This graph shows the number of students (per 10,000) diagnosed with autism (blue) and intellectual disability (red) in special-education programs in the US from 2000 to 2010. The increase in autism diagnoses during this period was offset by decreases in the diagnosis of intellectual disability, which suggests that shifting patterns of diagnosis may be driving increases in autism diagnoses.
Image credit: Penn State
Figures from the US Centers for Disease Control and Prevention (CDC) suggest that autism prevalence has increased from 1 in 5,000 in 1975 to 1 in 150 in 2002 to 1 in 68 in 2012. These increases have previously been attributed to there being a greater awareness of autism these days, combined with a broadening of diagnostic criteria.
However, the new study finds that much of the increase in prevalence may be due individuals with related neurological disorders being reclassified, rather than an increase in the rate of new autism cases.
"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," says Santhosh Girirajan, assistant professor of biochemistry and molecular biology and of anthropology at Penn State and the leader of the research team.
"The tricky part is how to deal with individuals who have multiple diagnoses," Girirajan adds, "because the set of features that define autism is commonly found in individuals with other cognitive or neurological deficits."
Under the United States Individuals with Disabilities Education Act (IDEA), individuals are classed into one of 13 disability categories. People may belong to multiple categories - such as autism spectrum disorders, intellectual disability and emotional disturbance - but children are only classified under one category.
The Penn State researchers found that although there were three times as many cases of autism in 2010 than there were in 2000, about 65% of these extra cases were as a result of individuals that had previously been classified under IDEA's intellectual disability category being reclassified under autism.
The authors further explain that the proportion of reclassification varied across age groups. For 8-year-old children, about 59% of the increase in autism prevalence is explained by reclassification. However, by the age of 15, the reclassification accounted for 97% of the increase in prevalence.
Shared genetic factors complicate classification
The authors say that the high rate of co-occurrence of other intellectual disabilities with autism leads to reclassification, and it is likely that this co-occurrence is a result of many neurodevelopment disorders sharing the same genetic factors. Girirajan explains:
"When individuals carrying classically defined genetic syndromes were evaluated for features of autism, a high frequency of autism was observed, even among disorders not previously associated with autism, suggesting that the tools for diagnosing autism lose specificity when applied to individuals severely affected by other genetic syndromes."
According to the study, variations on a state-by-state basis in the relationship between autism and other intellectual disabilities occur. For instance, in California, New Mexico and Texas, the researchers found no association between autism prevalence and prevalence of intellectual disability. The authors say this suggests that state-specific health policy is "a significant factor" in autism prevalence estimates.
The authors acknowledge that autism diagnosis is complicated both because there is so much individual variation between cases and because other neurodevelopment disorders co-occur at such a high rate. "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," Girirajan concludes.