Autistic Traits in Children with ADHD Index Clinical and Cognitive Problems

Background: Traits of autistic spectrum disorders (ASD) occur frequently in attention deficit hyperactivity disorder (ADHD), but the significance of their presence in terms of phenotype and underlying neurobiology is not properly understood.

Objectives: This analysis aimed to determine whether higher levels of autistic traits index a more severe presentation in a large, rigorously phenotyped sample of children with ADHD.

Methods: A clinical sample of children meeting research diagnostic criteria for ADHD with no known diagnosis of ASD (N=711) was assessed using parental report (interview and questionnaires) and psychological assessment (IQ and reading tests) on measures of autistic traits (via the Social Communication Questionnaire (SCQ)), as well as clinical comorbidities and cognitive and developmental features. Multivariate regression analyses were used to examine association of SCQ scores with core ADHD features and clinical, cognitive and developmental outcomes, with adjustment for putative confounders. For outcomes showing association with total SCQ score, post-hoc analyses explored levels of differential association of the three ASD sub-domains (social deficits, communication deficits and restrictive and repetitive behaviours (RRBs)).

Results: Results suggest that increasing ASD symptomatology within ADHD is associated with a more severe phenotype in terms of oppositional, conduct and anxiety symptoms, lower full-scale IQ, working memory deficits and motor problems. These associations persisted after accounting for ADHD severity, suggesting that autistic symptomatology independently indexes the severity of comorbid impairments in the context of ADHD. ASD sub-domain scores did not show unique contributions to most outcomes. The only exceptions were that social deficits were independently associated with oppositional symptoms and RRBs independently predicted hyperactive-impulsive symptoms and motor problems.

Conclusions: Autistic traits in children with ADHD appear to index higher levels of phenotypic complexity. As such, the results suggest that it is important to consider levels of socio-communicative and repetitive traits in children with ADHD who do not meet diagnostic criteria for ASD, as these may have implications for the efficacy of interventions.