Prevalence of Autism Spectrum Disorders in a Total Population Sample

by Matthew J. Carey

A long-awaited study of autism prevalence in Korea came out today in the The American Journal of Psychiatry. Most of the information we have about autism prevalence comes from the US, the UK and Europe, so many were looking at this as the “Korean Study”. It is that, and very much more.

The study team was led by Dr. Young Shin Kim of Yale, and included an international team of Korean and North American researchers. Those unfamiliar with this ongoing study might be surprised to see an anthropologist on the team: George Washington University professor, Roy Richard Grinker. This will be less of a surprise for those familiar with his book, Unstrange Minds, in which he compares cultural views of autism in the United States, Korea, India and Africa.

As you can see from the title, the team stresses the fact that this is a study of a “total population sample”, not that this is a “Korean” prevalence. The reason, I suspect, is because the results are so provocative as to strongly highlight how much methodology can influence a prevalence estimate. The researchers set out to screen school age children in an entire community in South Korea, and then conduct comprehensive diagnostic evaluations on children who screened positive for ASD:

This study was conducted between 2005 and 2009 in the Ilsan district of Goyang City, South Korea, a stable, residential community near Seoul (area, 102 km2; population, 488,590) and representative of the general South Korean population (Korean Statistical Information Service, Capital Region Population, 2006). The target population (N=55,266) included all children born from 1993 through 1999 (ages 7–12 years at screening) and attending Ilsan elementary schools, as well as children in the same age group enrolled in the Ilsan Disability Registry between September 2005 and August 2006. Thirty-three of 44 elementary schools agreed to participate; 36,592 children were enrolled in participating schools and 294 in the Disability Registry (see section 1 of the data supplement that accompanies the online edition of this article). Investigators built collaborative community partnerships through ethnographic interviewing, meetings, and lectures (see section 2 of the online data supplement).

The results? A much higher prevalence estimate than previously found, along with a large fraction of the autistic students previously unidentified and being educated in regular education programs.

Objective: Experts disagree about the causes and significance of the recent increases in the prevalence of autism spectrum disorders (ASDs). Limited data on population base rates contribute to this uncertainty. Using a population-based sample, the authors sought to estimate the prevalence and describe the clinical characteristics of ASDs in school-age children.
Method: The target population was all 7- to 12-year-old children (N=55,266) in a South Korean community; the study used a high-probability group from special education schools and a disability registry and a low-probability, general-population sample from regular schools. To identify cases, the authors used the Autism Spectrum Screening Questionnaire for systematic, multi-informant screening. Parents of children who screened positive were offered comprehensive assessments using standardized diagnostic procedures.
Results: The prevalence of ASDs was estimated to be 2.64% (95% CI=1.91–3.37), with 1.89% (95% CI=1.43–2.36) in the general-population sample and 0.75% (95% CI=0.58–0.93) in the high-probability group. ASD characteristics differed between the two groups: the male-to female ratios were 2.5:1 and 5.1:1 in the general population sample and high probability group, respectively, and the ratios of autistic disorders to other ASD subtypes were 1:2.6 and 2.6:1, respectively; 12% in the general-population sample had superior IQs, compared with 7% in the high-probability group; and 16% in the general-population sample had intellectual disability, compared with 59% in the high-probability group.
Conclusions: Two-thirds of ASD cases in the overall sample were in the mainstream school population, undiagnosed and untreated. These findings suggest that rigorous screening and comprehensive population coverage are necessary to produce more accurate ASD prevalence estimates and underscore the need for better detection, assessment, and services.

The number that will no doubt make the headlines is the overall prevalence estimate: 2.64%. This is more than double the current prevalence estimate in the United States or the United Kingdom. We often hear that the prevalence estimate for the US is currently 1 in 110. To put this new study into perspective, 2.64% would be about 1 in 38.

What is even more remarkable than the 2.64% figure is the fact that 2/3 of the autistic students were in the mainstream school population and were previously unidentified.

The question of how so many autistic students could be previously unidentified is likely due in part to Korean culture, as noted in the paper:

Indications generally suggest ascertainment bias toward lower prevalence in South Korea since Koreans consider autism to be a stigmatizing hereditary disorder; autism (chap’ae) impugns the child’s lineage on both sides and threatens the marriage prospects of unaffected relatives. As a result, autism is often untreated, misdiagnosed as attachment disorder, or unreported in records

In addition, the Korean school system may provide some level of built-in support for autistic students, which may explain why even amongst the general population group, 16% of the autistic students had intellectual disability:

We examined the striking finding that many of the children in our study were in regular schools, without having been diagnosed and without support. It is possible that higher-functioning Korean children with ASDs manage in mainstream classrooms because of the nature of South Korean education, in which the school day exceeds 12 hours, 5–6 days a week. Teaching is highly structured, with much behavioral regulation, in settings where socialization is subordinated to educational progress. For quiet, high-functioning children with ASDs, this environment may reduce the likelihood of referrals to special education programs. Therefore, it was especially important for our study to screen “mainstream” school populations; our screening of children in regular schools found a relatively high proportion of cases in the general-population sample.

Given the remarkable results of this study, I asked Prof. Grinker about the methods used:

Dr. Kim took great care to ensure that the methods were rigorous, that measures were applied consistently, and she assembled some of the best experts on autism, like Dr. Leventhal from the U.S. and Dr. Yun-Joo Koh from Korea. In addition, all the instruments and methods were previously validated. Of course, our confidence about the results will be limited until there are studies that replicate our findings.

What seems likely is that so many of these students were previously unidentified is because they were in the mainstream schools. Prevalence studies usually focus on a “high probability” population: children with existing medical records or special education IEPs. Such is the case for the Autism and Developmental Disabilities Monitoring Network (ADDM) which is the basis for the prevalence estimates from the CDC in the United States. Depending on the site, the ADDM reviews school records, medical records or both.

The ADDM also produces a separate study for each birth cohort. This is in contrast to the current study, which considered birth cohorts from 1993 to 1999 at one time. This, together with the difference in methodologies, may be behind another result of the current study: The prevalence in the Korea study is relatively flat over the birth cohorts from 1993 to 1999. A similar result was found in a U.S. based study, which again was performed in a single year. This is in contrast to the ADDM prevalence estimates in the United States, where prevalence estimated for 1990’s birth cohorts show an increase in estimated prevalence.

So much discussion goes into the rising prevalence estimates in the United States that we do need to pause to consider what insights this new study can give us about US prevalence. The CDC/ADDM estimates represent excellent work, but they are already acknowledged to be underestimates. Add to this the possibility of a very large pool of autistic students who may be “under the radar” in regular school settings (similar to that demonstrated in Korea), and we must question whether they represent a true increase in the number of autistic individuals in the U.S.. Being “under the radar” is not the same thing as not needing some additional support. We should not only acknowledge that the estimated U.S. autism prevalence will likely continue to rise, we should be working towards that goal.

Which brings us back to the methodology of this new study. In specific, the use of a total population sample. This type of study is more expensive to perform for a given population size, but should yield a higher prevalence by screening individuals who are not in the record sets. The authors note:

Our study highlights the importance of methodology in determining prevalence estimates. Previous studies have varied enormously in the methods associated with multistage case identification designs, study populations, survey areas, screening, participation, and quality of confirmative diagnoses; each factor may have led to different prevalence estimates in each study . Our study emphasizes the importance of proactive screening of all children in target populations to maximize the sensitivity of case identification and to reduce bias in prevalence estimates. Our experience suggests that the application of validated, reliable, gold-standard screening procedures and diagnostic methods in other total population samples is possible, and we believe it will strengthen the design of future prevalence surveys

The authors conclude:

While replication of our findings in other populations is essential, we conclude that the application of validated, reliable, and commonly accepted screening procedures and diagnostic criteria in a total population may yield an ASD prevalence exceeding previous estimates. Hence, we report an ASD prevalence estimate in the range of 2%–3% with due caution about the risks of over- and underestimation.

I was able to pose some questions to Prof. Grinker about the study:

Q. How were these autistic children doing in those schools?

A. Some well, some not so well. But the take home message is this: the fact that majority of children with autism in South Korea are in mainstream schools, in mainstream classes, and have average or above average intelligence, shows the extraordinary breadth of the autism spectrum. American clinicians might describe many of these children as “high-functioning.” Korean teachers and parents increasingly refer to them as “border children” (gyôngye-sôn): that is, children who, while disabled, may not be impaired enough to qualify for scarce government-funded medical and educational resources. And the problem is that many of these kids don’t get diagnosed, and therefore don’t get services.

Q.Is it possible that you are overestimating the prevalence of autism?

A. Yes, but it is also possible that we are underestimating it. There are biases in both directions. We might have seen more kids with autism because, in a society that has few special education services, parents saw us an opportunity to learn about their children and perhaps get help. We might have seen fewer kids, however, due to the intense stigma surrounding autism and other developmental and psychiatric conditions in South Korea.

Q. Is it ethical to diagnose children with a disorder for which there are few services available?

A. The question highlights an important contradiction. Diagnosis drives services, but unless you have diagnoses, there will be no public pressure to develop services. Our hope is that this prevalence study will be a wake up call for countries that do not have many services and treatments to develop them.

The paper:

Prevalence of Autism Spectrum Disorders in a Total Population Sample
American Journal of Psychiatry
Young Shin Kim, M.D., Ph.D., Bennett L. Leventhal, M.D., Yun-Joo Koh, Ph.D. , Eric Fombonne, M.D. Eugene Laska, Ph.D., Eun-Chung Lim, M.A., Keun-Ah Cheon, M.D., Ph.D. ,Soo-Jeong Kim, M.D., Young-Key Kim, M.D., HyunKyung Lee, M.A., Dong-Ho Song, M.D., Roy Richard Grinker, Ph.D.
doi: 10.1176/appi.ajp.2011.10101532