Total population study points to possible flaws in previous prevalence estimates, need for better epidemiological methods, screening, and services
• Science Update
The prevalence of autism spectrum disorder (ASD) among children in South Korea appears to be much higher than the range of estimates previously reported in other countries, according to a study partly funded by NIMH. The researchers found that two-thirds of ASD cases occurred in children attending mainstream schools; these children had not been previously diagnosed and had never received treatment for the disorder. The study was published online ahead of print on May 9, 2011, in the American Journal of Psychiatry.
Recent reports of increased prevalence of ASD have raised concerns among parents, researchers, and policymakers. However, it is still unclear whether these estimates reflect a true rise in ASD occurrence or improved rates of detection and diagnosis. And because different studies use different designs and methods, they may not be truly comparable. There are also limited data on the prevalence of ASD in countries outside of North America and Europe.
To address these issues, Young Shin Kim, M.D., Ph.D., of Yale School of Medicine, and colleagues targeted all children ages 7-12 in a South Korean community representative of the country's general population. The researchers asked parents and teachers about the children's social interactions, and whether they had communication problems or restricted and repetitive behaviors.
The researchers then evaluated 286 children suspected as having ASD based on the answers given. Of these children, 114 attended special education schools, had a history of mental health service use, or were listed in the local disability registry. For study purposes, the researchers considered these children to have a high probability of having ASD. The other 172 children attended regular schools, had never received special education or mental health services, and were not listed in the disability registry.
The study incorporated multiple measures to address potential cultural issues. For example, board-certified Korean child psychiatrists trained in both Korea and the United States conducted the diagnostic assessments using screening and diagnostic tools validated for Korean children. An anthropologist on the research team also organized focus groups with local parents and teachers to identify beliefs that may influence symptom reporting and to address stigma related to ASD.
Results of the Study
Based on diagnostic assessments, the prevalence of ASD among the total study population was 2.64 percent. Among the children attending regular schools, the prevalence was 1.89 percent and boys were 2.5 times more likely to have ASD than girls. Among the high-probability group the prevalence of ASD was 0.75 percent and boys were 5 times more likely to have ASD than girls.
Of the 2.64 percent of all ASD cases, 0.94 percent met diagnostic criteria for autism and 1.7 percent met criteria for other types of ASD, including Asperger's disorder and pervasive developmental disorder not otherwise specified.
Unlike previous studies that analyzed health records and registries, the researchers attempted to look at each child in every school in a particular community, even children who did not have a record of any special education need. According to the researchers, this method unmasked cases that could have gone unnoticed if they had relied solely on health records. As a result, this study's estimate of ASD prevalence is higher than previously reported estimates, which range from 0.6 percent to 1.8 percent.
However, according to the researchers, the prevalence in the high probability group is similar to reports in other studies that have focused on the same target populations. The major difference in this study was that two-thirds of ASD cases were identified in the general population among children who never had contact with care systems. This particular finding highlights the importance of screening mainstream school populations as well as clinical populations in future studies. The researchers also suggest that the highly structured educational system in South Korea may allow children with less severe ASD symptoms to manage in general education settings, despite their impairments.
More research is needed to find out whether these results can be repeated in other populations in Korea and other countries. The researchers note that more rigorous ASD screening may provide a more accurate estimate of the number of people with ASD, and that this number may exceed previous prevalence estimates. Additionally, this study only addressed ASD prevalence, or the current number of people with the disorder. Incidence studies—those that focus on the numbers of new cases—are essential to examine possible environmental and other potential causes of the rising ASD prevalence.
Kim YS, Leventhal BL, Koh YJ, Fombonne E, Laska E, Lim EC, Cheon KA, Kim SJ, Kim YK, Lee H, Song DH, Grinker RR. Prevalence of autism spectrum disorders in a total population sample . Am J Psychiatry. 2011 Sep;168(9):904-12. Epub 2011 May 9. PubMed PMID: 21558103.