Cognitive Profiles and Autism Symptoms in Comorbid Down Syndrome and Autism Spectrum Disorder.

Objective: The prevalence of comorbid autism spectrum disorder (ASD) in children with Down syndrome (DS) is estimated to be around 16%. This study aims to fill gaps in the literature by describing developmental and behavioral phenotypes in this group relative to those with DS or ASD in isolation.

Methods: Participants included 173 children (Mage = 73 months) with ASD, DS, or DS + ASD. Two 3 × 2 repeated-measures analyses of variance were completed with between-subject factors of the diagnostic group (DS, ASD, and DS + ASD) and within-subject factors of cognitive performance (verbal and nonverbal developmental quotient [DQ]) or ASD symptoms (social affect [SA] and restricted and repetitive behaviors [RRBs]).

Results: Significant group-by-scale interactions were found, and pairwise comparisons revealed that for verbal DQ, DS + ASD < DS, ASD, whereas for nonverbal DQ, DS + ASD < DS < ASD. For SA, DS < DS + ASD < ASD, whereas for RRB, DS, DS + ASD < ASD.

Conclusion: Findings suggest greater cognitive impairment in DS + ASD on both verbal and nonverbal measures. Despite these significant cognitive challenges, ASD symptoms appeared less severe in DS + ASD relative to peers with ASD in isolation, although SA symptoms were elevated over DS-only. This unique DS + ASD presentation has important implications for early identification and intervention. Given previous research suggesting relative social strengths in DS and less severe ASD symptoms documented in this study, future research may benefit from investigating different aspects of SA (i.e., components related to reciprocal social interaction vs. social communication) in those with DS + ASD to more clearly delineate the social phenotype in this group and potentially inform intervention efforts.