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Based on: King & Bearman (2011) American Sociological Review, 76(2), 320-346; Data from birth and diagnostic records for all children born in California 1992-2000 |
Everyone agrees there has been a remarkable increase in
autism diagnosis across the world. There is, however, considerable debate about
the reasons for this. Three very different kinds of explanation exist.
- Explanation #1 maintains that something in our modern environment has come
along to increase the risk of autism. There are numerous candidates, as indicated in this blogpost by Emily Willingham. - Explanation #2 sees the risks as largely biological or genetic, with changing
patterns of reproduction altering prevalence rates, either because of
assortative mating (not much evidence, in my view) or because of an increase in
older parents (more plausible). - Explanation #3 is very different: it says the
increase is not a real increase - it’s just a change in what we count as
autism. This has been termed ‘diagnostic substitution’ - the basic idea is that
children who would previously have received another diagnosis or no diagnosis are
now being identified with autism spectrum disorder (ASD). This could be in part
because of new conceptualisations of autism, but may also be fuelled by
strategic considerations: resources for children with ASD tend to be much
better than those for children with other related conditions, such as language
impairment or intellectual handicaps, so this diagnosis may be preferred.
In 2008, my research group published a study that
documented one kind of diagnostic substitution. We contacted people who had
taken part in our studies of children with specific language impairment years
ago. We carried out a standard diagnostic observation procedure for autism with
the young adults themselves and, where possible, interviewed their parents
about their early history. We found a number of individuals who had been
regarded as cases of specific language impairment ten or twenty years ago but who
would nowadays be diagnosed with ASD. Although it’s possible that some people
develop autistic symptomatology as they get older, in our cases the autistic
symptoms appeared to have been present from early childhood - as indicated by
the parental interviews. Around half of the sample had been identified as having
‘semantic-pragmatic disorder’ in childhood, but autism had been excluded because
at that time, prior to publication of DSM-IV diagnostic guidelines, it was
regarded as a very rare condition in which there were severe social and
behavioural impairments. How many children would have qualified for ASD
diagnoses had they been seen today? Well, it depends. I suspect few people
appreciate just how flexible the diagnostic criteria are for autism, even when lengthy standardized diagnostic instruments are used. Although we used the gold
standard diagnostic procedures (ADOS-G and ADI-R) we found they seldom gave the
same answer. If we diagnosed ASD only when both diagnostic instruments agreed,
21% of cases met criteria. If we included anyone who met criteria for autism or
PDDNOS on either ADI-R or ADOS, the rate shot up to 66%.
Last year, a fascinating study by Brugha and colleagues attacked the same question from a different angle. They did an epidemiological
survey of a representative sample of adults from the English population, using
the ADOS-G, and found that the rates of ASD were similar to those recently
reported in children. Within the adult population, rates of ASD did not change
with age. Thus, provided we stick to the same diagnostic criteria, then the
prevalence of autism is the same for those born several decades ago, as it is
for the current generation of children. Importantly, none of these adults with
ASD had received a formal diagnosis.
Recently, we conducted a study with another group: children with an additional sex chromosome (i.e. trisomy). We had not intended to study
diagnostic substitution: the goal was rather to understand more about the
language difficulties that had previously been described in children with sex
chromosome trisomies. The effect of an extra sex chromosome is relatively mild:
most of these children attend mainstream schools and they do not have any
obvious physical abnormalities. Indeed, they can be hard to study because many
individuals with trisomies will be unaware of their condition. We gathered
information by parental report, and did not do any direct evaluation of the
child, but we did ask about whether the child had had any kind of diagnosis by
a medical or psychological expert. We confirmed that there was a strong
association with language problems in all three kinds of trisomy (girls with
XXX, and boys with XYY or XXY), many of whom had had speech-language therapy.
But we also found that 2/19 (11%) of boys with XXY and 11/58 (19%) of those
with XYY had received an ASD diagnosis.
It is important to emphasise that most children with a sex
chromosome trisomy did not have an ASD diagnosis, and many were not giving any
cause for concern. Nevertheless, although they are only a minority of cases,
the proportion with ASD is much higher than in the general population. We were
really surprised at this because before publishing our study we had done a
systematic review of the literature on children with sex chromosome trisomies,
focusing on studies that avoided ascertainment bias. In these studies, not a
single case of autism had been mentioned when discussing outcomes. So was our
study a fluke? We are confident this is not the case, because this year two
further studies from the USA
have been reported (Ross et al and Lee et al, in press), both of which got results very similar to ours, though
using different methods.
This research provides further evidence that diagnostic
substitution has occurred, suggesting that children who in the past would have
been diagnosed with language impairment are now being diagnosed with ASD. The
only other way to explain the increased diagnosis rate in children with a known
chromosomal abnormality would be if the trisomy acted as a risk factor, making
children more sensitive to environmental factors that could cause autism. That’s
a possibility, but it seems more likely that cases of ASD were missed in the
past because more stringent diagnostic criteria were used, just as was found in our follow-up of children with SLI and in the epidemiological study
of adults by Brugha and colleagues.
It is becoming clear that changing diagnostic criteria,
increased awareness of ASD, and strategic use of diagnosis to gain access to
services, have had a massive effect on the numbers of children with ASD. When I
started studies in this area, I thought diagnostic substitution had happened
but I did not think it would be sufficient to explain the increase in numbers of ASD
diagnoses. But now, on the basis of studies reviewed here, I think it could be
the full story.
PS: a slightly extended version of this blogpost was featured on PLOS Blogs on 8th June 2012.
References
Bishop, D., Jacobs, P., Lachlan, K., Wellesley, D., Barnicoat, A., Boyd, P., Fryer, A., Middlemiss, P., Smithson, S., Metcalfe, K., Shears, D., Leggett, V., Nation, K., & Scerif, G. (2010). Autism, language and communication in children with sex chromosome trisomies Archives of Disease in Childhood, 96 (10), 954-959 DOI: 10.1136/adc.2009.179747
Bishop, D., Whitehouse, A., Watt, H., & Line, E. (2008). Autism and diagnostic substitution: evidence from a study of adults with a history of developmental language disorder Developmental Medicine & Child Neurology, 50 (5), 341-345 DOI: 10.1111/j.1469-8749.2008.02057.x
Brugha, T. (2011). Epidemiology of Autism Spectrum Disorders in Adults in the Community in England Archives of General Psychiatry, 68 (5) DOI: 10.1001/archgenpsychiatry.2011.38
Lee, N. R., Wallace, G. L., Adeyemi, E. I., Lopez, K. C., Blumenthal, J. D., Clasen, L. S., & Giedd, J. N. (2012, in press). Dosage effects of X and Y chromosomes on language and social functioning in children with supernumerary sex chromosome aneuploidies: Implications for idiopathic language impairment and autism spectrum disorders. Journal of Child Psychology and Psychiatry.
Ross, J. L.,et al (2012). Behavioral and social phenotypes in boys with 47, XYY syndrome or 47, XXY Klinefelter syndrome. Pediatrics, 129(4), 769-778. doi: 10.1542/peds.2011-0719
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