To Örebro University

A significant cross-sectional association between asthma and ADHD was reported from a recent meta-analysis and population based study. Asthma and ADHD both aggregate in families, but whether and to what extent the phenotypic association can be explained by familial liability remain unknown. In this Swedish national register-based study, we investigated how asthma and ADHD co-aggregate within individuals and within families to test for the shared familial liability. We studied 2 714 000 individuals born in Sweden between 1983 and 2009, and extracted clinical diagnosis records of asthma and ADHD from Swedish registers. Logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (95% CI) the association between clinically diagnosed asthma and ADHD both within individuals and families. Individuals with asthma had higher risk of having ADHD compared to individuals without asthma (OR = 1.85, 95% CI 1.81–1.90). Within families, the association was stronger in monozygotic twins (OR = 1.96, 95% CI 1.25–3.07) than in dizygotic twins (OR = 1.62, 95% CI 1.36–1.92), full-siblings (OR = 1.50, 95% CI 1.46–1.55), maternal-half siblings (OR = 1.27, 95% CI 1.21–1.32), paternal half-siblings (OR = 1.11, 95% CI 1.06–1.16), full-cousins (OR = 1.12, 95% CI 1.10–1.14) and half-cousins (OR = 1.06, 95% CI 1.03–1.09). The pattern of associations estimated from different degrees of relatives supports shared familial liability, especially genetic overlap, underlying clinically diagnosed asthma and ADHD.