Much of the increase in autism diagnoses in recent decades may be tied to changes in how the condition is reported. Sixty percent of the increase in autism cases in Denmark can be explained by these changes, scientists report November 3 in JAMA Pediatrics.
The researchers followed all 677,915 people born in Denmark in 1980 through 1991, monitoring them from birth through the end of 2011. Among children born in this period, diagnoses increased fivefold, until 1 percent of children born in the early 1990s were diagnosed with autism by age 20.
During these decades, Denmark experienced two changes in the way autism is reported. In 1994, the criteria physicians rely on to diagnose autism were updated in both the International Classification of Diseases manual used by Denmark and in its American counterpart, the Diagnostic and Statistical Manual of Mental Disorders.
Then in 1995, the Danish Psychiatric Register began reporting diagnoses where doctors had only outpatient contact with children, in addition to cases where autism was diagnosed after children had been kept overnight.
The researchers estimated Danish children’s likelihood of being diagnosed with autism before and after the two reporting changes. These changes accounted for 60 percent of the increase in diagnoses.
The updates in diagnostic criteria and including outpatient cases still leave a considerable amount of the rise in reported autism prevalence unaccounted for.
“We still need to look for other factors that can explain increases in autism,” says Stefan Hansen, a coauthor and biostatistician at Aarhus University in Denmark. He says that growing awareness among the public and children being diagnosed younger may also play a role.