Shared and Distinct Presentations of ADHD and ASD: An Examination of the Autism Spectrum Continuum

Background: Current DSM-IV-TR diagnostic criteria do not allow for a comorbid diagnosis of Attention-Deficit/Hyperactivity Disorder (ADHD) and Autism Spectrum Disorder (ASD). While it is widely accepted that children with ASD often present with ADHD symptoms, only recently have reports of elevated rates of autistic traits in a subgroup of children with ADHD (ADHD⁺) emerged. Initial evidence suggests that increased ratings of autistic traits in ADHD do not solely result from ADHD symptoms and may be qualitatively similar to ASD. However, direct comparisons between ADHD⁺ and children with ASD (with or without ADHD traits) are limited.

Objectives: We aimed to characterize children identified as ADHD⁺ with respect to ASD and ADHD symptom profiles, communicative skills, and general psychopathology relative to children with ASD with and without ADHD traits (ASD⁺ and ASD^-, respectively).

Methods: We included 264 children (164 with ADHD and 100 with ASD) aged 7.0 to 17.8 years. The Social Responsiveness Scale (SRS) was used to identify ASD traits in children with ADHD (ADHD⁺; T-Score≥65). The Conners’ Parent Rating Scale (CPRS-R:L) was used to identify children with ASD and ADHD traits (ASD+; DSM-IV Total T score≥65). In accordance with prior work (Grzadzinski et al. 2010), SRS consensus coding was used to examine the contribution of specific symptom domains identified by the consensus of 8 ASD experts. The Children’s Communication Checklist (CCC-2) was used to measure communication profiles consistent with ASD; the Child Behavior Checklist (CBCL) to examine general psychopathology including behavioral and emotional difficulties. We used the likelihood ratio test to examine the contribution of each SRS category to the total SRS score. ANOVAs were conducted comparing the three groups with respect to CPRS-R:L, CCC-2 and CBCL scales.

Results: Consistent with prior evidence, 21% of the ADHD group was identified as ADHD⁺ and 61% of the ASD sample as ASD⁺. The contribution of the SRS consensus categories was similar for the ADHD⁺ and both ASD groups, except for significantly higher contribution from the Restricted/Repetitive Behaviors category to the ASD⁺ (p<0.001). Results of the CCC-2 revealed three patterns of impairments: an increasing gradient of severity in social relatedness going from ADHD to ASD (i.e., ADHD⁺ < ASD^- < ASD⁺); shared impairment between ADHD⁺ and ASD^{+ or -} (e.g., stereotyped language); and shared impairments between ADHD⁺ and ASD⁺ (e.g., inappropriate initiation). ADHD⁺ and ASD⁺ did not differ on ADHD symptoms severity. In terms of other psychopathology, compared to ASD^-, ADHD⁺ and ASD⁺ showed similarly elevated scores for CBCL-Externalizing problems as well as for CPRS Oppositional, Anxious/Shy, and Emotional Lability.

Conclusions: We found that children with ADHD⁺ present with social-communicative impairments qualitatively similar to ASD, albeit less severe. The ADHD⁺ and ASD⁺ subgroups show more severe patterns of behavioral and emotional difficulties than the ASD^- subgroup. These results illustrate the complexity of the overlap between ADHD and ASD and confirm the need for objective examinations of the distinct and shared characteristics of ADHD and ASD through neuroimaging and genetics studies.