To Örebro University

Objectives: Observational studies have reported associations between ADHD and PTSD. But these observed associations can reflect causal relationships in either direction or be confounding. We thus aimed to estimate the potential causal effect of ADHD on the risk of subsequently developing PTSD conducting a 2-sample Mendelian randomization (MR) and a population-based sibling comparison study.

Methods: This 2-sample MR and population-based sibling comparison study assessed the association between ADHD and PTSD using a genome-wide association study (GWAS) summary data of European individuals and a population-based cohort of 2,082,118 individuals. The ADHD GWAS data consisted of 20,183 cases and 35,191 controls. PTSD GWAS data sets included up to 320,369 individuals. Two-sample MR tested potential causal associations of genetic variants associated with ADHD and PTSD symptoms adjusting for potential confounders. In the population-based comparisons, Cox regression models were fitted to account for time at risk, a range of sociodemographic factors, and unmeasured familial confounders (via sibling comparisons).

Results: Based on large GWAS data sources, regression (rg) analyses revealed consistent associations (rg range, 0.43-0.52; p < .001) between ADHD and PTSD. ADHD genetic liability was causally linked with increased risk for PTSD (inverse variance weighted [IVW] OR = 1.45; 95% CI, 1.20-1.74; p = 7.68x10-5). This result was not affected by heterogeneity or horizontal pleiotropy (MR Egger intercept = 4.34x10-4; p = .961), and was consistent across PTSD data sets. However, we found no consistent associations between PTSD genetic liability and ADHD risk. Results from the sibling comparison showed an increased risk for developing PTSD in individuals diagnosed with ADHD compared to their undiagnosed full siblings (hazard ratio = 2.64 [95% CI, 1.98-3.53]) after adjustments. Adjusting for plausible mediators did not affect the MR results but attenuated the estimates in sibling comparisons.

Conclusions: Our findings add impetus to the need for early and effective treatment of ADHD because the condition may put individuals at risk for PTSD later in life.