Autism: Rising Rates Mostly Due to Reclassification

Rates of autism diagnoses more than tripled Between 2000 and 2010, but this increase in diagnoses was caused by reclassification of people with related neurodevelopment disorders, researchers from Pennsylvania State University suggest. THose individuals previously would have been diagnosed with other intellectual disability disorders, according to the study.

The scientists analyzed 11 years of special-education enrollment data, on an average of 6.2 million children per year. They found no overall increase in the number of students enrolled in special education.

They also observed that the increase in students diagnosed with autism was offset by a nearly equal decrease in students diagnosed with other intellectual disabilities that often co-occur with autism. Thus, the researchers conclude that the large increase in the prevalence of autism is likely the result of shifting patterns of diagnosis that are complicated by the variability of autism and its overlap with other related disorders.

Research team leader Santhosh Girirajan, assistant professor of biochemistry and molecular biology and of anthropology at Penn State, said:

“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.”

The researchers found that the relationship between autism cases and other intellectual disabilities varied state-by-state. When assessed individually, states such as California, New Mexico, and Texas showed no relationship between the prevalence of autism and that of intellectual disability, suggesting that state-specific health policy may be a significant factor in estimates of autism prevalence.

“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 Girirajan. “Every patient is different and must be treated as such. Standardized diagnostic measures incorporating detailed genetic analysis and periodic follow up should be taken into account in future studies of autism prevalence.”

Original Study:

Andrew Polyak, Richard M. Kubina and Santhosh Girirajan
Comorbidity of intellectual disability confounds ascertainment of autism: Implications for genetic diagnosis
American Journal of Medical Genetics Part B DOI: 10.1002/ajmg.b.32338

Illustration:Penn State University