Comorbidity of asthma and attention deficit hyperactivity disorder in childhood: causal, shared early-life risk factors, or shared genetic liability?

BACKGROUND: The positive association between childhood asthma and attention deficit hyperactivity disorder (ADHD) has not been adequately explained. We aimed to investigate whether it is causal or whether shared genetics and/or shared early-life risk factors explain the link. METHODS: We used a two-sample Mendelian randomization (MR) to test whether there is a causal relationship between asthma and ADHD. In the Avon Longitudinal Study of Parents and Children (ALSPAC), we defined doctor-diagnosed asthma, its endotypes according to atopy determined by skin-prick tests, and the risk of ADHD by using the Strengths and Difficulties Questionnaire, all at 7 years of age. We explored whether the asthma-ADHD association was attenuated when controlling for an extensive number of early-life risk factors (N = 7165). Child polygenic risk scores (PRSs) for asthma and ADHD were calculated by using published genome-wide association studies across seven P-value thresholds (