The dramatic increase in the rate of autism among children enrolled in special education programs in the United States between 2000 and 2010 may be due in large part to the reclassification of students who previously would have been given other disability labels, according to new research. In a paper published online in the American Journal of Medical Genetics, scientists at Penn State reported their analysis of 11 years of data from the United States Individuals with Disabilities Education Act (IDEA) for students enrolled in special education programs. Under IDEA, individuals are classified into one of 13 disability categories including autism, intellectual disability, emotional disturbance, other health impairment and specific learning disability.
The researchers found that between 2000 and 2010, the number of children in the autism category more than tripled from 93,624 in 2000 to 419,647 a decade later. Yet nearly 65 percent of this increase was significantly associated with a corresponding decline in the number of students classified under the intellectual disability category in the IDEA data. The researchers estimate that, for 8 year-olds, approximately 59 percent of the observed increase in autism is accounted for by reclassification, but by age 15, reclassification accounts for as much as 97 percent of the increase in autism. Likewise, the researchers found the category of specific learning disability as another potential contributor to diagnostic reclassification. They also note that the relationship between autism cases and disability categories varied state-by-state, suggesting that state-specific health policy may be a significant factor in estimates of autism prevalence.
Although a broadening of diagnostic criteria, improved assessment practices, increased awareness of autism, and strategic use of diagnosis to gain access to services have all had a significant effect on the prevalence of autism, this new research provides the first direct evidence that much of the increase may be attributable to a reclassification of children with related disorders (or diagnostic substitution) rather than to an actual increase in the rate of new cases of autism. The researchers conclude that the large increase in the prevalence of autism is likely the result of students being moved from one category to another and is complicated by the variability of autism and its overlap with other related disorders. Current prevalence estimates are based on specific clinical features of autism and do not consider co-occurring disorders that may confound diagnosis and classification. "Because features of neurodevelopmental disorders co-occur at such a high rate and there is so much individual variation in autism, diagnosis is greatly complicated, which affects the perceived prevalence of autism and related disorders," said Santhosh Girirajan, assistant professor of biochemistry and molecular biology and of anthropology at Penn State and the leader of the research team. "For quite some time, researchers have been struggling to sort disorders into categories based on observable clinical features, but it gets complicated with autism because every individual can show a different combination of features. The tricky part is how to deal with individuals who have multiple diagnoses, because the set of features that define autism is commonly found in individuals with other cognitive or neurological deficits."
Polyak A, Kubina RM, Girirajan S. 2015. Comorbidity of intellectual disability confounds ascertainment of autism: implications for genetic diagnosis. Am J Med Genet Part B 9999:1–9.
Lee A. Wilkinson, PhD, NCSP is author of the award-winning book, A Best Practice Guide to Assessment and Intervention for Autism and Asperger Syndrome in Schools, published by Jessica Kingsley Publishers. He is also editor of a best-selling text in the American Psychological Association (APA) School Psychology Book Series, Autism Spectrum Disorder in Children and Adolescents: Evidence-Based Assessment and Intervention in Schools and author of the book, Overcoming Anxiety and Depression on the Autism Spectrum: A Self-Help Guide Using CBT. His latest book is A Best Practice Guide to Assessment and Intervention for Autism Spectrum Disorder in Schools (2nd Edition).