Cognitive and ASD Symptom Profiles in Comorbid Down syndrome and Autism Spectrum Disorder

Individuals with a comorbid presentation of Down syndrome (DS) and autism spectrum disorder (ASD) are a misunderstood and vulnerable group who have received little attention in the literature despite a prevalence estimated at 20% of the DS population. There is a great need to characterize the cognitive and ASD symptom profiles, as currently very little is known. Given the relative social strengths in DS and defining weaknesses in ASD, previous studies have focused on documenting social features in DS+ASD relative to either group in isolation. However, few studies have included all three groups and only one has used gold-standard evaluations to characterize children. As such, this study aims to fill the large gap in the literature that prevents us from better understanding the developmental and behavioral phenotype for these children by exploring cognitive and ASD symptoms within and between all three clinical groups. Participants included 173 children (Mean age= 60 months) with ASD (n=102), DS (n=56), and DS+ASD (n=15). Two 3x2 repeated measures ANOVA using semi-parametric statistics in R statistical software were completed. Between-subjects factor was diagnostic group (DS, ASD, DS+ASD) and within subjects factors were either cognitive (verbal DQ and nonverbal DQ) or ASD symptoms (social-affect (SA) and restricted and repetitive behaviors (RRBs)). Significant group-by-scale interactions were revealed for both cognitive and ASD symptom models. Pairwise comparisons revealed profiles of Verbal DQ: DS+ASD < DS, ASD whereas nonverbal DQ: DS+ASD < DS < ASD. For SA, a profile of DS < DS+ASD, ASD emerged. A RRB profile of DS, DS+ASD < ASD emerged. Findings suggest greater cognitive impairment for children with DS+ASD on both verbal and nonverbal measures of functioning. Despite this severe presentation, ASD symptom profiles appeared less impairing for those with comorbid DS+ASD relative to their peers with ASD in isolation. Specifically, SA was significantly lower in DS+ASD, though still elevated over DS-only. Discrepancies in overall severity for both cognitive functioning and ASD symptoms have important implications for early identification and intervention, as youth with DS+ASD appear to have a unique presentation. Future research should investigate whether traditional ASD interventions, which have been shown to alter developmental trajectories and improve lifetime outcomes, will be beneficial in this group. Finally, given prior research suggesting social abilities are a relative strength for those with DS and the documentation of less severe ASD symptoms in this study, future research should investigate the potentially protective role in DS.