Dec 21 2009

Autism Prevalence

The Centers for Disease Control (CDC) has published the results of their latest study on the prevalence of autism. There is no question that in the last 20 years the number of autism spectrum disorder (ASD) diagnoses has increased. What is also clear is that during this time there has been increased surveillance and a broadening of the diagnosis of ASD. Whether or not this accounts for the entire increase in ASD numbers, or if there is a true increase in there as well, is unknown.

Into that context, the CDC adds their most recent numbers, concluding:

In 2006, on average, approximately 1% or one child in every 110 in the 11 ADDM sites was classified as having an ASD (approximate range: 1:80–1:240 children [males: 1:70; females: 1:315]). The average prevalence of ASDs identified among children aged 8 years increased 57% in 10 sites from the 2002 to the 2006 ADDM surveillance year. Although improved ascertainment accounts for some of the prevalence increases documented in the ADDM sites, a true increase in the risk for children to develop ASD symptoms cannot be ruled out. On average, although delays in identification persisted, ASDs were being diagnosed by community professionals at earlier ages in 2006 than in 2002.

That 1 in every 110 children on average now carry an ASD diagnosis is not new news. This CDC data was actually released ahead of publication in October. At the same time a phone survey published in Pediatrics found 110 in 10,000 children carried an ASD diagnosis – or a little more than 1%. This 1% figure seems to be highly replicated – a National Health Services survey released in September also found a prevalence of 1% for ASD in the UK.

Of interest, Autism Speaks chose to release their own press release regarding the CDC numbers, writing:

The CDC report, published in this week’s Morbidity and Mortality Weekly Report (MMWR), states that 1% or 1 in every 110 children has been diagnosed with autism, including 1 in 70 boys. This represents a staggering 57 percent increase from 2002 to 2006, and a 600 percent increase in just the past 20 years. Other significant findings include that a broader definition of ASDs does not account for the increase, and while improved and earlier diagnosis accounts for some of the increase, it does not fully account for the increase. Thus, a true increase in the risk for ASD cannot be ruled out….

“This study provides strong evidence that the prevalence of autism spectrum disorder is, in fact, dramatically increasing,” said Geraldine Dawson, Ph.D., Autism Speaks chief science officer, who noted that recent research indicates that a significant amount of the increase in autism prevalence cannot be explained by better, broader or earlier diagnosis.

This is somewhat misleading. It is true that increased surveillance alone cannot account for the increase, neither can the broadening of the diagnosis by itself. But it is possible that the two together can account for the entire increase. The fact that a real increase “cannot be ruled out” is purely a statement about the statistical power of the epidemiological evidence – it is not the equivalent of evidence for a real increase.

The CDC study argues that broadening of diagnosis does not account for the 2002-2006 increase in ASD prevalence, according to their data. But this is not surprising, as the changes in diagnosis had occurred prior to 2002. The increase from broadening the definition of autism had already occurred.

The epidemiological evidence, in fact, argues against a real increase. That same National Health Service survey I linked to above found that the prevalence of ASD was about 1% in every age group they surveyed. If true prevalence were increasing, then we would expect that autism prevalence would increase with younger age groups. That every age group they looked at had about the same prevalence speaks powerfully to the conclusion that true autism prevalence is static.

But that is not all. Recent evidence suggests that ASD caseloads increase at times when the diagnostic criteria are changed.

Another study published in Pediatrics found that diagnosis rates are probably below the true estimated prevalence rates, leaving room for increase through better diagnosis. They also found what is called “diagnostic substitution” – as the ASD diagnosis increased, other related diagnoses decreased, concluding:

“Prevalence findings from special education data do not support the claim of an autism epidemic because the administrative prevalence figures for most states are well below epidemiological estimates. The growing administrative prevalence of autism from 1994 to 2003 was associated with corresponding declines in the usage of other diagnostic categories.”

Notice the timeframe – 1994-2003 – before the recent CDC data where they conclude there was no diagnostic substitution.


ASD diagnostic prevalence has increased in the last 20 years has increased and is now around 1%. However, the epidemiological evidence strongly suggests that most or all of this increase is due to broadening the diagnosis and increased surveillance – leading to more and earlier diagnosis.

This still means that autism is a huge health issue that deserves funding and attention. It does not imply, however, that there is some causal factor, such as an environmental factor, that is increasing the prevalence of autism.

However, it is also undeniably true that a real increase in prevalence cannot be ruled out, although it would only account for a part of the total increase. The fact that older age groups also have about a 1% prevalence strongly suggests that any real increase in ASD must be small. But it could be there.

A real increase could be due to many factors. One strong possibility is increasing paternal age. People are having children later in life, and older age is associated with an increased risk of many disorder. A recent study found that increasing paternal age also increases the risk of autism:

In adjusted models that included age of the other parent and demographic covariates, a 10-year increase in maternal age was associated with a 38% increase in the odds ratio for autism (odds ratio = 1.38, 95% confidence interval: 1.32, 1.44), and a 10-year increase in paternal age was associated with a 22% increase (odds ratio = 1.22, 95% confidence interval: 1.18, 1.26).

We would therefore expect a real increase in ASD from this factor alone. There may be other factors as well. Much attention is paid to environmental factors, but so far none have been proven – including vaccines.

If the increase in ASD prevalence is largely due to increased surveillance and a broadened definition, with perhaps a small real increase thrown in, then we would expect the measured prevalence rates to eventually level off. We may be getting close to that point now. Only time, and further epidemiology, will tell.

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