Autism Spectrum Disorder Symptoms in Individuals with a Primary Diagnosis of Attention-Deficit/Hyperactivity Disorder: A Systematic Review

Background and Aims: Attention-Deficit/Hyperactivity Disorder (ADHD) and Autism Spectrum Disorder (ASD) represent two of the most prevalent neurodevelopmental conditions, with prevalence rates of 5–7% and 0.6–1%, respectively. Despite their conceptually distinct phenotypes, ADHD and ASD are highly comorbid. This review is aimed at examining and synthesising the prevalence and profile of ASD-like symptoms in individuals with a primary diagnosis of ADHD across the lifespan in accordance with the Diagnostic and Statistical Manual, Fifth Edition (DSM-5; APA, 2013) diagnostic criteria. Methods: This systematic review was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, as discussed by (Moher et al. in PLOS Medicine, 6(7), e1000097, 2009; Page et al. in Systematic Reviews, 10(1), 89, 2021). Studies were identified through a systematic literature search of the databases: PsycINFO, Medline, CINAHL, Psychology and Behavioural Sciences Collection, and Scopus (N = 12,633). Articles were critically appraised according to established quality methodology criteria, and findings were summarised using narrative synthesis. Results and Conclusion: A total of nine studies (ADHD individuals n = 548) were identified and included in the review. Of these studies, three were classified as having ‘good’ methodological quality, five as ‘fair’, and one as ‘poor’. These studies varied considerably in their methodological approach and assessment procedures. Overall, higher levels of ASD symptoms were found in individuals with ADHD compared to the general population. There was considerable variation in the reported prevalence of clinically significant ASD symptoms in individuals with ADHD, ranging from 15 to 64.3%. Clinical implications and considerations for further research are discussed.