ADHD Brain Delay

The research is part of a large study of kids with and without ADHD from age five through young adulthood. This analysis included more than 800 MRI scans of 450 kids, half of whom had ADHD.

"One of the strengths of it is that it's an ongoing study so we get a chance to scan children repeatedly as they grow up," Shaw says. The kids in the study group are now at ages 17 to 18.

The researchers used advanced software to measure the thickness of cortex, or grey matter, of kids' brains and compare them over time. Previous NIMH research had established how the cortex thickens and thins during childhood and adolescence.

"It starts off relatively thin, it gets thicker and it reaches a peak thickness before getting thinner. This peak thickness is sort of like the first milestone, or one of the first milestones of brain development," says Shaw.

He says past studies of ADHD brain size and development used much cruder measures.

"Previously, most studies have looked at change in the entire lobe– there are only four lobes," he explains. "So before, you were looking at change in four big measures, and now we're looking at change at 40,000 points across the brain. So we're now able, due to advances in technology, to pick this up at a much finer brain level. So the other stuff isn't wrong, it's just now we can look at a much more detailed level."

Shaw explains MRI scan.

The researchers statistically matched the children in each group for factors like intelligence. In another earlier study, they had found that children with very high IQs have a different rate of brain development. "It was important to make sure that the kids with ADHD had the same IQ as the healthy kids we were comparing them with, to make sure that the differences we find don't just reflect differences in intelligence, they're actually something to do with the disorder of ADHD," says Shaw.

The ADHD delay was longest in specific brain areas. "The delay that we find in the ADHD kids overall was about three years, but it was much more marked in certain areas at the front of the brain which are important for the control of action and attention. Here the delay could be as much as five years," he says.

Shaw says the finding might explain why many kids outgrow the disorder. The next step is to look at why some don't.

"We're very interested in recovery, the mechanisms of getting over ADHD," he says. "Most kids with ADHD– probably as many as three quarters– grow out of the disorder by the time they're in early adulthood. So what we're looking at is, is this something to do with the degree of delay in these kids that get better? Are they a bit less delayed than kids who have very severe ADHD which persists? And if this is the case, is there something we can do to help kids who've got a very persistent form of ADHD– something we can do to help them catch up? "

This study was published in early edition the week of November 12-16, 2007 and funded by the National Institute of Mental Health of the National Institutes of Health.