Research funded by the Wellcome Trust suggests that many children diagnosed with severe language disorders in the 1980s and 1990s would today be diagnosed as having autism.
The research supports the theory that the rise in the number of cases of autism may be related to changes in how it is diagnosed.
Professor Dorothy Bishop, a Wellcome Trust Principal Research Fellow at the University of Oxford, led a study which revisited 38 adults, aged between 15-31, who had been diagnosed with having developmental language disorders as children rather than being autistic. Professor Bishop and colleagues looked at whether they now met current diagnostic criteria for autistic spectrum disorders, either through reports of their childhood behaviour or on the basis of their current behaviour. The results are published this month in the journal Developmental Medicine & Child Neurology.
Developmental language disorders, which include specific language impairment, are diagnosed when a child has unusual difficulty in his or her grasp of the spoken language, despite normal development in other areas. This may range from a child who has very limited ability to produce or understand spoken sentences, to one who does speak in long and complex utterances, but nevertheless has problem communicating effectively because of problems in conveying a point or grasping what others mean.
Autistic spectrum disorders, which include autism and Asperger syndrome, are developmental disorders affecting how a person communicates with and relates to other people and how they make sense of the world around them.
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, paediatricians 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 the Special Needs and Autism Project, the figure until the 1990s was widely accepted as being about 5 people per 10,000; even using the narrowest definition of autism, this rose to almost 40 in 10,000 by 2006