But More Aggressive Diagnosis May Explain Increase in Cases

Dec. 31, 2002 -- New research from the Centers for Disease Control and Prevention suggests that autism rates are at least 10 times higher than three previous studies in the U.S. have suggested. But experts say it is not yet clear if the increase is real or reflects changes in reporting and diagnostic practices.

CDC investigators have tracked autism cases in the metropolitan Atlanta area since 1996, finding that 34 children per 10,000 either have autism or disorders linked to it. That is far higher than the average estimate of roughly three autism cases per 10,000 children in studies conducted in the U.S. before 1990. But it is in line with another recently reported CDC investigation from New Jersey and studies from the United Kingdom and Canada.

The Atlanta surveillance is the first ongoing investigation of autism within a population in the United States, but the CDC has recently begun to track autism cases at a dozen other sites around the country. CDC epidemiologist Marshalyn Yeargin-Allsopp, MD, who led the Atlanta investigation, tells WebMD that the new surveillance network will provide a much better picture of the true incidence of autism within the U.S.

"Despite all of the talk of an epidemic of autism, there is really no way to know if there is a real increase in cases because so many things have changed within the last 10 years or so," she tells WebMD.

The definition of autism has been expanded to include a wider spectrum of disorders, including Asperger syndrome and pervasive developmental disorder. In addition, an increase in services for autistic children has prompted more aggressive diagnosis, as has increased media attention and parental awareness.

The CDC report, published in the Jan. 1, 2003, issue of The Journal of the American Medical Association, examines autism prevalence among children aged 3 to 10 in metropolitan Atlanta. In the 1996 investigation, 987 children with autism or related disorders were identified, resulting in 3.4 cases per 1,000 children. Four times more boys than girls had the developmental disorder, but no difference in the rates of autism was seen between black and white children.

Autism researcher Eric Fombonne, MD, tells WebMD that the new CDC figures probably still underestimate the true incidence of autism. A study he conducted in Canada and several other recent investigations suggest the prevalence rate may be closer to 60 cases per 10,000 children -- almost double the CDC's Atlanta findings. In an editorial accompanying the CDC study, he writes that underestimated cases may be partly to blame on the fact that children with milder autism may have been missed.

Fombonne notes that there is, so far, little clinical evidence to back up claims that environmental influences play a role in the development of autism. He writes that claims of an association with measles-mumps-rubella vaccine have not been supported by recent studies, and there is also little evidence for causal association with other exposures, such as mercury-containing vaccines.

"Although claims about an epidemic of autism and about its putative causes have the most weak empirical support, the subsequent controversy has put autism on the public agenda," he writes. "In recent years, children with autism, their families, and professionals involved in their care and in research have seen welcome and legitimate increases in public funding. Yet, ironically, what has triggered substantial social policy changes in autism appears to have little connection with the state of the science."

SOURCES: The Journal of the American Medical Association, Jan. 1, 2003 • Marshalyn Yeargin-Allsopp, MD, developmental pediatrician and medical epidemiologist, CDC • Eric Fombonne, MD, professor of psychiatry, head of the division of child psychiatry, McGill University, Montreal, Canada.

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