Danish Study Explains Most of Autism’s Rise
By The Carlat Psychiatry Report
It’s now estimated that about one in 68 children in the US have been diagnosed with autism spectrum disorder (ASD), a 123% increase since 2002, when a monitoring network funded by the US Centers for Disease Control and Prevention (CDC) began its reporting.
There is disagreement about the causes of this increased incidence of autism. Debate has focused on whether the rise in cases is an artifact caused by increased diagnosis and reporting, or if there is some unknown pathogenic factor in the environment that is causing an actual increase in new cases. A new study out of Denmark provides support for the artifact argument.
Researchers analyzed information from nearly 678,000 children born in Denmark from 1980 to 1991, who were followed until 2011. Of those children, 3,956 were diagnosed with autism, with a sharp increase after 1994. Researchers found that there were only 192 diagnoses reported from 1980 to 1993; 100 from 1994 to 1995; and an astonishing 3,665 (95% of the total) were reported from 1996 to 2011.
What happened in 1994? That was when the ICD-10 was introduced, in which the criteria for diagnosis were changed in ways that made it easier to diagnose. These changes included recognizing autism as a spectrum of disorders (rather than as a subgroup of schizophrenia in ICD-8, which was the previous version used in Denmark) and various changes in the diagnostic criteria. Then, in 1995, a change in reporting practices occurred. Previously, autism could be diagnosed only in inpatient settings; after 1995, outpatient diagnoses were allowed.
Using statistical techniques that predicted changes in diagnostic rates based on past trends, the researchers estimated that about 60% of the increase in autism prevalence in Denmark can be explained by changes in diagnostic criteria and in reporting practices. This means that 40% of the increase remained unexplained. Researchers suggested that generally growing awareness about autism might contribute, but that further studies are needed to explain those changes (Hansen SN et al, JAMA Pediatr 2014; Epub ahead of print).