Review articleExecutive functioning deficits among adults with Bipolar Disorder (types I and II): A systematic review and meta-analysis
Introduction
Bipolar Disorder affects nearly 4.4% of the global population, and is ranked by the World Health Organisation as the seventh leading cause of years of life lost due to disability in males, and eighth in females (Sole et al., 2011a). The Diagnostic and Statistical Manual of Mental Disorders – 5th Edition (DSM-5) (APA, 2013) recognises two subtypes, BD1 and BD2: BD1 is characterised by the experience of a full manic episode, and often (but not always) accompanied by major depressive, or hypomanic episodes; and BD2 is defined by alternating periods of milder manic symptoms (hypomania) and depression. A significant body of evidence suggests that much of the difficulties in BD can be attributed to weaknesses in cognitive capacity, particularly those of the executive faculties (Goswami et al., 2006, Levy and Manove, 2012, Thompson et al., 2005, Torres et al., 2007).
Numerous definitions of EF have been rehearsed in the literature (see, Baddeley, 1996, Baddeley, 1998; Jurado and Rosselli, 2007; Lezak, 2004). Today, the term ‘executive function(s)’ is often used as an umbrella term to encapsulate an array of complex cognitive processes and sub-processes (Elliot, 2003). As defined by Miyake and Friedman (2012), the EFs are ‘a set of general-purpose control mechanisms, often linked to the pre-frontal cortex of the brain, that regulate the dynamics of human cognition and action’.
Numerous theoretical frameworks have been posited for understanding the executive system (Baddeley, 2002, Banich, 2009, Barkley, 1997, Fuster et al., 1985, Lezak, 2004, Miller and Cohen, 2001, Norman and Shallice, 1986, Zelazo et al., 1997), most of which can be distinguished by the degree of responsibility attributed to a ‘central executive’ for the control of executive and related processes. From these foundations, modern theorists conceptualise executive system as a ‘macro-system’, made up of executive ‘sub-functions’ that work together to achieve specific goals (Elliot, 2003, Jurado and Rosselli, 2007, Miyake et al., 2000, Miyake and Friedman, 2012). It is now generally accepted that the EFs operate across a vast network of neural circuitry, rather than within prefrontal areas alone (Elliot, 2003, Miyake et al., 2000). Due to such complexity, however, attempts to define EF often result in lists of cognitive abilities (e.g., SS, WM, PLA), reflecting the notion that EF is not a unitary, or straightforward concept (Elliot, 2003).
Section snippets
Executive functioning in Bipolar Disorder
Various executive impairments have been identified in the BD literature, impacting functioning in the domains of WM (Dias et al., 2008), SS (Trivedi et al., 2008a, ma et al., 2008b), PLA (Xu et al., 2012), ATT (Pattanayak et al., 2012), VF (Erol et al., 2014), and updating (Miyake and Friedman, 2012), for example. Importantly, executive deficits have been shown to have broad and significant implications in ‘real life’, often demonstrated as severe problems in the control and regulation of
Study design and inclusion criteria
A systematic review and meta-analysis of studies were undertaken. Studies included were randomised controlled trials (RCTs) conducted between 1994 and 2015 that included an experimental group with BD, and a healthy control group. All included studies assessed EF using neuropsychological assessment measures appropriate for that purpose. Studies based on subjective (self) report were excluded. Studies were published in full-text, in English, and included the results of the study as descriptive
Results of literature search
An online search of CINAHL Plus, ERIC, MEDLINE, PsychInfo, and Google Scholar databases for published studies that explored EF among individuals with BD returned 327 articles. Hand searching raised a further 113 articles for consideration. Of these 440 studies, 404 were excluded for not meeting inclusion criteria. Details of the search strategy and results are depicted in Fig. 1.
Characteristics of included studies
Thirty-six studies met criteria for inclusion in the current review; see Table 1. Studies were published between 1999
Discussion
The current project sought to investigate the EF of individuals with remitted BD. A systematic review and meta-analyses were undertaken to synthesise the available evidence, and quantify the magnitude of impairment for each bipolar subtype (BD1s and BD2s) when compared to HCs and each other. As a result, six core domains of EF important to the study of individuals with BD were identified: SS, ATT, VF, WM, PLA, and INH. The magnitude of impairment within each domain was calculated; individuals
Conclusion
Robust evidence of a generalised executive deficit for individuals with BD1 came from the results of the present review. For BD2s, impairment was noted in the domains of VF, WM, SS and ATT. With regard to whether a difference in performance between BD1s and BD2s exists, the evidence was mixed. Importantly, the results of this review indicate that, as far as EF is concerned, BD2 may not be the ‘milder’ subtype as previously believed, with BD2s noting deficit as or more severe than BD1s in some
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