The brains of children with attention deficit hyperactivity disorder develop more slowly than those of other children but eventually catch up, according to a government study published Monday that suggests ADHD may be a transient condition, at least for some.
Using advanced imaging techniques, scientists found that the cortices of children with ADHD reach peak thickness an average of three years later than children without the disorder.
The cortex is involved in decision-making and supports the ability to focus attention, remember things moment to moment and suppress inappropriate actions -- functions often deficient in children with ADHD.
Dr. Philip Shaw of the National Institute of Mental Health, lead author of the report, published in Proceedings of the National Academy of Sciences, said the results might help explain why many children with ADHD appear to grow out of the disorder and become less impulsive and fidgety as they mature.
Shaw said that although brain development was slower among those with ADHD, it followed a normal pattern, which should reassure parents.
"There has been a debate about whether ADHD is a delay or deviance from normal brain development," he said. "This study comes down strongly in favor of delay."
About 4.4 million school-age children in the U.S. have ADHD, which can lead to poor school performance and behavior problems. Half of children diagnosed with the disorder are treated with stimulants, such as Ritalin, or other medicines.
Researchers used magnetic resonance imaging equipment to scan the brains of 223 children and adolescents with ADHD and 223 youngsters without the disorder. The scans were repeated two, three or four or more times at three-year intervals.
Scientists focused on the cortex, which becomes thicker as the brain builds new connections to process all the things children are learning -- a key milestone in brain development.
They measured cortical thickness at 40,000 points on each scan, creating a detailed map of brain development in the two groups.
In general, they found that the parts of the cortex involved in sensory and motor processing reached peak thickness earlier than the areas responsible for decision-making and other higher-order functions.
In children with ADHD, developmental lags were most pronounced in the prefrontal cortex, which supports attention and working memory, among other things. Half of the cortical points in ADHD children reached peak thickness at an average age of 10.5, contrasted with age 7.5 in children without the disorder.
The primary motor cortex reached peak thickness at age 7.4 in children with ADHD, about five months earlier than in normal children, researchers found. Shaw said it was possible that the early maturation of the primary motor cortex contributed to the fidgety behavior characteristic of ADHD.
Dr. F. Xavier Castellanos of New York University said the research helps explain why children with ADHD often choose younger playmates, and it should reassure parents who are worried about their children fitting in.
"They may be 11 but their brain is 8. They can't act their chronological age," he said. "This lets parents know that having younger playmates is OK and to be expected," said Castellanos, a former National Institute of Mental Health researcher involved in the early stages of the study.
The study, which focused on one aspect of brain development, did not explain why some people continue to experience ADHD symptoms as adults.
Dr. Bradley S. Peterson of Columbia University, who was not connected to the study, said that although the brains of children with ADHD reached the appropriate thickness, there was no way of knowing from the study whether individual cells were normal.
"Billions of cells make up brain tissue, and we cannot measure all the cells and all the connections between the cells," he said. "Subtle deficits could easily remain."
In addition, he said, the study did not examine the process of cortical thinning that takes place in late adolescence -- a second developmental milestone in which unneeded connections are pruned to shape the adult brain.
Government researchers plan to continue tracking some study participants through adulthood, Shaw said.
"We have not captured this later transition," he said. "It is possible some people never quite get there and that is what accounts for the persistent" ADHD.