Patterns of psychiatric comorbidity and genetic correlations provide new insights into differences between attention-deficit/hyperactivity disorder and autism spectrum disorder

Abstract Background Attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) share common genetic factors, but seem to have specific patterns of psychiatric comorbidities. There are few systematic studies on adults, therefore we compared psychiatric comorbidities in adults with these two neurodevelopmental disorders using population-based data, and analysed their genetic correlations to evaluate underlying factors. Methods Using data from Norwegian registries, we assessed patterns of psychiatric disorders in adults with ADHD (n=38,636; 2.3%), ASD (n=7,528; 0.4%) and both diagnoses (n=1,467; 0.1%), compared to the remaining adult population (n=1,653,575). We calculated their prevalence ratios (PRs) and differences using Poisson regression, also examining sex-specific relations. Genetic correlations (r g ) between ADHD, ASD and the examined psychiatric disorders were calculated by linkage disequilibrium score regression, exploiting summary statistics from relevant genome-wide association studies. Results For all psychiatric comorbidities, PRs differed between ADHD and ASD. Associations were strongest in individuals with ADHD and ADHD+ASD for most comorbidities, both in men and women. The relative prevalence increase of substance use disorder (SUD) was three times larger in ADHD than in ASD (PR ADHD 6.2; 95% CI, 6.1-6.4, PR ASD 1.9; 95% CI, 1.7-2.2, p ASD =13.9, 95% CI, 12.7-15.2, PR ADHD =4.4; 95% CI, 4.1-4.7, p p 0.006 for all). Conclusions Adults with ADHD, ASD or both, have specific patterns of psychiatric comorbidities. This may partly be explained by differences in underlying genetic factors.