The prevalence of autism.

THE NUMBER OF EPIDEMIOLOGICAL STUDIES OF AUtism has increased in recent years, including in the United States, where investigators are now catching up in what has traditionally been a weak area of child psychiatric research in North America. In this issue of THE JOURNAL, Yeargin-Allsopp et al report the findings of a survey, which was funded by the Centers for Disease Control and Prevention, that found a rate of 34 per 10000 for autism spectrum disorders (ASDs) among 3to 10-year-old children in metropolitan Atlanta. The strengths of the survey include use of multiple ascertainment sources and large sample size (ie, 987 confirmed ASD cases compared with a median sample size of 50 in 32 previous studies), thereby allowing the authors to have good precision in the estimates and to conduct meaningful subgroup analyses. In addition, this study is the first to derive a robust population-based estimate for the rate of ASD in black children, which is comparable to other racial groups. Other findings are typical of those found in previous surveys with ASD cases, with a strong overrepresentation of boys, cognitive impairments in more than two thirds of cases, and a relatively high rate (8%) of epilepsy. Approximately 18% of the sample did not have a previous diagnosis or were not suspected of having ASD, and children from black, younger, or less educated mothers were more often identified through schools as the only source of case finding. These findings highlight the need to rely on multiple ascertainment sources in epidemiological studies of ASD and caution against findings that are based on single service provider databases. The prevalence rate of 34 per 10000 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 3and 4-year-olds may reflect lower sensitivity of case identification among younger children for developmental disorders that often are diagnosed later. Third, there was an unexpected decrease in prevalence among 9and 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 10000 obtained for the 5to 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 10000. High prevalence rates from more recent epidemiological surveys have fueled the debate about a possible epidemic of autism. However, 4 separate issues need to be addressed. The first issue concerns the best current estimate for the prevalence of autism and related disorders. Increasing and consistent evidence from recent surveys shows that the prevalence rate for ASDs (including not only autism disorder but also Asperger disorder and pervasive developmental disorder–not otherwise specified) is approximately 60 per 10000; the study results from Yeargin-Allsopp et al concur with this conclusion. This estimate translates to approximately 425000 children younger than age 18 years with ASDs in the United States, including 114000 children younger than 5 years. The second issue is whether the prevalence of ASD has increased over time. Surveys conducted in the 1960s and 1970s only dealt with autism disorder (as opposed to ASD) and with a rather narrow definition of autism, as per Kanner’s descriptions, and not accounting for autism occurring in subjects who are not mentally retarded. Thus, comparisons of rates over time generally deal with studies that have used different case definitions, making interpretation of time trends difficult. The closest estimate of ASD prevalence available in the late 1970s was 20 per 10000 in a survey from the United Kingdom that was limited to the severely impaired children with ASD. Comparing rates for subtypes of ASD provide another avenue for estimation over time especially for autism disorder, but as shown by Yeargin-Allsopp et al and other surveys, the breakdown in ASD subtypes is not always reli-