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Researchers said the findings may offer new clues about the underlying mechanisms of autism. A larger head size probably means the children also have a larger brain.
Boys with brain and body "overgrowth" tend to have more severe autism symptoms, particularly involving social skills, than autistic children who don't grow faster than normal. So, it's also possible the overgrowth is one of the causes of autism; that it somehow makes symptoms worse or represents a subtype of autism that's marked by both accelerated growth and severe social deficits, said study author Katarzyna Chawarska, an associate professor of child psychology at the Yale University Child Study Center.
Prior research has also found an association between accelerated head growth and autism. This study adds to that by showing that boys with autism have a tendency toward accelerated growth throughout the rest of the body.
"We found these children tend to have accelerated growth patterns in skeletal growth, including length or height, and a little later we see them getting a little heavier, suggesting enhanced muscle growth," Chawarska said.
While the focus of research has been on understanding why the brain grows faster in autistic kids, "now we need to extend our search to other factors that affect multiple morphological [structural] features," she said. "We need to ask why growth factors may be dysregulated in autism. And that's something we have no answer to now."
The study is published in the October issue of the Archives of General Psychiatry.
Autism is a neurodevelopment disorder that's characterized by problems with social interaction, verbal and nonverbal communication and restricted interests and behaviors. An estimated one in 110 U.S. children has the disorder, according to the U.S. Centers for Disease Control and Prevention.
In the study, researchers analyzed pediatricians' medical records on 64 children with autism; 34 boys with pervasive developmental disorder-not otherwise specified (considered an autism spectrum disorder); 13 boys with global development delay, 18 boys with other development problems and 55 typically developing kids.
Though they were of normal size at birth, compared to typically developing kids, boys with autism were longer at about 5 months old, had a larger head circumference at 9.5 months and weighed more by their first birthday.
The growth of the head was not out of proportion to the rest of the body; instead, the whole child grew bigger than normal, according to the study.
None of the children with other types of developmental problems showed similar growth patterns.
Boys who were in the top 10% for overall physical size in infancy had more severe social deficits and were lower functioning, according to the study.
Still, researchers stressed that "overgrowth" should not be used to diagnose autism, since not all kids who are later diagnosed with autism grow more rapidly than normal, and a large head circumference can indicate conditions other than autism.
However, pediatricians should play close attention to kids with an accelerated growth pattern, and perhaps refer them for genetic testing, Chawarska said.
Chawarska and her colleagues plan on conducting a similar study in girls, and investigating the possibility that interfering with overgrowth might help ease autism symptoms.
"That is highly speculative, and at this point we don't have a good model explaining how this might work," she noted.
Geraldine Dawson, chief science officer for Autism Speaks, an advocacy organization, said the study suggests interesting new areas of research, including the role that "neuroinflammatory processes" may play in excessive head growth.
"These findings indicate that autism is not a static condition. Rather, there are dynamic changes in the brain during early postnatal life," Dawson said. She noted that the abnormal head growth typically happens around the time that autism symptoms begin to appear.
"Therefore, there may be a direct connection between the abnormal neural processes that are accounting for abnormal head growth and the emergence of autism symptoms," Dawson said, although what is causing the brain enlargement remains unclear.
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