Is atopic disease a risk factor for attention-deficit/hyperactivity disorder? A systematic review

The increase in prevalence and burden of atopic diseases, i.e. eczema, rhinitis, and asthma over the past decades was paralleled by a worldwide increase in attention-deficit/hyperactivity disorder (ADHD) diagnoses. We systematically reviewed epidemiologic studies investigating the relationship between atopic diseases and ADHD. Electronic literature search in PubMed and PsycINFO (until 02/2010) supplemented by handsearch yielded 20 relevant studies totaling 170,175 individuals. Relevant data were abstracted independently by two reviewers. Six studies consistently reported a positive association between eczema and ADHD with one study suggesting effect modification by sleeping problems. Twelve studies consistently found a positive association between asthma and ADHD, which, however, appeared to be at least partly explained (confounded) by concurrent or previous eczema. Rhinitis and serum-IgE level were not related to ADHD symptomatology. We conclude that not atopic disease in general, but rather that eczema appears to be independently related to ADHD. Conclusions about temporality and whether the observed association constitutes a causal relationship are impossible, as most studies were cross-sectional (n = 14; 70%) or case-control studies without incident exposure measurement (n = 5; 25%). Another methodological concern is that the criteria to define atopic disease and ADHD were inadequate in most studies. A failure to adjust for confounders in the majority of studies was an additional limitation so that meta-analysis was not indicated. Future interdisciplinary high-quality prospective research is needed to better understand the mechanisms underlying the relationship between eczema and ADHD and to eventually establish targeted preventive and treatment strategies.