1. Prevalence Rates and Home Schooling.

I received a thoughtful email about the impact of home schooling on the CDC prevalence rate and autism research in general, given that many children with ASDs may be home schooled. Here is my response:

Regarding the CDC:
The prevalence was obtained from health records and, in some States, also educational records. States that used educational records had higher prevalence rates, and those records only included public school records. So theoretically, the prevalence would be even higher once home/private school cases are added. While education records may have included some children in private/home schools (many children in home school still receive special education services in some States and would therefore be identified by the CDC teams), many cases are likely being missed.

Interestingly however, the new CDC numbers are in line with the national autism prevalence study published in Pediatrics. This study was not based on educational or health records reviews, but instead it was based on detailed phone screenings of a representative sample of US families. Both of these studies however, would miss some children with ASD that are undiagnosed (and maybe home schooled) due to limited contact with health workers (pediatricians, etc). These children would not have any records showing that they have ASD symptoms and these parents would also respond ‘no’ to the basic phone screening question “have your child ever been diagnosed with an autism spectrum disorder?”

Regarding Research in General:
Fortunately, most research on autism is not conducted via the school systems. Most research is conducted at medical and university centers with families recruited from the community. In my neuropsychology assessment experience, I would say that at least 30% of the ASD kids we see are home schooled, and many of these children are active participants in our research programs. So the news is a bit better for general research, in that it is unlikely that home schooled kids are underrepresented in those studies.

2. Vaccines.

I really dislike writing anything about vaccines, mostly because regardless of how factual I aimed to be, any mention of vaccines is usually followed by a dozen of  ’friendly’ emails. But I’ve received several emails asking how the CDC numbers affect the vaccine theory. The CDC study does not address this issue at all, and the data say little about this theory. However, some reasonable conclusions can be made.

- If the increases in diagnoses among 8 year olds from 2002 to 2006 are due to real increases in true prevalence

and

- If vaccines play a role in the incidence of autism

- Then a 50% increase in the prevalence during the 4 year period should be accompanied by a noticeable change in vaccination practices during key years.

Specifically, the 2002 CDC  study was based on children born in 1994 and the new CDC study was done with children born in 1998.  Thus, given the striking increases in prevalence rates among the 1998 children, you would expect that compared to those born in 1994, children born in 1998 received higher vaccination dosages, received more harmful dosages, or simply were vaccinated at a higher rate. I have some data on vaccination rates:

I took a look at the CDC vaccination rates for MMR for those born in 1994 and 1998 by the time they were 2 years of age. You can take a look at the data here. The National vaccination rate for MMR for those born in 1994 was 90%. For those born in 1998, the vaccination rate was also 90%. For the states included in the CDC autism study, the vaccination rate for those born in 1994 was 90% and for those born in 1998 was also 90%. At the State and National level, there were no changes in vaccination rates for kids born in 1994 and 1998 that could help explain the 50% jump in autism prevalence.

12/28/09 UPDATE: Please note that in the paragraphs above I presented a simple logical argument for the vaccines debate. If vaccines played a role in the 1994 to 1998 autism rate change, then there must be a change in vaccination practices between 1994 and 1998-2000. Potential changes may have involved higher vaccination rates, changes in vaccine cocktails or contents, changes in schedules, etc etc. I then provided data for vaccination rates for one single vaccine as an example: MMR. Clearly such data are very limited and does not cover all possible changes that may have taken place during that time.

3. What’s up with Missouri?

Missouri had the highest autism rates of all states assessed (albeit it was a tie with Arizona in many measures), with rates that were often more than twice that of other States. One of my readers asked whether this was due to demographic differences in the target counties in Missouri. For example, is it possible that the data from Missouri came mostly from urban St Louis with a higher proportion of ethnic minorities or lower SES families? The data does not seem to support this theory. The Missouri sample was close to 70% white, and other States with significantly higher % of ethnic minorities in urban settings had significantly lower autism rates (e.g., Colorado – all from metro Denver with only 55% white; Florida – all from Miami with only 23% white; Georgia – all from metro Atlanta with only 38% white). We do not know why the high prevalence of autism in Missouri and Arizona, but it is very unlikely that it is due to demographic differences between these States and the other States included in the study.

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I thought it would be appropriate to start the month with some thoughts about the new Autism prevalence study that was just published in Pediatrics and which hit the news cycle this morning.

In sum, the study found that the prevalence of parent-reported rates of autism was higher than previous estimates. Specifically, they found that 1.1% of all children aged 3 to 17 had autism (1 in 91) as compared to previous estimates of approximately 1 in 150.

Does this suggest an increase in autism rates between 2003 and 2007? Maybe, in that the finding is consistent with the possibility that autism rates are increasing. However, there are a number of issues that must be taken into account, especially the methodological differences between this study and the CDC study that reported prevalence rates for 2002.

For a more detailed explanation of the CDC report from where the 1 in 150 rate comes from read my previous post on the topic: Autism rates in the USA: where did the 1 in 150 number came from?

How was the current study conducted?
The study include an analysis of the 2007 National Survey of Children’s Health (NSCH). The NSCH consists of a random and representative telephone survey of parents of 78,037 children. Each family provided data on a single child, so that if a family had more than one child, only one of the children was selected as the target child for interview purposes. During the interview the parents were asked whether “they had ever been told by a doctor or other health care provider that their child had ‘autism, Asperger’s disorder, pervasive developmental disorder, or other autism spectrum disorder.’ “ If the parent said yes, then they were asked whether the child currently had autism or another ASD.

They found a prevalence of 110 cases of current ASDs per 10,000 children, or 1 in 91 children. As expected rates were 4 times higher in boys than in girls.

There are two key differences between this study and the previous CDC report:

1. Unlike the CDC report, this study was nation-wide and included all US regions. In contrast, the CDC report was based on autism rates observed in only 15 states. Therefore, the current report appears to be more representative of the US population than the previous study.
2. The current study was based on parental reports during a phone survey with no corroboration of the diagnoses endorsed by the parents. That is, we assume that parents are correct when asked… does your child current have Autism or ASD? The authors could not verify the veracity of these parental reports through review of medical or educational records. In contrast, the CDC report was based on a review of medical and educational records, and documented diagnoses were doubled checked by trained clinicians to make sure that there was sufficient evidence for each diagnosis. Therefore, the CDC study was much more conservative in estimating autism rates because diagnoses were obtained from medical and educational records rather than from parental reports.

Clearly, neither the CDC nor the current Pediatric study is near perfect, and they provide only a rough estimation of autism cases in the US. Unfortunately, the very significant differences in methodology between these two studies make it impossible to determine with certainty whether the new rate of 1 in 91 reflects a true increase in autism or is simply a byproduct of different estimation procedures not used in previous studies.

The reference: Michael D. Kogan, PhD,a Stephen J. Blumberg,, PhD,b Laura A. Schieve, PhD,c Coleen A. Boyle, PhD,c, James M. Perrin, MD,, Reem M. Ghandour, DrPH,, Gopal K. Singh, PhD,, Bonnie B. Strickland, PhD,, Edwin Trevathan, MD, MPH,, & Peter C. van Dyck, MD, MPH (2009). Prevalence of Parent-Reported Diagnosis of Autism Spectrum Disorder Among Children in the US, 2007 Pediatrics, 124 (4) : 10.1542/peds.2009-1522

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