Reporting may explain rise in Danish autism cases

By Kathryn Doyle

NEW YORK (Reuters Health) - More than half of the apparent increase in childhood autism cases in Denmark between 1980 and 2011 may be due to changes in how those cases are reported, according to a new study.

Researchers from Aarhus University and the Initiative for Integrative Psychiatric Research in Denmark studied nearly 678,000 children born there between 1980 and 1991. By December 2011, 3,956 of the children had been diagnosed with autism spectrum disorder.

Between 1980 and 1993, 192 kids were diagnosed with autism, compared to 100 between 1994 and 1995 and 3,664 between 1996 and the end of the study.

Researchers compared groups of children by age, and found a considerable spike in autism diagnoses around 1995 for every group, whether they were age three or 15 at the time.

"In 1994, the diagnostic criteria used by clinicians to establish psychiatric diagnoses was changed," upgrading from the International Statistical Classification of Diseases and Related Health Problems version 8 (ICD-8) to ICD-10, said lead author Stefan N. Hansen of the section for biostatistics in the department of public health at Aarhus University.

"In 1995, the national health registries in Denmark, which are often used in Danish health research, began to also include diagnoses given in connection with outpatient consultations, whereas before 1995 only diagnoses given in connection with hospitalization was reported to the registries," Hansen told Reuters Health by email.

These changes accounted for 60% of the increase in reported autism cases, according to results in JAMA Pediatrics.

"The definition of autism is more precise today compared to the 1980's," said Hjordis Osk Atladottir, who also works in the department of public health at Aarhus University but was not involved in the new study. "We got the ICD-10 in 1994 in Denmark and before that we had ICD-8, but autism was very poorly defined in the ICD-8, resulting in almost no autism diagnoses prior to 1994."

The apparent increase in autism prevalence in Denmark in recent years is in large part attributable to changes in reporting practices over time, the authors write. However, a considerable part of the increase is not explained by the two changes in reporting practices, so the search for the remaining 40% is important.

These findings are likely applicable in other developed countries, where diagnostic criteria have changed in a similar way, said Professor Helen Leonard of the Telethon Kids Institute in Perth, Australia, who was not involved in the new study.

"Other Nordic countries, e.g. Sweden and Finland, have similar national health registries that are used in health research and their registries have had similar changes throughout the 1990s, i.e. changes in the diagnostic criteria and the addition of outpatient diagnoses to the registries," Hansen said. "So we expect these changes to also have had an impact on autism prevalence in said countries but exactly how much of the increase in those countries that can be attributed to the changes is something we can only speculate about."

Over the past 30 years, parents have become more and more aware of autism and its symptoms, and children are diagnosed with autism at an earlier age today, which may help explain the rest of the increase, he said.

"Several studies have also found an association between advanced parental age and autism which might explain some part of the increase, since the general pattern is that parents are getting older," he said.

In addition, some of the stigma of an autism diagnosis may be dispelled over time, Leonard said. In some countries, a diagnosis of autism attracts better funding for early intervention than a diagnosis of intellectual disability, she added.

"Therefore there is the issue of diagnostic substitution where children who would previously have been given an alternative diagnosis such as intellectual disability or developmental delay are now attracting a diagnosis of autism," she told Reuters Health by email.

Environmental factors like using certain medications during pregnancy might also contribute, but more research is required to quantify the contribution of hypothesized risk factors, she said.

SOURCE: http://bit.ly/1tUudR4

JAMA Pediatrics 2014

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