I have heard a lot of people say that. It does make sense for Aspergers but not for the "low functioning" end of things. Certainly they did not just get by in the past without being noticed. I do agree that more awareness leads to discovery and diagnosis but it seems to me that that would only apply to people who could have passed as typical. My niece, who is 13, non-verbal and in diapers would have definitely been diagnosed as would others who are similarly affected.
If there has indeed been an increase in the actual rates of Autism, then it is more useful to let the research into the reasons be done rather than to cast around looking at everything that has changed in the world as a potential cause. Given time, the answer will surely be determined, while anything done in a panic is likely to be wrong.
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There are two basic hypotheses: 1) That the true incidence of autism is rising due to an environmental cause, 2) That the rise in incidence is mostly or completely an artifact of increased surveillance and broadening of the definition of autism.
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The confusion about the epidemiology of autism is one common to scientific medicine. Whenever historical comparisons are made it is possible that changing definitions and practices over time will distort those comparisons. This is why medical scientists are often reluctant to change nomenclature (disease names) and definitions – doing so immediately renders the literature obsolete. All subsequent literature must now have a footnote. But the progress of our understanding of biology and disease makes such changes unavoidable.
In the 1990′s the diagnosis of autism was changed to autism spectrum disorder (ASD) – the new name reflecting the changing concept of autism to include a broader spectrum of symptoms, including much more subtle manifestations. In particular a diagnostic entity known as Aspergers syndrome, which is essentially a subtle manifestation of autism features, was classified as part of ASD. Any time you broaden a category the number of individuals that fit into that category is likely to increase.
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Another prediction that flows from the second hypothesis is that if we compare apples to apples – meaning if we look at the same community and apply the same diagnostic methods that were used in the past as documented in a published study, then the incidence should be the same. In other words – if we control for any changes in the diagnostic criteria and surveillance methods the incidence of autism should be stable over time. Chakrabarti and Fombonne did exactly that, comparing the incidence of autism in 2002 (looking at a cohort of children born between 1996-1998) to the same population using the same methods as a previous study looking at the cohort of children born between 1992-1995. They found:
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The rate in this study is comparable to that in previous birth cohorts from the same area and surveyed with the same methods, suggesting a stable incidence. (Chakrabarti 2005)
If the broadened diagnosis hypothesis is true than it must also be true that as other diagnoses shifted over to autism they would decrease as autism numbers increased. This is exactly what Jick et al found when they reviewed a cohort of boys with and without autism. What was previously diagnosed as language disorder is now being diagnosed as autism, with a corresponding decrease in non-specific language disorders. Shattuck found the exact same effect, so called “diagnostic substitution,” when he studied the prevalence of disabilities among children in US special education from 1984 to 2003. He found that in locations where the prevalence of autism had increased there was a corresponding decrease in the prevalence of other disabilities. (Shattuck 2006)
12 Dec 2012, 1:09 pm
