Friday, June 15, 2012

Study : Not All Historically Missed Cases of Autism are High-Functioning

If you have been exposed to almost any discussion about why the number of children with a diagnosis of autism has risen sharply over the past several decades, I am sure you have run across the idea that a large part of the increase is in the high-functioning group.  And, to an extent, this idea is true - there are more children being diagnosed today than in the past that fit into the high-functioning bucket.

But what is often not mentioned is that there has been an equally large (if not larger) increase in the number of middle and lower functioning children as well.  This pattern is plainly visible in many of the recent estimates of autism prevalence (I hope to have a post up about that soon, time permitting).

Of course the reason or reasons for the sharp increase are still unknown; whether it be more awareness, better services, an expanding definition of autism, an actual increase in the number of cases, or some other reason entirely.

So, in light of all of this, I found this recent re-examination of a historical data set from 1980 to be of interest. I haven't had a chance to look at the full text of the paper yet, but it does raise some questions.  The immediate thing that jumps out at me is that the "missed" group is comprised of children who would likely be very low functioning, having an average IQ of 35 (!) .

The original group was solidly in the intellectually disabled range (i.e. low functioning) and yet the group identified in this re-examination had an even lower average IQ than that.  For reference, an "average" person's IQ is about 100 and intellectual disability starts at about 70.

The abstract is below.
Autism Spectrum Disorder Reclassified: A Second Look at the 1980s Utah/UCLA Autism Epidemiologic Study.
The purpose of the present study was to re-examine diagnostic data from a state-wide autism prevalence study (n = 489) conducted in the 1980s to investigate the impact of broader diagnostic criteria on autism spectrum disorder (ASD) case status. Sixty-four (59 %) of the 108 originally "Diagnosed Not Autistic" met the current ASD case definition. The average IQ estimate in the newly identified group (IQ = 35.58; SD = 23.01) was significantly lower than in the original group (IQ = 56.19 SD = 21.21; t = 5.75; p < .0001). Today's diagnostic criteria applied to participants ascertained in the 1980s identified more cases of autism with intellectual disability. The current analysis puts this historic work into context and highlights differences in ascertainment between epidemiological studies performed decades ago and those of today 
Of course, the real question about this result is what exactly is the "current ASD case definition" that was used and how it was determined whether an original participant met this definition.  As we saw with other recent studies, it is not always easy (or possible) to map the data collected years ago into what the criteria for autism look like today.


Miller JS, Bilder D, Farley M, Coon H, Pinborough-Zimmerman J, Jenson W, Rice  CE, Fombonne E, Pingree CB, Ritvo E, Ritvo RA, McMahon WM. Autism Spectrum Disorder Reclassified: A Second Look at the 1980s Utah/UCLA Autism Epidemiologic  Study. J Autism Dev Disord. 2012 Jun 13. [Epub ahead of print]
PubMed PMID: 22696195. DOI: 10.1007/s10803-012-1566-0


  1. Interesting post on what may turn out to be quite an important analysis (re-analysis) bearing in mind that Utah has been pretty good at collecting/estimating data on numbers of cases of autism down the years.

    Hopefully not too far off topic but there was an interesting paper delivered at IMFAR 2011 on the issue of ASD across the DSMs (well at least across DSM-III, DSM-IIIR & DSM-IVTR) which might be relevant to this and other discussions:

    They estimated that about 18% of the change among the more general PDD category was due to diagnostic changes.

    1. Hi Paul,

      I have to wonder if Utah has been that good at keeping track. The CDC's prevalence estimates for Utah jumped substantially between 2002 an 2008 - the majority of that increase coming in higher functioning children (4.1 per 1,000 to 12.3 per 1,000).

      If Utah is under counting the lower functioning group and it really has been growing in proportion to the higher functioning group like the rest of the US has, then that would mean that Utah has a rate that is around 30 to 40 per 1,000...

      Anyway, thanks for the reference to the paper, I will have to take a look.

  2. I imagine the lower the IQ the more difficult it becomes to apply diagnostic tools effectively?

    1. I'm not sure but I would guess it would depend upon the reason the person's IQ is that low and the actual diagnostic involved.

      Most of the (many, many) autism specific tests I have seen or participated in are filled out or answered by the parents/caregiver. For example, the ADI-R, one of the gold standards, is entirely based upon parental interview.

      So if the parent/caregiver is responding to questions based on the person's actions over time, I don't know that IQ would be as much of a factor.

      But if it was a test for autism that involved the person themselves, such as the ADOS, then their functional level of IQ might make a difference.