Autism’s Rise: why Cases are increasing in Denmark

Explaining the Increase in the Prevalence of Autism Spectrum Disorders

A new study, by the Aarhus University, Department of Public Health, finds that most – 60% – of the autism rise over the last three decades in Denmark cases is due to changes in reporting practices. But still, changes in reporting practices fail to explain 40 percent of the rise, the researcher noted.

Abstract:

Aarhus Universitet
New study shows that you cannot really talk about an autism epidemic, even though Denmark and other countries are currently experiencing a dramatic increase in the number of cases of autism spectrum disorders.

Importance
The prevalence of autism spectrum disorders (ASDs) has increased markedly in recent decades, which researchers have suggested could be caused in part by nonetiologic factors such as changes in diagnosis reporting practices. To our knowledge, no study has quantified the degree to which changes in reporting practices might explain this increase. Danish national health registries have undergone a change in diagnostic criteria in 1994 and the inclusion of outpatient contacts to health registries in 1995.

Objective
To quantify the effect of changes in reporting practices in Denmark on reported ASD prevalence.

Design, Setting, and Participants
We used a population-based birth cohort approach that includes information on all individuals with permanent residence in Denmark. We assessed all children born alive from January 1, 1980, through December 31, 1991, in Denmark (n = 677 915). The children were followed up from birth until ASD diagnosis, death, emigration, or the end of follow-up on December 31, 2011, whichever occurred first. The analysis uses a stratified Cox proportional hazards regression model with the changes in reporting practices modeled as time-dependent covariates.

Exposures
The change in diagnostic criteria in 1994 and the inclusion of outpatient diagnoses in 1995.

Main Outcomes and Measures
Autism spectrum disorders.

Results
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.

Conclusions and Relevance
Changes in reporting practices can account for most (60%) of the increase in the observed prevalence of ASDs in children born from 1980 through 1991 in Denmark. Hence, the study supports the argument that the apparent increase in ASDs in recent years is in large part attributable to changes in reporting practices.

Sources and more information:
  • Altered diagnosis has led to growth in autism, Aarhus University News, 2014.11.04.
  • Explaining the Increase in the Prevalence of Autism Spectrum Disorders, JAMA Pediatr. Published online November 03, 2014. doi:10.1001/jamapediatrics.2014.1893.
  • Autism’s Rise: Researchers Look at Why Cases Are Increasing, livescience, November 03, 2014.
  • Beyond Vaccines: 5 Things that Might Really Cause Autism, livescience, January 07, 2011.

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