Evidence Links Asthma and ADHD

Samuele Cortese, MD

A suspected link between asthma and attention-deficit hyperactivity disorder (ADHD) appears to have strengthened, according to recent analysis which controlled for a range of shared risk factors.

Samuele Cortese, MD, Center for Innovation in Mental Health, Academic Unit of Psychology, and Clinical and Experimental Sciences, Faculty of Medicine, University of Southampton, UK, and colleagues explained their rationale for investigating the possible association between these apparently disparate conditions.

Researchers noted that, from both a clinical and public health perspective, awareness of a significant association between the conditions would drive ADHD specialists to refer patients with early forms of asthma, and asthma specialists to refer patients with early ADHD symptoms, “thus helping to reduce the diagnostic delay that is concern for both ADHD and asthma.”

Following their investigation into the putative association via a systematic review and meta-analysis, Cortese and colleagues tested the association further by conducting a large population-based study that accounted for a range of possible confounders.

The systematic review and meta-analysis comprised 49 datasets including 210,343 subjects with ADHD and 3,115,168 without. This was a substantial increase from the 5 or 6 datasets included in previous meta-analyses, Jessica Agnew-Blais, PhD, research fellow at the MRC Social, Genetic & Developmental Psychiatry Centre at King’s College, London, noted in accompanying editorial.

Finding that the included studies had used different potential confounders to calculate adjusted odds ratios (ORs) for the association between asthma and ADHD, Cortese and colleagues designed their population-based study to account for all of the studies’ covariates.

Their cohort was composed of 1, 575,377 individuals from several Swedish national registers, born between 1992 and 2006. Diagnoses of ADHD and asthma were determined by the corresponding ICD-9 or ICD-10 codes in the medical record, or, for asthma, record of filling 2 prescriptions for asthma medication.

Among the covariates were birth year, parental education, family income, birthweight and gestational age. The researchers used a directed acyclic graph to visually establish the relative strength of association between the covariates. Each was then classified as either “confounder”, for possible common cause of asthma and ADHD; “mediator”, for lying on the causal pathway from asthma to ADHD; or “collider,” as a possible result in common to either asthma or ADHD. Their OR calculation was then based on the potential confounders, they indicate, to avoid introducing bias related to the mediator and collider categories.

From the meta-analysis, Cortese and colleagues determined a pooled adjusted OR for the association of the 2 conditions of 1.53 (95% Confidence Interval [CI] 1.41-1.65). The population-based cohort study provided a similar adjusted OR of 1.45 (CI 1.41-1.48).

“The combined results of the meta-analysis and the population-based study support a significant association between asthma and ADHD,” Cortese and colleagues concluded.

Agnew-Blasis agreed, noting the researchers provided “robust evidence of a cross-sectional association between asthma and ADHD.”

“Clarification of the nature of this association is needed from future studies to identify potential pathways for prevention and treatment,” Agnew-Blasis wrote.

The study, "Association between attention deficit hyperactivity disorder and asthma: a systematic review and meta-analysis and a Swedish population-based study," was published online in The Lancet Psychiatry last week.