*Article* **Symptoms of Autism Spectrum Disorder in Individuals with Down Syndrome**

**Amanda Dimachkie Nunnally 1,\* , Vivian Nguyen <sup>1</sup> , Claudine Anglo <sup>1</sup> , Audra Sterling <sup>2</sup> , Jamie Edgin <sup>3</sup> , Stephanie Sherman <sup>4</sup> , Elizabeth Berry-Kravis <sup>5</sup> , Laura del Hoyo Soriano <sup>1</sup> , Leonard Abbeduto 1,6 and Angela John Thurman 1,6**


**Abstract:** There is a growing body of evidence to suggest that individuals with Down syndrome (DS) are diagnosed with autism spectrum disorders (ASD) at a higher rate than individuals in the general population. Nonetheless, little is known regarding the unique presentation of ASD symptoms in DS. The current study aims to explore the prevalence and profiles of ASD symptoms in a sample of individuals with DS (*n* = 83), aged between 6 and 23 years. Analysis of this sample (*MAge* = 15.13) revealed that approximately 37% of the sample met the classification cut-off for ASD using the Autism Diagnostic Observation Schedule 2 (ADOS-2) Calibrated Severity Score (CSS), an indicator of the participants' severity of ASD-related symptoms. Item-level analyses revealed that multiple items on Module 2 and Module 3 of the ADOS-2, mostly in the Social Affect (SA) subdomain, differentiated the children with DS who did not meet ASD classification (DS-only) from those who did (DS + ASD). Lastly, comparisons of individuals with DS-only and those with DS + ASD differed significantly on the syntactic complexity of their expressive language. These findings shed light on the unique presentation of ASD symptoms in a sample of individuals with DS and suggest that expressive language abilities may play a pivotal role in the presentation of ASD symptoms in DS.

**Keywords:** Down syndrome; autism spectrum disorder; co-occurring; prevalence
