Archive for the ‘prevalence’ Category

The Centers for Disease Control and Prevention (CDC) today reported that 1 in 68 children is diagnosed with an autism spectrum disorder. This new estimate is roughly 30 percent higher than previous estimates reported in 2012 of 1 in 88 children. The number of children identified with ASD ranged from 1 in 175 children in Alabama to 1 in 45 children in New Jersey.

For the full press release, please visit our website.

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By Matthew Maenner

Early identification of autism spectrum disorders (ASD) continues to be an important public health objective.  Research has shown that ASD can be reliably identified in children by around 2 years of age, and public health campaigns promote the detection of developmental “early warning signs”  that may indicate ASD.  Despite these efforts, there is a considerable gap between the age ASD is detected in clinical research, and the age at which children are identified as having ASD in typical community settings.  Previous population-based studies have shown that the average age of ASD identification in the community is less than ideal (at 5.7 years), and there is little information about whether these “early warning signs” lead to earlier ASD identification in everyday practice.

Our new study uses data from the CDC Autism and Developmental Disabilities Monitoring (ADDM) Network to answer two questions about how ASD behavioral features are described by community professionals and whether these behaviors are associated with the age of ASD identification. The ADDM Network identified 2,757 8-year-old children that met the surveillance case definition for ASD (based on the DSM-IV-TR criteria) in 2006.

616 combination

First, we examined the frequency and patterns of diagnostic behaviors that lead to a child meeting the diagnostic criteria for ASD (based on the DSM-IV-TR).  There are many different ways to meet the diagnostic criteria for ASD.  For example, there are 616 combinations of the 12 behavioral criteria that fulfill the minimum number (6) and pattern needed for “Autistic Disorder” alone.  Although there was considerable variability between individuals with ASD, boys and girls had similar patterns of documented behaviors, as did black and white children overall.  Among the 2,757 children, the most commonly documented behaviors were impairments in emotional reciprocity (90%), delays in spoken language (89%), and impairments in the ability to hold a conversation (86%).  The least frequently documented behaviors were lack of sharing enjoyment or interests (49%) and lack of spontaneous or pretend play (57%).

Our second question was whether particular ASD behaviors (such as those highlighted by the CDC’s “Learn the Signs” campaign) are actually associated with earlier ASD identification in typical community settings.  We found that the both the total number and types of diagnostic behaviors in a child’s record were strongly associated with the age that they were identified as having ASD.  Children with all 12 behavioral symptoms were diagnosed at a median age of 3.8 years of age, compared to 8.2 years for children with only 7 of the 12 behaviors.  Additionally, children with documented impairments in nonverbal communication, pretend play, inflexible routines, or repetitive motor behaviors tended to have an earlier age at ASD identification than children who did have these features in their records.  Children with impairments in peer relations, conversational ability, or idiosyncratic speech were more likely to be identified as having ASD at a later age.

These findings give us a clearer understanding of how ASD diagnoses are made in the community, and help inform efforts to maximize early identification and intervention among children with ASDs.  It may be more difficult to detect ASD at an early age among children with fewer symptoms, or symptoms that are most apparent at later ages (such as getting along with peers or conversational ability). A recent national telephone survey reported an increase in ASD prevalence among young teenagers, and parents were more likely to describe their later-diagnosed children as having “mild” ASD.  It’s possible that increased awareness and intensified screening for ASD could lead to more individuals being identified at both earlier and later ages. Strategies to improve early ASD identification and interventions could benefit by considering the manner in which individuals may meet ASD criteria.

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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:

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.

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.

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.

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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by Matt Carey

There has been a great deal of media coverage recently about the new autism prevalence estimate released by the CDC. The CDC provides a good summary page on prevalence data as well as the full report. If those who may see the report as a bit long, here is a bit of a summary of the findings.

The United States Centers for Disease Control (CDC) releases autism prevalence estimates as part of their MMWR (Morbidity and Mortality Weekly Report). They also maintain a page of information on autism. Today the CDC released the latest MMWR on autism: Prevalence of Autism Spectrum Disorders — Autism and Developmental Disabilities Monitoring Network, 14 Sites, United States, 2008. The one number from it that will be quoted most often is “1 in 88”, the new prevalence estimate.

The researchers working for the CDC use existing records: school, medical or both. So, in one area they may use school records. In another they may use school and medical records. What they don’t do is actually screen individual children and give them tests like the ADOS. This means that if a kid is not flagged somewhere in the records, they won’t find him/her. On the other hand, they don’t just count which kids already have autism diagnoses. They review the records and evaluate them to determine which kids are autistic or not. They cross check, meaning that for some fraction of the kids they use more than one person to check the records and they see how well the various researchers agree.

The CDC works with groups in a subset of states in the U.S.. For this report they used Alabama, Arizona, Arkansas, Colorado, Florida, Maryland, Missouri, New Jersey, North Carolina, Pennsylvania, South Carolina, Utah, West Virginia, and Wisconsin, most of which were used in previous reports.

Overall, the prevalence was 1 in 88 (11.3 per 1,000). This continues the upward trend in prevalence estimates from the CDC. This figure (here for bigger) is from the CDC:

This varied a great deal state-to-state. Alabama had the lowest estimated prevalence at 4.1 per 1,000. Utah the highest at 21.2 per 1,000. Or, there is about a five fold variation in autism prevalence estimates, state-to-state.

Prevalence estimates also varied by race/ethnicity. The report states “the estimated prevalence among non-Hispanic white children (12.0 per 1,000) was significantly greater than that among non-Hispanic black children (10.2 per 1,000) and Hispanic children (7.9 per 1,000). ” The estimate for Hispanic in Alabama was 1.4 per 1,000 and for whites in Utah as 40 per 1,000. More than a 20 fold difference.

This figure (click to enlarge)was interesting in showing two things. First in showing the state-to-state variability in prevalence estimates. The second interesting point to me is the difference between sites with just medical records and those with medical and education records. The sites with health-only records have lower prevalence estimates. i.e. more kids are picked up by their school records.

As with previous CDC reports, a large fraction of the children identified were not classified as autistic previously. This figure (click to enlarge) shows state-by-state and year-by-year what percent were previously unidentified. The figure also shows how many were previously unidentified but where a suspicion of autism was noted. In 2002, as many as 40% in some states were not classified as autistic before their records were reviewed. In general, over time the fraction previously unidentified has gone down. This would be consistent with schools and medical personnel getting better over time with identification of autism.

Many children identified had IQ test scores (or examiner statements) showing “normal” or borderline-normal values. This figure (click to enlarge) shows the percentages in many states with IQ>85, IQ=71–85 and IQ<70 (for children where the IQ data were available).

In Utah, for an extreme example, over 70% of those identified as autistic have IQ scores above 85. The CDC report reads:

When data from these seven sites were combined, 38% of children with ASDs were classified in the range of intellectual disability (i.e., IQ >70 or an examiner’s statement of intellectual disability), 24% in the borderline range (IQ 71–85), and 38% had IQ scores >85 or an examiner’s statement of average or above-average intellectual ability.

I.e. most children were borderline or above. Of course, the other way to read this is most children were borderline or below. Intellectual Disability is roughly defined as IQ below 70, so most children (about 62%) identified as autistic in this report were not intellectually disabled. States with higher prevalence estimates had higher percentages of non-intellectually disabled children.

The prevalence estimates are going up with time.

While ASD prevalence estimates in the overall population increased 23% for the 2-year period 2006–2008, and 78% during the 6-year period 2002–2008, the largest increases over time were noted among Hispanic children and non-Hispanic black children and among children without co-occurring intellectual disability. Better identification in these specific groups explains only part of the overall increase, however, as estimated ASD prevalence increased in all groups when data were stratified by sex, race/ethnicity, and intellectual ability.

The CDC report does have some limitations, and they note two primary limitations:

First, increases in awareness and access to services have improved the ability of the ADDM Network to identify children with ASD over time, and this likely contributes to the increase in estimated prevalence. The proportion of the increase that is attributable to such changes in case ascertainment or attributable to a true increase in prevalence of ASD symptoms cannot be determined. Ongoing monitoring is an important tool to learn why more children are being identified with ASDs and can provide important clues in the search for risk factors.

This study can’t say if there is an increase in the number of autistic children, or if there is, what would be the cause.


Second, the surveillance areas were not selected to be representative of the United States as a whole, nor were they selected to be representative of the states in which they are located. Limitations regarding population size, surveillance areas, and the consistency of these attributes were considered when analysts evaluated comparisons across multiple time points.

So, these numbers may not represent the United States as a whole.

It is valid to say that while these factors limit the ability of the CDC to define a true autism rate for the United States, the factors that go into these limitations are valid research concerns in themselves. It is very much worthwhile and valuable to ask why there are such variations state-to-state, for example. Answering this could lead to better identification and service provision overall. Likewise, understanding the effects of rising awareness could feed back into more efficient awareness campaigns to, again, help in identifying more autistic children and providing support and services to them.

The CDC concludes:

ASDs continue to be an important public health concern. The findings provided in this report confirm that prevalence estimates of ASD continue to increase in the majority of ADDM Network communities, and ongoing public health surveillance is needed to quantify and understand these changes over time. Further work is needed to evaluate multiple factors affecting ASD prevalence over time. ADDM Network investigators continue to explore these factors in multiple ways, with a focus on understanding disparities in the identification of ASDs among certain subgroups and evaluating temporal changes in the prevalence of ASDs. CDC also is engaged with other federal, state, and private partners in a coordinated response to identify risk factors for ASDs and meet the needs of persons with ASDs and their families. Additional information is available at http://www.cdc.gov/autism.

We need these data. Limitations and all. We need to know what the autism prevalence is, what the makeup is of the autistic population, and where we can do better identifying autisics. Most countries have no autism prevalence information. Other countries have few or even just one study. In my opinion we are fortunate to have the CDC and other researchers focusing on these questions here in the United States.

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