mandelli et al. (2006) found impaired performance on the WAIS-R subscales in a sample of depressed inpatients that included both responders and non-responders to pharmacological treatment. Furthermore, this impairment has been identified across a number of specific cognitive domains including executive function (e.g. (Baudic et al., 2004, Beats et al., 1996, Merriam et al., 1999, Nebes et al., 2003, Purcell et al., 1997, Reppermund et al., 2007), episodic memory (e.g. (Austin et al., 1992, Beats et al., 1996, Brebion et al., 1997, Butters et al., 2004, Porter et al., 2003, Smith et al., 1994), semantic memory (e.g. (Bhalla et al., 2005, Cataldo et al., 2005, Naismith et al., 2003, Sheline et al., 2006), visuo-spatial memory (e.g. (Abas et al., 1990, Elliott et al., 1996), and information processing speed (e.g. (Austin et al., 1992, Baune et al., 2006, Butters et al., 2004, Cataldo et al., 2005, Reppermund et al., 2007, Sheline et al., 2006).
However, conflicting patterns of cognitive impairment have been reported across studies. For example, Grant et al. (2001) found impaired performance of executive function, but no significant impairment of attention, memory and motor function. In terms of individual cognitive functions, contradictory results have been reported for episodic memory (Airaksinen et al., 2004, Baune et al., 2006), for semantic memory (Bhalla et al., 2005, Ilsley et al., 1995), for executive function (Baudic et al., 2004, Elderkin-Thompson et al., 2007), and for processing speed (Boone et al., 1995, Portella et al., 2003).
Different levels and patterns of cognitive impairment reported across studies have been attributed, in part, to different mean depression scores of patient samples (Austin et al., 2001, Herrmann et al., 2007). Increased severity of depression may be related to greater impairment on overall cognitive ability (e.g. (Austin et al., 1999), but this is not regularly replicated (e.g. (Kyte et al., 2005).
The relationship between severity of depression and cognitive impairment may, of course, differ across individual cognitive domains. Cataldo et al. (2005) reported that severity of depression was associated with performance in executive functioning and processing speed, but not with semantic memory. Naismith et al. (2003) found depression severity to be associated with semantic memory in addition to executive functioning. Contradictory results have been reported for the association between depression severity ratings and executive function (Degl'Innocenti et al., 1998, Grant et al., 2001), episodic memory (Brebion et al., 1997, Smith et al., 1994), semantic memory (Cataldo et al., 2005, Naismith et al., 2003), visuo-spatial memory (Abas et al., 1990, Elliott et al., 1996), and processing speed (Austin et al., 1992, Cataldo et al., 2005).
Narrative reviews have described no consistent pattern of association between depression severity and cognitive impairment in specific cognitive domains. We therefore conducted a systematic review and meta-analysis in order to quantify to what extent severity of depression may be associated with the level of impairment across cognitive domains. We included cross-sectional studies examining depression severity and performance on neuropsychological tests. Our hypothesis was that greater severity of depression would be associated with greater impairment in certain cognitive domains. Specifically it was predicted that depression severity would be associated with executive function, as despite conflicting findings review evidence has reported that in general, significant impairment in executive function has been observed in subjects with more severe depression (Austin et al., 2001). It was also predicted that processing speed would be associated with depression severity, due to the fact that psycho-motor slowing which is a feature of more severe depression may reduce subjects' performance in this function.
Section snippets
Data acquisition
Studies which investigated severity of depression in relation to performance on neuropsychological tests, and report a correlation score between the two variables were included in the meta-analysis.
Studies were obtained by a systematic search of MEDLINE, EMBASE and PsychINFO, using the search terms (‘depression’ OR ‘major depression’, OR ‘depressive illness’, OR ‘major depressive disorder’, OR ‘depressed’) AND (‘cognitive function’ OR ‘cognition’ OR ‘cognitive’ OR ‘neuropsychological’ OR
Results
In total, sixty-nine papers were identified which met the inclusion criteria. This number was reduced to a total of fourteen studies which reported a correlation value between depression severity and individual neuropsychological test scores (Abas et al., 1990, Austin et al., 1992, Baudic et al., 2004, Brebion et al., 1997, Butters et al., 2004, Cataldo et al., 2005, Elliott et al., 1996, Grant et al., 2001, Merriam et al., 1999, Naismith et al., 2003, Porter et al., 2003, Purcell et al., 1997,
Discussion
Our findings indicate that severity of depression is related to cognitive performance in episodic memory, executive function, and processing speed. Specifically, increased severity of depression was significantly associated with reduced cognitive performance across these domains. This result has implications for the inconsistent findings previously reported of the relationship between cognitive impairment and depression: contradictory reports of which cognitive tests may be affected by
Role of funding source
Gordon Small Charitable Trust.
Conflict of interest
No conflict declared.
Acknowledgement
We are grateful to the Gordon Small Charitable Trust for financial support.
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