Researchers Suggest Asthma, ADHD Linked in 2 Studies

Asthma and attention-deficit/hyperactivity disorder (ADHD), both chronic conditions, have been associated with each other in previous studies, but questions remained as to the strength of the association because of other possible confounders. A new review of published data, followed by a population-based study, says the association remains after controlling for possible confounders.

The research, based on data from Sweden, was published recently in The Lancet Psychiatry. The authors said awareness of the association could help to reduce the delay in the diagnosis of both diseases, and that longitudinal studies are needed to determine whether this association is causal or due to shared factors, and to investigate a possible role of allergic mechanisms in ADHD.

ADHD affects around 5% of children and 2% to 5% of adults worldwide. Asthma is the most prevalent chronic respiratory disease, estimated to affect around 358 million people worldwide. In the United States, the economic burdens of asthma are estimated to be at least $80 billion annually, according to a CDC study published earlier this year.

The researchers identified 2649 potentially eligible citations, obtaining 49 datasets, including a total of 210,363 participants with ADHD and 3,115,168 without. The pooled unadjusted odds ratio (OR) was 1.66 (95% CI, 1.22-2.26) and the pooled adjusted OR was 1.53 (1.41-1.65), indicating a significant association between asthma and ADHD.

The population-based study included 1,575,377 individuals born between January 1, 1992, and December 31, 2006, of whom 259,253 (16.5%) had asthma and 57,957 (3.7%) had ADHD.

Asthma was significantly associated with ADHD (OR, 1.60; 95% CI 1.57-1.63) in the crude model adjusting for sex and year of birth, and this association remained significant after simultaneous adjustment for all covariates (1.45; 1.41-1.48).

Confounders included maternal age at birth, mother’s country of birth, mother’s cohabitation status at childbirth, highest parental education status, and family disposable income.

Individual-level confounders included sex, year of birth, birthweight, gestational age, and a diagnosis of another atopic disease, eczema, in childhood.

The meta-analysis had some limitations. The researchers said they could not fully address the role of possible confounders because most potential confounders were missing from several studies. In addition, the diversity of the studies indicates that the pooled OR cannot appropriately summarize results from all datasets. The heterogeneity of the studies decreased when studies with lifetime and current prevalence of asthma were analyzed separately, suggesting that the time factor in the asthma diagnosis might affect the heterogeneity of the results.

Despite those limitations, the results were consistent with those from the population-based study.

Additionally, the increase in the prevalence of asthma in individuals with versus those without ADHD was similar in the meta-analysis (about 50%) and the population-based study (about 60% when restricted to clinical diagnosis of asthma in the National Patient Register), suggesting the results are clinically meaningful, the researchers wrote.

The study mentioned a previous meta-analysis that concluded that childhood asthma predicted future occurrence of ADHD, and the researchers theorized that inflammatory mechanisms might mediate a link between asthma and future ADHD.

An increase in inflammatory cytokines following the allergic inflammation that characterizes asthma might affect certain regions in the prefrontal cortex and neurotransmitter systems that have been implicated in ADHD, they wrote.

Reference

Cortese S, Sun S, Zhang J, Almquvist C, Lrsson H, Faraone SV. Association between attention deficit hyperactivity disorder and asthma: a systematic review and meta-analysis and a Swedish population-based study. [published online July 24, 2018]. Lancet Psychiatry.

doi: 10.1016/S2215-0366(18)30224-4