Review articleSystematic review of environmental risk factors for Obsessive-Compulsive Disorder: A proposed roadmap from association to causation
Introduction
Despite dedicated research and some breakthroughs in the scientific understanding of relevant neurobiological and psychosocial factors, the causes of Obsessive-Compulsive Disorder (OCD) remain largely unknown. There is a pressing need to understand the causal mechanisms implicated in the disorder, with views to improving early detection and clinical outcomes, reducing chronicity and societal costs (Fineberg et al., 2013).
Controlled family studies have consistently found that OCD is a familial condition, with first degree relatives of OCD probands being approximately 4–5 times more likely to have OCD themselves, compared to relatives of unaffected controls (Mataix-Cols et al., 2013, Taylor, 2013). This risk diminishes as the genetic distance to the relative increases (Mataix-Cols et al., 2013). For instance, the risk for second- and third-degree relatives drops to approximately 2-fold and 1.5-fold, respectively. Twin studies of obsessive-compulsive symptoms (OCS) measured dimensionally in nonclinical populations suggest that both additive genetic effects and non-shared environment effects account for most of the variance in OCS (38–40% and 50–52%, respectively), with shared environment making a negligible contribution (5–6% of the variance; Taylor, 2011). Collectively, these lines of evidence indicate that OCD has a genetic basis. Gene-searching efforts are well underway, with two modestly sized genome-wide association studies (GWAS) having recently been published (Mattheisen et al., 2015, Stewart et al., 2013). Though these genomic efforts have met with limited success to date, they are likely to continue, and eventually succeed, with a dramatic increase in sample sizes expected in the next few years. While these gene-searching endeavours are undoubtedly important, it is also clear, based on available evidence, that OCD is only moderately heritable. The identification of the specific environmental risk factors that confer risk to OCD and interact with genetic factors is therefore as important as identifying genes, and could be regarded as a priority because some environmental risk factors may be potentially amenable to early intervention or even prevention strategies.
The literature on environmental risk factors for OCD, broadly defined here as any factors that are not directly genetic in nature, is methodologically diverse and difficult to reconcile. In this context, we thought it timely to conduct the first systematic review of a broad range of potential environmental risk factors for OCD and to propose a roadmap to guide future research in the area. We will argue that the field is ripe to move from largely descriptive research to the identification of causal factors conferring risk to the disorder, either directly or via genetic influences.
Section snippets
Methods
The systematic literature review was conducted following the PRISMA guidelines (Moher et al., 2009). The search strategy, as well as inclusion and exclusion criteria, and data to be extracted, were defined in advance. First, we conducted a structured literature search in PubMed, Scopus and Embase databases, using relevant keywords and Medical Subject Heading (MeSH) search terms (see Supplement 1); encompassing both concepts pertaining to risk factors in a general sense and more specific areas
Results
The literature search yielded 9950 records. After the screening and assessment process, 128 papers were ultimately included in this review, including 12 papers from sources other than the initial search (Fig. 1).
Discussion
In an attempt to summarise a large and disperse literature, we conducted the first systematic review of potential environmental risk factors for OCD and critically appraised the methodological quality of research in this area. Taken together, our review identified a range of potential environmental risk factors for OCD, particularly in the broad areas of perinatal complications, reproductive cycle events and stressful or traumatic life events but none of these factors can be conclusively said
Conclusions
The body of literature on putative risk factors for OCD is severely limited by both amount of research and methodological issues, but have provided some indications of areas of high interest for further research, such as birth complications, parental age, pregnancy and the postpartum period, infection as well as stressful life events. However, because of the limited methodological quality of the studies, we propose that, at this stage, all environmental risk factors remain potential candidates
Conflicts of interest
All authors declare no competing interests.
Financial support
This study was partially funded by a Karolinska Institutet PhD stipend (KID-funding).
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