Professor Dorothy Bishop, a Wellcome Trust Principal Research Fellow at the University of Oxford in England, led a study that checked back in with 38 adults (currently between the ages of 15 and 31), who had been previously diagnosed as having "developmental language disorders" as children. At that point, these kids were not considered autistic.
Bishop and her colleagues wanted to find out whether these people would now be said to meet the current diagnostic criteria for autistic spectrum disorders. To determine this, they studied reports of the kids' childhood behaviors or discussed current behavior.
"Our study shows pretty direct evidence to support the theory that changes in diagnosis may contribute towards the rise in autism," says Professor Bishop. "These were children that people were saying were not autistic in the 1980s, but when we talk to their parents now about what they were like as children, it's clear that they would be classified as autistic now.
Much of the differentiation between now and decades past is based on the degree of symptoms. "Criteria for diagnosing autism were much more stringent in the 1980s than nowadays and a child wouldn't be classed as autistic unless he or she was very severe," says Professor Bishop. "Now, children are being identified who have more subtle characteristics and who could in the past easily have been missed."
Still, don't assume these stats mean the rising number of cases of autism being diagnosed is purely attributable to a change in medical semantics. "We can't say that genuine cases of autism are not on the increase as the numbers in our study are very small," Bishop points out. "However, this is the only study to date where direct evidence has been found of people who would have had a different diagnosis today than they were given 15 or 20 years ago."
Participants in the study were drawn from a pool of children who had participated in a series of studies of developmental language disorder conducted during the period 1986 to 2003 and about whose conditions detailed information was known. All attended special schools or classes for children with language impairments, and would have been diagnosed by educational psychologists, pediatricians or speech therapists as having developmental language disorders and none had previously been diagnosed as autistic. However, when reassessed by Professor Bishop and colleagues using current criteria, around a quarter were identified as having autistic spectrum disorder.
In recent years, the criteria for diagnosing developmental language disorders and autism have changed. This has coincided with a marked rise in the rates of diagnosis of autism. According to England's Special Needs and Autism Project, until around 1990, the accepted autism statistics said that 5 people per 10,000 were autistic. And even when using only the narrowest definition of autism, this figure increased to almost 40 in 10,000 by 2006. (All statistics are based on the UK.)
One of the main theories suggests that the true prevalence of autism has been constant, and simply that changes to the diagnostic criteria mean that more children are being classified now as autistic. A UK study using the General Practice Research Database supports this belief, as the researchers discovered that the rise in autism was coupled with decline in the frequency of language disorder diagnoses.
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