Vol.:(0123456789) 1 3 Journal of Autism and Developmental Disorders https://doi.org/10.1007/s10803-021-05016-6 ORIGINAL PAPER Co‑occurring Down Syndrome and Autism Spectrum Disorder: Cognitive, Adaptive, and Behavioral Characteristics Kathryn R. Bradbury 1,4  · Emily I. Anderberg 1,5  · Lark Huang‑Storms 2,3  · Iulia Vasile 1  · Rachel K. Greene 1  · Susanne W. Duvall 1 Accepted: 6 April 2021 © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021 Abstract The current study explores functioning in individuals with co-occurring Autism Spectrum Disorder and Down Syndrome (ASD+DS; n = 23), individuals with ASD and cognitive impairment (ASD+ID; n = 99) and individuals with idiopathic ID (n = 38). ANCOVA results revealed that individuals with ASD+DS showed strengths in behavioral functioning compared to individuals with ID and more similar behavioral functioning to those with ASD+ID (η 2 = 0.12), with the exception of disruptive behaviors. Cognitive functioning (ɸ c = 0.41) and ASD symptomatology (η 2 = 0.11) were more comparable for children with ASD+DS and ASD + ID than for individuals with ID. Individuals with ASD+DS had the lowest overall adap- tive skills (η 2 = 0.11). Findings highlight similarities between ASD+DS and ASD+ID groups, emphasizing the importance of ASD identifcation within the DS population to provide access to specifc interventions. Keywords Down syndrome · Autism spectrum disorder · Dual diagnosis · Emotional and behavioral functioning · Cognitive functioning · Adaptive functioning Introduction Down Syndrome (DS) is a genetic disorder characterized by trisomy of chromosome 21 and is the leading genetic cause of intellectual disability (ID), with most individuals falling in the moderate to severe ID range (Grieco et al., 2015). Recent estimates of individuals with DS in the United States are about 8.27 per 10,000 people (Presson et al., 2013). As reviewed in Grieco et al. (2015), individuals with DS tend to have a relative weakness in verbal skills compared to nonverbal abilities, have stronger receptive language skills than expressive language skills, and tend to exhibit difculties with attention and externalizing behaviors as children. In general, common personality features such as being cheerful and friendly, reduced risk for psychopathol- ogy, and lower reported parenting stress compared to those of other disabilities (including idiopathic intellectual dis- ability), may all serve as protective factors for individuals with DS (Grieco et al., 2015). However, when individuals with DS have additional co-occurring diagnoses, such as Autism spectrum disorder (ASD), the picture may be more complicated. Down Syndrome and Autism Spectrum Disorder ASD is a neurodevelopmental disorder characterized by def- cits in social communication and the presence of restricted and repetitive behaviors, with current prevalence estimates of 1 in 54 8-year-old children and higher rates in males than females (Maenner et al., 2020). Despite the increased preva- lence of ASD in a variety of genetic syndromes (Richards * Kathryn R. Bradbury bradbury@childneuropsych.com 1 Division of Pediatric Psychology, Department of Pediatrics, Institute on Development and Disability, Oregon Health & Science University and Doernbecher Children’s Hospital, Portland, OR, USA 2 Oregon Center for Children and Youth with Special Health Needs (OCCYSHN), Oregon Health & Science University, Portland, OR, USA 3 Center for Integrative Brain Research and Autism Center, Seattle Children’s Hospital, Seattle, WA, USA 4 Present Address: Children’s Neuropsychological Services, Andover, MA, USA 5 Present Address: Lurie Center for Autism at Massachusetts General Hospital, Boston, MA, USA