Children with autism show key "patches of disorganization" in the outer layers of the brain, according to a new study said to offer more evidence that the developmental disorder begins in the womb.
Experts have long believed autism involves disruptions in typical brain development, going back to pregnancy. The new study, reported online March 27 in the New England Journal of Medicine, offers more direct evidence of such early origins.
For the study, researchers examined samples of brain tissue from 22 children after death -- 11 with autism and 11 without. They were able to spot tiny patches of disrupted development dotting the outer layers of the brain in the children with autism.
Differences like that would take shape during prenatal development, said Ed Lein, a researcher at Seattle's Allen Institute for Brain Science, who worked on the study.
"This is pretty direct evidence of a prenatal origin," Lein said.
An autism researcher who reviewed the study agreed. "The foundation for this would likely be prenatal," said Dr. Walter Kaufmann, a neurologist at Boston Children's Hospital. "How early in the prenatal period? That's hard to say."
An even bigger question is, What causes the early disruptions in brain development? Lein and Kaufmann said it's impossible to pin down.
"We still need to try to understand that," Lein said.
In general, however, experts believe autism arises from genetic susceptibility and yet unknown environmental factors. "Ultimately, it's an interplay between genes and environment," Kaufmann said.
In the United States, an estimated one in 88 children has an autism spectrum disorder, which affects the ability to communicate and interact with others. Some kids are profoundly affected, speaking very little or not at all and focusing obsessively on just a few interests. Others have milder problems communicating and reading social cues, such as other people's gestures and facial expressions.
Researchers have managed to find a few hundred genes that are linked to autism risk. And although there is no definite environmental culprit, studies have tied certain factors during pregnancy to an increased risk, including exposure to high levels of air pollution, low intake of the B vitamin folate and viral infections.
For the new study, Lein and his colleagues examined small samples of the neocortex -- the outer surface of the brain. During fetal development, the neocortex forms six layers, each with its own specialized brain cells. As those cells develop, they take on a "genetic signature" that can be visualized in tissue samples, using sophisticated techniques.
Overall, the study found, brain tissue from children with autism showed tiny patches where certain genetic signatures were absent from brain cells.
What's more, those patches were concentrated in areas associated with higher order brain functions, such as understanding language and social cues.
"That makes sense," Kaufmann said. "Those are the areas where you would expect to find abnormalities."
The phenomenon, he said, was seen in 10 of the 11 autistic children, even though the severity of their symptoms varied. Some, for example, had been diagnosed with intellectual disability, while others had not.
Lein said the fact that the brain tissue showed small patches of disruption, rather than pervasive abnormalities, is "potentially good news." It suggests that much of the neocortex is actually typical in children with autism, he said.
That might help explain why autistic toddlers who get early behavioral therapy often show significant improvements, Lein said. It's possible the brain is able to "rewire," to an extent, to get around some of the trouble spots seen in this study.
In general, experts say the earlier such therapy starts, the better. The problem is, most children are not diagnosed with autism until after they reach age 4, according to the U.S. Centers for Disease Control and Prevention.
Kaufmann said researchers are working on finding objective "biomarkers," such as proteins in the blood, that could be used to detect autism earlier. But any such tests are a long way off, he said.
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