By Matt Carey
The prevalence of autim spectrum disorders in Sweden is estimated at about 1% (1.15%) based on a study just released using the Stockholm Youth Cohort. This prevalence is consistent with current estimates in the U.S. and the U.K., and with a subset of the population from a previous Swedish study. Autism prevalence is relatively flat with age, especially for children born in the 1990’s.
Most autism prevalence data is from Europe and the United States, with the U.S. as the largest source of data. CDC prevalence estimates are reported every two years. CDC estimates use a record-review methodology. While this methodology has its own limitations, including likely underestimating autism prevalence, relying upon administrative registries (such as the California Department of Developmental Services datasets or medical registries) are likely to give an even greater underestimation of autism prevalence. A whole-population approach, such as that used in a recent study of autism prevalence in Korea, should give the most accurate estimates, but are more costly to perform and typically limited to the number of study subjects.
The recent study presents autism prevalence in Sweden, using the Stockholm Youth Cohort (a medical registry). The paper, Autism Spectrum Disorders in the Stockholm Youth Cohort: Design, Prevalence and Validity, is in the journal PLoS ONE, which means the full article is available online.
A previous report, from 2006, gave the prevalence of autistic disorder in Sweden (from data in 2001) at 20.5 per 10,000, with other ASD’s at 32.9 per 10,000. This for a population born from 1977-1994. The prevalence in the youngest population in the study (7-12 years old at time of the study) was 1.23%. Yes, 1.23% for kids born in 1989-1994. That’s a prevalence comparable to the recent CDC estimate for U.S. kids born in 2000.
Back to the present Sweden study. The authors note the potential problem with using medical registries:
Furthermore, Scandinavian studies have frequently ascertained ASD cases via health care registries [8], [9], [10], [11]. This approach may underestimate the prevalence of ASD, since affected children require social and educational interventions more often than health care.
Stockholm county has a very active surveillance program:
All ASD related services, including diagnosis and follow-up health, special educational and social care are provided by services run by, or contracted with the Stockholm County Council and available free of charge. Referrals for diagnostic evaluation of suspected ASD are commonly made by child healthcare centres, whose health- and developmental surveillance program engages 99.8% of all preschool children [15]. Developmental surveillance is performed by specially trained child healthcare centre nurses at regular intervals (1, 2, 6, 10–12, 18, 36, 48 and 60 months of age), with examination by a paediatrician at key ages (2, 6, 10–12 months) and in case of developmental deviation or according to need at other age intervals. Speech abilities and language comprehension are evaluated by nurses at 36 and 48 months, and examination of sight and hearing is made at 48 months.
Interestingly, even with this tight surveillance effort, the median age of diagnosis is 8. Children with intellectual disability were identified as autistic earlier. Girls were identified later.
Where such information was available (n = 148), the median age at diagnosis was 8.0 years for ASD overall (range 1–19, interquartile range [IQR] 8.0). For cases without and with intellectual disability (n = 80 and 68), the corresponding ages were 11.5 (range 4–19, IQR 6.0) and 6.0 (range 1–17, IQR 4.0) years. Girls (n = 48) were older than boys (n = 100) at diagnostic assessment (median age 11.0 as compared to 8.0 years).
The authors were able to test the validity of the diagnoses by checking on a subset of autistic children (starting from a sample of 100 with and 100 without intellectual disability). The researchers were able to check records on 85% of this subset, and 96% of the records checked were consistent with a diagnosis of ASD.
Unlike U.S. CDC prevalence estimates, this study gives prevalence estimates for a range of ages. The graph below (and larger here) is part of Figure 1 from the study:
The prevalence is given for children born between 1983 and 2003, with a peak ASD prevalence of about 1.5% for children born between 1990 and 1997. Lower autism prevalence for younger children is likely due to underdiagnosis: the average age of diagnosis being about 8. The lower prevalence for older autistics is possibly due to “key registers used for case ascertainment only being started in 1997 and 2001, respectively, may have deflated the observed ASD prevalence among older children.”
For those interested in how this ties into the failed notion of thimerosal causing an autism epidemic: Swedish children had low exposures to thimerosal in the 1980’s and it was phased out in 1993. The flat prevalence though the 1990’s speaks strongly against the notion. In fact, the data, especially for autistics without intellectual disability, speaks against a strong rise in autism prevalence, especially during the 1990’s.
The prevalence of ASD without intellectual disability is higher than that with ID throughout the entire age range of the study. The Male:Female ratio (not shown in the graph above) changes with age. It is often about 4-5 to 1, with males predominant. For the older individuals, the ratio decreased from 5.1:1 at age 8 to 1.9:1 at age 18.
What’s most important is that this is only the first report on the autistics in the Stockholm Youth Cohort. The researchers now have a cross section of ages to work with to look at outcomes, risk factors and other studies.
Here is the abstract for the study:
Objective
Reports of rising prevalence of autism spectrum disorders (ASD), along with their profound personal and societal burden, emphasize the need of methodologically sound studies to explore their causes and consequences. We here present the design of a large intergenerational resource for ASD research, along with population-based prevalence estimates of ASD and their diagnostic validity.
Method
The Stockholm Youth Cohort is a record-linkage study comprising all individuals aged 0–17 years, ever resident in Stockholm County in 2001–2007 (N = 589,114). ASD cases (N = 5,100) were identified using a multisource approach, involving registers covering all pathways to ASD diagnosis and care, and categorized according to co-morbid intellectual disability. Prospectively recorded information on potential determinants and consequences of ASD were retrieved from national and regional health and administrative registers. Case ascertainment was validated through case-note review, and cross validation with co-existing cases in a national twin study.
Results
The 2007 year prevalence of ASD in all children and young people was 11.5 per 1,000 (95% confidence interval 11.2–11.8), with a co-morbid intellectual disability recorded in 42.6% (41.0–44.2) of cases. We found 96.0% (92.0–98.4) of reviewed case-notes being consistent with a diagnosis of ASD, and confirmed ASD in 85.2% (66.2–95.8) of affected twins.
Conclusions
Findings from this contemporary study accords with recently reported prevalence estimates from Western countries at around 1%, based on valid case ascertainment. The Stockholm Youth Cohort, in light of the availability of extensive information from Sweden’s registers, constitutes an important resource for ASD research. On-going work, including collection of biological samples, will enrich the study further.
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