Trends Over Time in the Prevalence of Autism by Adaptive and Intellectual Functioning Levels

The autistic community is a large, growing, and heterogeneous population, and there is a need for improved methods to describe their diverse needs. Measures of adaptive functioning collected through public health surveillance may provide valuable information on functioning and support needs at a population level. We aimed to use adaptive behavior and cognitive scores abstracted from health and educational records to describe trends over time in the population prevalence of autism by adaptive level and co-occurrence of intellectual disability (ID). Using data from the Autism and Developmental Disabilities Monitoring Network, years 2000 to 2016, we estimated the prevalence of autism per 1000 8-year-old children by four levels of adaptive challenges (moderate to profound, mild, borderline, or none) and by co-occurrence of ID. The prevalence of autism with mild, borderline, or no significant adaptive challenges increased between 2000 and 2016, from 5.1 per 1000 (95% confidence interval CI: 4.6-5.5) to 17.6 (95% CI: 17.1-18.1) while the prevalence of autism with moderate to profound challenges decreased slightly, from 1.5 (95% CI: 1.2-1.7) to 1.2 (95% CI: 1.1-1.4). The prevalence increase was greater for autism without co-occurring ID than for autism with co-occurring ID. The increase in autism prevalence between 2000 and 2016 was confined to autism with milder phenotypes. This trend could indicate improved identification of milder forms of autism over time. It is possible that increased access to therapies that improve intellectual and adaptive functioning of children diagnosed with autism also contributed to the trends.