Rate of autism higher than previously believed

March 2007

The rate of autism may be higher than previously thought – about one in 150 children in the United States has autism, according to recent findings reported by the CDC.

The rate was based on multistate reviews of medical and school records for children participating in an autism spectrum disorder (ASD) surveillance project in 2000 and 2002. The CDC study includes children with behaviors consistent with autism, Asperger’s and pervasive developmental delays not otherwise specified.

The results showed an average of 6.7 children out of 1,000 had an ASD in the six communities assessed in 2000, and an average of 6.6 children out of 1,000 having an ASD in the 14 communities included in the 2002 study.

“Our estimates are becoming better and more consistent, though we can’t yet tell if there is a true increase in ASDs or if the changes are the result of our better studies,” said CDC Director Julie Gerberding, MD, MPH, in a press release. “We do know, however, that these disorders are affecting too many children.”

The six states assessed in 2000 are: Arizona, Georgia, Maryland, New Jersey, South Carolina and West Virginia. A total of 1,252 8-year-olds were identified as having autism in that survey.

The 14 states assessed in 2002 are: Alabama, Arizona, Arkansas, Colorado, Georgia, Maryland, Missouri, New Jersey, North Carolina, Pennsylvania, South Carolina, Utah, West Virginia and Wisconsin. A total of 2,685 8-year-olds in that group were identified as having autism.

Overall, the 2000 CDC study found ASD rates ranged from one in 222 children to one in 101 8-year-old children in the six communities studied.

The 2002 study found ASD rates ranging from one in 303 to one in 94 among 8-year-old children.

Regional variation

CDC officials said that the estimate for West Virginia (4.5 per 1,000 children, aged 8 years) was significantly lower, and the estimate for New Jersey (9.9 per 1,000 children, aged 8 years) was significantly higher than the mid-range estimates. However, they noted that etiologic differences might contribute to across-site variation in prevalence.

“It’s important to note that these studies don’t provide a national estimate, but that they do confirm that ASDs in the areas surveyed are more common in these communities studied than previously thought,” said Marshalyn Yeargin-Allsopp, MD, chief of the CDC’s autism program. “We need to continue efforts to monitor the prevalence of ASDs and to improve our understanding of these disorders. Good estimates of how many children in a community may have an ASD will also help school and health officials in their planning and intervention efforts.”

The CDC said that conclusions about specific etiologic factors “cannot be drawn from prevalence data alone and will need to be addressed through future studies.”

For example, they said following concern about high prevalence of autism in Brick Township, N.J., “health officials launched an investigation to assess specific environmental exposures in that township, and no specific associations were identified between hazard exposure from the municipal drinking water supply, town river, or landfill and autism cases.”

In addition, the CDC noted, to better understand the consistency and variance in autism prevalence in different communities, “specific studies are needed to investigate the complex genetic and environmental interactions likely at play in the development of heterogeneous conditions.”

The study also found higher prevalence in boys than girls (a range of 2.8-6 boys to girls, depending on the state) and no statistically significant difference among non-Hispanic whites and non-Hispanic black children.

Age of diagnosis

Colleen Boyle, PhD, of the CDC noted that the study showed the age of diagnosis (between 4 to 5 years of age) was much later than the age at which developmental concern was registered (before 24 months), underscoring the need for earlier identification of autism.

The CDC noted that 51% to 88% of children with ASDs had documented developmental concerns before age 3. The most commonly reported concerns were in language development, followed by social development.

The researchers noted that they studied the issue because “ASDs are diagnosed on the basis of behavioral criteria, and clinicians might apply criteria differently to arrive at a diagnosis of autism and related subtypes, determining prevalence is challenging.”

The researchers noted some limitations to their data, specifically, “although the intent was to use comparable methods, multiple sources of variability might be responsible for differences in prevalence across study sites. These include variation in state standards for evaluation and classification of children with ASDs, exclusion of certain sources (eg, private schools), use of special education child count data from one rather than two school years, and inability to locate source files. Sites also varied in the percentage of records abstracted and in the quantity and quality of information available in some of the abstracted files. Because the ADDM methodology is records-based, the number and content of the records abstracted at any site could influence prevalence estimation.”

The researchers also noted another limitation of a records-based approach, specifically that the amount and quality of information available varied across sites.

“Two sites (Maryland and West Virginia) screened special education files from one rather than two school years, which appears to account, in part, for the lower prevalence rates from these sites. After the impact of screening one year rather than two years of special education data was taken into account, prevalence estimates from Maryland and West Virginia (6.3 and 5.1 per 1,000 children aged 8 years, respectively) were similar to the majority of the other sites,” the researchers wrote.

One factor that affected all sites was the number of files not located at data sources during the case-ascertainment phase. Sensitivity analyses conducted by all sites to determine the effect of files that were not located indicated a possible prevalence underestimation of 3.4% to 16.7%, with the greatest impact in Maryland, researchers noted.

The Autism Society of America (ASA) welcomed the new CDC studies as “tremendously significant data that will help the cause of improving the lives of all those affected by autism.”

“Finally, we can end the debate on the prevalence of autism in our nation and focus on getting the services and supports the families need,” said Lee Grossman, ASA president and CEO. “Autism is a treatable lifelong condition that affects tens of millions of Americans today. It is time to aggressively address this national health crisis.”

The initial findings presented in the report were derived from data collected by the first comprehensive U.S. monitoring network – the Autism and Developmental Disabilities Monitoring (ADDM) Network – devoted to ASDs.

“Prevalence of ASDs in five of the six sites was consistent with the upper end of recent community and survey-based epidemiologic study estimates, and one site demonstrated a significantly higher (P<.001) prevalence similar to two non-U.S. studies. These findings indicate that ASDs are more common than were once believed and are conditions of urgent public health concern,” the researchers concluded. “Since its inception, the ADDM Network has expanded to include additional sites that will continue to provide baseline ASD prevalence data and descriptive information regarding the ASD population in the United States. Ongoing surveillance to monitor trends in ASD prevalence and studies that include clinical validation of cases and noncases will be particularly useful for furthering the understanding of the prevalence of ASDs and of the characteristics of persons with ASDs.”

For more information:

• CDC. Prevalence of autism spectrum disorders – autism and developmental disabilities monitoring network, six sites, United States 2000. MMWR. 2007;56(SS01):1-11.