TUESDAY, Jan. 2, 2018 -- Autism rates are much higher than originally thought but may have stabilized in recent years, a new study suggests.
An estimated 2.41 percent of children in the United States have autism spectrum disorder, according to a new analysis of data from the U.S. National Institutes of Health, or NIH. The most recent previous estimate put autism rates at 1.47 percent in 2010, researchers from the new study said.
"The prevalence of autism spectrum disorder is much higher than previously thought," said senior author Dr. Wei Bao, an epidemiologist with the University of Iowa College of Public Health.
Autism spectrum disorders now affects about 1 of every 41 children, a huge increase in autism from previous decades, Bao said.
"Autism now is not something rare," he said. "It's not as rare as 1 per 1,000, as it was in the 1970s and 1980s. With these data, now we can see it is already 1 per 41. The prevalence is much higher than previously thought."
However, the autism rate might have leveled off in recent years.
From 2014 to 2016, the rate did not experience a statistically significant increase, according to the new study, published as a research letter in the Jan. 2 issue of Journal of the American Medical Association.
"After many years of seeing a slow but steady increase in the prevalence of autism spectrum disorders, it is encouraging that the most recent national data failed to find any increase during the most recent three-year period," said Dr. Andrew Adesman. He is chief of developmental and behavioral pediatrics at Cohen Children's Medical Center of New York in New Hyde Park.
"Although it is encouraging that the prevalence of autism spectrum disorders is not increasing further, we don't have a good understanding of why the prevalence increased in recent previous years, and it remains concerning that the prevalence is as high as it is," Adesman said.
Bao thinks it's too soon to cheer the perceived leveling off of autism rates.
"This is just a three-year duration," he said. "It is hard to see a big increase or decrease over just three years. It is not safe to firmly conclude whether this really indicates stable or not."
Previous estimates of autism relied on the Autism and Developmental Disabilities Monitoring Network, a group of programs funded by the U.S. Centers for Disease Control and Prevention to track autism rates.
But for their study, Bao and his colleagues instead analyzed data from the National Health Interview Survey, an annual poll conducted by the NIH.
Results from that survey indicate that previous attempts might have underreported autism rates, Bao said.
Experts have argued that the increase in autism might be due to new diagnostic criteria that broadened the definition of the condition. For example, high-functioning children with Asperger syndrome now are diagnosed as having autism spectrum disorder.
A 2015 study in JAMA Pediatrics argued that nearly two-thirds of the increase in autism among Danish children could be chalked up to changes in how autism is diagnosed and tracked.
That could explain some of the increase, but most cases of autism spectrum disorder are still children diagnosed with traditional autism, Bao said.
"I don't think the change in diagnostic criteria can fully explain this kind of increase," Bao said. "It should be part of the reason."
Boys are more likely to have autism than girls, by a 3-to-1 margin, Bao and his colleagues found. No one knows why this is, but it could have something to do with gender differences in genetics or hormones, he said.
The new study also found that white and black children are more likely than Hispanic children to be diagnosed with autism. Again, Bao said there's no clear reason why this might be.
Autism Speaks has more on new diagnostic criteria for autism.
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