Autism epidemic and symptom substitution

Study provides evidence for the symptom substitution theory explaining the dramatic increases in autism during the last 20 years. A review of: 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 and Child Neurology, 50, 1-5.

One theory that has been proposed as a possible explanation of the dramatic increase in autism diagnoses during the 1990’s and 2000s is the Diagnostic Substitution phenomena. The basic premise of this position is that increases in autism diagnoses are not due to a true increase in the number of ‘cases’ of autism, but instead to a change in diagnostic practices so that individuals who are now diagnosed with autism would have been diagnosed with a different condition 20 or 30 years ago.

What are the basic assumptions about this theory that can be tested? First, an increase in diagnoses of autism should be accompanied by an equally dramatic decrease in diagnoses of other related disorders that are believed to drive the substitution. Sullivan at Gray Matter – White Matter provides a great example of this phenomenon by looking at the rates of autism and mental retardation in Alabama. Another way to test this theory would be to examine retrospectively the clinical presentation of adults who have been diagnosed with a non-autism disorder, and determine if the clinical presentation of these individuals during childhood would have led to a diagnosis of Autism today. That is, would children with the same presentation receive a diagnosis of autism today?

In this study, the authors examined 38 adults (age 15 to 31) who had received a diagnosis of language disorder during childhood but not a diagnosis of Autism. The authors were mostly interested in a particular type of language disorder diagnosis called pragmatic language impairment (PLI), since this disorder has many similarities with autism. The authors conducted full ADOS and ADI evaluations of these individuals. They found that 55% of the participants with PLI met the criteria of autism as indicated by the ADOS or the ADI, and 40% met the criteria of autism as indicated by both, the ADOS and the ADI.

These findings are consistent with the diagnostic substitution theory. The implication is that a significant percentage of people who were diagnosed with PLI in the past would now receive a diagnosis of autism instead. Likely this substitution is not sufficient to explain, in its entirety, the dramatic increase in autism diagnoses; but it is reasonable to conclude that such substitution could partially explain such increase. In addition, the PLI substitution is just one of several proposed substitutions (see for example MR as described here). Finally, it could be argued that these individuals received the correct diagnosis of PLI as children and developed autism symptoms as adults. Although this is a plausible explanation, it is not consistent with what we know of the developmental progression of autism symptoms.

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