Wednesday 8 August 2018

Autism prevalence in California: 1931 to 2014

Credit: Nevison et al, 2018
'Autism prevalence' is a topic of real discussion both inside and outside of autism research circles. It's not so much a debate about whether there has been an increase in the numbers of people being diagnosed with autism or autism spectrum disorder (ASD) - there has and continues to be (see here and see here) - but rather the extent of the increase and the possible reason(s) behind such an increase.

The paper published by Cynthia Nevison and colleagues [1] adds something further to the autism prevalence discussions. They focused on how much of an increase in cases of autism has been noted in one part of the United States (US) and how some notable birth years have been seemingly driving the increase.

OK, first things first, Nevison et al are no strangers to the [peer-reviewed] debates around the autism numbers (see here and see here). Their previous analyses have concluded that the increase in cases of autism in the US is NOT solely driven by changing diagnostic criteria or diagnostic switching/substitution, although these factors have probably played some role. Yes folks, there might have been a very real increase in the numbers of people being diagnosed with autism (see here).

On this most recent research occasion, Nevison and co-author Mark Blaxill are joined by a noteworthy figure in the field of autism prevalence tracking, Walter Zahorodny. Zahorodny is intricately involved in tracking autism prevalence in New Jersey (see here) and was also listed as an author on the most recent CDC stats regarding estimated autism prevalence that were quite quietly published a few months back [2]. In short, he's an expert in such matters.

Authors started with the California Department of Developmental Services (CDDS) data on autism which "provides services to eligible individuals living in California who meet the DSM diagnostic criteria for autism." In the age of the (very) wide autism spectrum, the CDDS has been pretty reserved in terms of who gets the services it offers. The term 'code 1 autism' has been used to refer to CDDS recipients eligibility, where autism or autistic disorder was typically the diagnosis listed and importantly: "individuals applying for CDDS services must demonstrate significant functional disability in 3 out of 7 life challenges, which include self-care, language, learning, mobility, self-direction, capacity for independent living and economic self-sufficiency." We are told that: "Milder subtypes such as Asperger’s syndrome and PDD-NOS [pervasive developmental disorder - not otherwise specified] have not been eligible for services unless they have another qualifying disability" (authors words not mine). With the onset of the DSM-5 description of autism, where individual diagnoses like PDD-NOS and Asperger syndrome have been 'rolled' into one definition (autism spectrum disorder), the CDDS has had to adapt and change. The years 2016 and 2017 have been particularly important for the DSM-5 change to the CDDS.

Added to their examination of the CDDS data, authors also looked at a couple of other initiatives with autism prevalence data: The Individuals with Disabilities Education Act (IDEA) and the Autism and Developmental Disabilities Monitoring (ADDM) Network. As I've already mentioned, Zahorodny has been quite 'active' in ADDM research circles for some time.

From all this [estimated] prevalence of autism data, authors set about to 'visualise' autism prevalence data extracted from the CDDS and other information sources and further examine whether "ASD is truly a constant prevalence condition" taking into account the various methods for collecting data on autism prevalence.

Credit: Nevison et al. 2018
Results: "The data are consistent across methods in showing a strong upward trend over time." The long-term trend of autism prevalence based on the CDDS data show "an apparent ~ 1000-fold increase in CDDS autism prevalence between birth year 1931, when prevalence was only ~ 0.001%, and birth year 2012, when prevalence had increased to 1.18% among 5 year-olds born in that year." Nevison et al talk about "age-resolved snapshot and constant-age tracking method[s]" as tools in their research, but the long-and-short of it is that the only way was up when it came to autism prevalence across the years. They further note: "The increase from ~ 0.001 to 1.18% in the 2012 birth cohort has occurred gradually, with a slow upward creep starting as far back as the 1940s, but with several change points along the way, around ~ 1980, ~ 1990, and ~ 2007, when the rate of growth accelerated."

I don't think there is too much more to say about the Nevison findings aside from reiterating that autism prevalence seems to have quite dramatically changed over the last 80 years or so. Insofar as those upticks in diagnoses around birth years 1980, 1990 and 2007, I'd be interested to know a little more about what factors might have been contributory to those particular years' growth or whether they are just statistical blips.

And before you say it, no, it's probably not just better awareness and the like...


[1] Nevison C. et al. California Autism Prevalence Trends from 1931 to 2014 and Comparison to National ASD Data from IDEA and ADDM. J Autism Dev Disord. 2018. July 5.

[2] Baio J. et al. Prevalence of Autism Spectrum Disorder Among Children Aged 8 Years — Autism and Developmental Disabilities Monitoring Network, 11 Sites, United States, 2014. Morbidity and Mortality Weekly Report (MMWR). 2018; 67(6): 1-23.


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