Autism’s relationship to head size, explained – AutismSTEP
Some people with autism have an unusually large head: This fact has been known since autism was first described in the 1940s. But debate about this finding has raged ever since. How many people with autism have a large head? What causes the enlargement? And does it have any bearing on outcome?
Here is what researchers do and do not know about head size in autism.
What proportion of people with autism have a large head?
When Leo Kanner first described 11 children with autism in a 1943 paper, he noted many unusual features. “Five had relatively large heads,” he reported, and he said no more on the matter. But the sample size was small.
In 2011, the Autism Phenome Project refined this estimate to 15 percent of autistic boys2. The team followed boys with autism from their diagnosis throughout childhood. They focused on whether head size is disproportionate to the rest of the body, rather than simply large. The researchers call this ‘disproportionate megalencephaly’ and say it marks a distinct subgroup of autistic people. “We’ve defined a big-brain form of autism,” says lead investigator David Amaral, distinguished professor of psychiatry and behavioral sciences at the University of California, Davis MIND Institute.
No one contests the 15 percent figure, but scientists differ in their interpretation of the finding.
“It only applies to a small proportion of children with autism,” says Katarzyna Chawarska, Emily Fraser Beede Professor of Child Psychiatry at Yale University.
Neuroscientist Eric Courchesne of the University of California, San Diego argues instead that unusually large brains are a near-universal feature in autistic people, and the 15 percent represent the most extreme cases of this trait. He points to a 2015 review of more than 8,000 people, which concluded that head size is larger in autistic people than in their typical peers3.
What about girls on the spectrum?
Multiple studies have suggested that large brains are much rarer among girls with autism than among boys with the condition.
“We found overgrowth in terms of head [size] in boys, but definitely not in girls,” Chawarska says.
Earlier studies did not include many girls, so when Amaral and his colleagues set out to study the phenomenon, they preferentially recruited autistic girls. “Even though we have a much larger number now, we still don’t see it in the same frequency with girls as we do with boys,” says Amaral. “It’s still very, very rare.”
The reason for this sex difference is unclear, but autism is thought to affect girls differently than boys, with girls being somehow protected from the condition.
Do autistic children who have a large head also have a large brain?
Yes. Researchers have scanned the brains of autistic people by using technologies such as magnetic resonance imaging (MRI) and have found that those with a large head also tend to have an unusually large brain. However, the link between the two is not entirely straightforward — some autistic children with an enlarged brain don’t have a large head — so it is best for researchers to scan the brain rather than rely on head measurements.
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