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Changing diagnoses may account for increased prevalence of autism

From 2000 to 2011, autism diagnoses among children in special education programs increased more than threefold, potentially due to the reclassification of children who previously would have been diagnosed with intellectual disability.

“A recent study by the United States CDC estimated the prevalence of autism among 8-year-old children, within the Autism and Developmental Disabilities Monitoring (ADDM) network sites in 2010, to be one in 68 children. This estimate was a documented 123% increase in prevalence when compared with the data from 2002 (one in 150 children),” Andrew Polyak, an undergraduate student and research assistant at the Pennsylvania State University, and colleagues wrote.

To investigate the effect of comorbidity on diagnosis and prevalence of autism, researchers analyzed 2000 through 2011 enrollment data from the U.S. Individuals with Disabilities Education Act for approximately 6.2 million children per year.

From 2000 to 2011, autism prevalence within special education increased by 331%. This increase is potentially due to a diagnostic reclassification from frequently comorbid features such as intellectual disability, according to researchers.

A decrease in intellectual disability prevalence accounted for an average of 64.2% of the increase in autism.

Potential reclassification from intellectual disability to autism occurred more frequently among older children compared with younger children (75% vs. 48%; P = .007).

Approximately 59% of the increase in autism among children aged 8 years was attributed to reclassification. By age 15 years, 97% of the increase in autism was attributed to reclassification, according to researchers.

Researchers observed negative correlations between the prevalence of autism and intellectual disability in some states, suggesting state-specific health policy may play a significant role in autism categorization.

“The high rate of co-occurrence of other intellectual disabilities with autism, which leads to diagnostic reclassification, is likely due to shared genetic factors in many neurodevelopmental disorders,” study researcher Santhosh Girirajan, MBBS, PhD, said in a press release. “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. 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.” – by Amanda Oldt

Disclosure: Please see the full study for a list of all other authors’ relevant financial disclosures.