Explaining the Rise in Autism Diagnoses
A recent study looks at the prevalence of autism spectrum disorder (ASD) in Denmark over the last 20 years and finds that 60% of the increased in prevalence can be accounted for by changes in diagnostic practices.
ASD is a neurological condition characterized by reduced social and verbal skills. Since the mid 1990s, in various countries, the diagnosis has been expanded to include very mild manifestations. This expanded diagnosis is responsible for a portion of the apparent increase in its prevalence.
In Denmark specifically the diagnosis was expanded in 1994. In addition, in 1995 outpatient records were included in the counting of cases of ASD. The authors of the recent study found that:
For Danish children born during the study period, 33% (95% CI, 0%-70%) of the increase in reported ASD prevalence could be explained by the change in diagnostic criteria alone; 42% (95% CI, 14%-69%), by the inclusion of outpatient contacts alone; and 60% (95% CI, 33%-87%), by the change in diagnostic criteria and the inclusion of outpatient contacts.
These two factors alone could account for 60% of the increase in the number of ASD diagnoses being counted. This does not mean that the other 40% represent a real increase in the incidence of ASD. There are other factors at work, such as diagnostic substitution – children who 20 years ago would be diagnosed with a nonspecific language disorder are today diagnosed with ASD. Another factor is increased awareness among parents, educators, and health care professionals.
A 2014 study, reviewing the world’s literature on ASD prevalence, found that once these factors were accounted for, there has been no change in ASD prevalence in the last 20 years. A 2006 review came to the same conclusion.
This most recent study adds to the growing literature showing that the prevalence of ASD has been stable over the last two decades, and the increase in the number of diagnoses being made is simply an artifact of awareness and diagnostic methods.
This data contradicts claims that environmental factors have been increasing ASD over the last two decades.
Source: Explaining the Increase in the Prevalence of Autism Spectrum Disorders, in JAMA Pediatrics