Brain Size, Early Growth: Clues to Autism’s Causes
Tuesday, May 3, 2011
There’s been a lot of news recently about efforts to detect signs of autism in children earlier — even before age 2, which is when doctors typically make the first diagnosis based on toddlers’ behavior and development. Now a new study sheds light on another key issue — why autistic children tend to develop larger brains than those without the condition.
The brain changes in autism have been increasingly documented in studies, but the phenomenon is still largely a mystery. Does having autism make children’s brains grow more quickly than those of unaffected children? Does the enlarged size precede the hallmark changes in behavior, personality and learning? And how do their brains become so big? Where is the additional volume coming from?
Heather Hazlett, in the department of psychiatry at the Carolina Institute for Developmental Disabilities, and her colleagues studied MRI images of 38 children diagnosed with autism spectrum disorders (ASD) at 2 years old and compared them with the scans from 21 unaffected youngsters of the same age. All the children were scanned again at age 4 or 5, and at all stages, the children with ASD had on average 6% more total brain volume and 9% more volume in the cerebral cortex, the region of the brain that contains the “newest” sprouting of neurons and is responsible for everything from receiving signals and input from the environment to processing memory and attention.
The findings suggest that although autistic brains are bigger, their rate of growth is relatively normal, at least after age 2. Some time before 2, however, autistic children may experience a spurt in brain growth that is strongly linked to their later behavioral and developmental symptoms.
The group also found that the children with autism had more surface area in their cortex, in the form of greater convolutions and in-foldings of the tissue. That suggests that new nerve cells are being produced and pushed to the cortex surface, something that autism researchers had not known before.
“This study strongly suggests that much of the enlargement in the brains of children with autism takes place before the age of two,” says Dr. Joseph Piven, a psychiatrist and director of the Carolina Institute for Developmental Disabilities and senior author of the study. “And the results suggest a mechanism not thought of before in autism, that there is increased production of neurons in the [cerebral cortex] of the brain.”
Exactly what that means isn’t clear yet, but the results open a new window into understanding the critical early phases of the disorder, when, scientists hope, doctors may be able to intervene and perhaps even reverse the abnormal course of development that leads to autism’s social and behavioral symptoms. There is evidence, for example, that children with autism have larger head circumferences than those without the disorder, and if, as Hazlett and Pivens study shows, the larger size is due to growth in cortex volume, then focusing on ways to control that growth well before the first behavioral symptoms appear may be helpful.
Piven, who led the previous study on head circumference, says that the changes in head size start to appear when infants are about 6 months old, which suggests that the internal changes responsible for that expansion may be occurring around the same time as well. He is currently studying children who haven’t been diagnosed yet but have older siblings with autism and are, therefore, at high risk of developing the disorder, beginning at 6 months.
“We have a large study involving infants from around the country and we are conducting behavioral tests and brain imaging at six, 12 and 24 months to start narrowing down the interval where we can see changes,” he says. “Are there certain circuits or tracts in the brain that show abnormalities early on that can help us predict later behavioral changes? Because of findings like our current study, we are focusing much more intently on the six-to-24-month-old period.”
The earlier researchers can detect autism, the more effective any interventions designed to treat symptoms, or possibly even reverse them, can be. And because autism’s symptoms can range from relatively mild to more debilitating, for some, such early detection can make the difference between being able to function normally in school or at a job or needing institutional care.
“We all think we could eventually intervene by changing the environment or direct treatments aimed at particular behaviors and ultimately change the brain [from its abnormal developmental path],” says Piven. His findings strongly suggest there may be a biological reality behind that hope.