Prevalence of Autism in a US Metropolitan Area

Yeargin-Allsopp M, Rice C, Karapurkar T, Doernberg N, Boyle C, Murphy C
National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention (F-15), 4770 Buford Hwy NE, Atlanta, GA

Abstract: JAMA. 2003 Jan 1;289(1):49-55

Findings

Given the complexity of autism and the broad spectrum of related disorders, current prevalence rates and whether rates are increasing are highly debated topics. Before 1985 in the U.S., publish rates were 4 to 5 per 10,000 children for a broader autism spectrum and 2 per 10,000 for “classic autism.” Since then, these rates have risen 1.5 to 2.0 times. However, little is known about the prevalence of autism in the U.S. because only 4 U.S. population studies have been carried out, and results have not been conclusive.

In this study, Dr. Yeargin-Allsopp and colleages determined the prevalence of autism among children aged 3 to 10 years in the 5 counties of metropolitan Atlanta, Ga in 1996. Children were identified through screening and studying records through many medical and educational sources.

A total of 987 children displayed behaviors consistent with standard criteria for autistic disorder, PDD-NOS, or Asperger disorder. The prevalence for autism was 3.4 per 1000 (ie, 34 per 10,000), with a 4:1 male-female ratio. This prevalence was comparable for black and white children (black, 3.4 per 1000 and white, 3.4 per 1000), which is consistent with other studies. A total of 68% (880 children) with IQ or developmental test results showed some cognitive impairment. As severity of cognitive impairment increased from mild to profound, the male-female ratio decreased from 4.4 to 1.3. Forty percent of children with autism were identified only at educational sources. Schools were the most important source for information on black children, children of younger mothers, and children of mothers with less than 12 years of education. Finally, prevalence rates varied with the child’s age, varying from1.9 per 1,000 in 3-year-olds to 4.7 per 1,000 in 8-year-olds.

Conclusions

This study showed that in several counties of a major metropolitan area, the rate of autism was 10 times higher than published rates from studies conducted in the U.S. in the 1980s and early 1990s. The prevalence rate of 34 per 10 000 is, however, likely to be an underestimate. First, as the authors point out, children with milder or high-functioning (ie, normal IQ) ASD subtypes are likely to have been missed. Second, the lower prevalence in 3- and 4-year-olds may reflect lower sensitivity of identification among younger children for developmental disorders that often are diagnosed later. Third, there was an unexpected decrease in prevalence among 9- and 10-year-olds. Although it would be tempting to interpret this age trend as indicative of a secular increase in the rate of ASD (ie, the younger the birth cohort, the higher the prevalence), such an explanation is both unlikely and biologically implausible because rates plateaued for birth cohorts aged 5 through 8. Rather, the authors suggest that these differences might reflect new diagnostic criteria for autism and increased availability of developmental disability services for children with autism in the 1990s. What this means, however, is that the rate of 41 to 45 in 10 000 obtained for the 5- to 8-year-olds might be more accurate. This rate also is more in line with those of 3 recent surveys that yielded prevalence estimates in the range of 60 per 10 000.

The investigators suggest several reasons for this increase in autism rates. For one, the symptoms used to diagnose the illness have been expanded over the years and there is heightened public awareness of the condition. This is "due in large part to efforts of parent and advocacy groups, availability of more medical and educational resources, increased media coverage of affected children and families, and more training and information for physicians, psychologists and other service providers," they write. And in 1991, the US Department of Education included autism as category for special education services, which may have also increased diagnoses.

The lower prevalence of autism in younger children may simply mean that they have not yet come to the attention of professionals. "The combined influence of these factors has probably contributed to the identification of more individuals with autism," the authors write. "However, it remains unclear whether specific environmental, immunologic, genetic or unidentified factors also may have contributed to these higher reported prevalence rates," they conclude.

“The study is the first to get a good population-based estimate of autism in black children, showing that the rates are similar to those found in other youngsters”, noted Dr. Fombonne of the McGill University and Montreal Children's Hospital in Canada in an editorial accompanying the study. He also remarks that the centers for disease control (CDC) in Atlanta, has "recently funded a surveillance network across several states."

"This and other initiatives should help address more directly hypotheses about...changes in the incidence of autism spectrum disorders," he writes.