Patterns of Psychiatric Comorbidity and Genetic Correlations Provide New Insights Into Differences Between Attention-Deficit/Hyperactivity Disorder and Autism Spectrum Disorder.

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 analyzed 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 = 7528; 0.4%), and both diagnoses (n = 1467; 0.1%) compared with 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) among 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, in both men and women. The relative prevalence increase of substance use disorder was three times larger in ADHD than in ASD (PR, 6.2; 95% confidence interval [CI], 6.1-6.4; PR, 1.9; 95% CI, 1.7-2.2; p < .001); however, the opposite was true for schizophrenia (PR, 13.9; 95% CI, 12.7-15.2; PR, 4.4; 95% CI, 4.1-4.7; p < .001). Genetic correlations supported these patterns but were significantly different between ADHD and ASD only for the substance use disorder proxies and personality traits (p < .006 for all).

Conclusions: Adults with ADHD, ASD, or both ADHD and ASD have specific patterns of psychiatric comorbidities. This may partly be explained by differences in underlying genetic factors.