A study, published in the online edition of the Proceedings of the National Academy of Sciences, seems to have cracked the mystery. It says some brain regions in children with ADHD tend to mature later than normal.
Scientists hadn't realized this before because they usually rely on MRI scans of the brain that are like single snapshots — they only capture what's happening at a particular moment in time.
But for this study, a team at the National Institute of Mental Health used multiple snapshots of a child's brain taken at regular intervals as he or she grew up. They assembled these snapshots into time-lapse movies.
'You can see a wave of brain maturation going from this middle line,' says lead author Philip Shaw, pointing to an animated brain image on his laptop. 'The last bit to mature are these bits in the middle, which are important for the control of action and attention.'
Action and attention are key functions that are disrupted in children with ADHD.
Shaw's team studied more than 400 children — half of them with ADHD. The team measured changes in the thickness of the brain's gray matter, or cortex. Shaw says that for all the kids, the cortex followed the same sequence of change.
'It starts off relatively thin in early childhood,' he says. 'It gets thicker, then reaches its peak cortical thickness, and then it gets thinner throughout adolescence.'
But Shaw says that when they compared the time-lapse images from the two groups, the timing of these changes was different.
'The big finding was the difference in the age at which they reached this milestone of peak cortical thickness,' he says. 'The healthy kids had a peak at around age 7 or 8, the kids with ADHD a couple of years later around the age of 10.'
The delay in this developmental milestone was most apparent in the area of the brain that controls action and attention.
Judith Rapoport, another scientist involved in the study, says the scans showed that the brains of many of the kids with ADHD did eventually catch up.
'It's long been known that some kids, even if they seem very hyperactive and are a lot of trouble in early grades, seem to grow out of it and do extremely well, as their grandmother said all along they would,' she says.
The delays, which put areas of the brain used in higher order decision making behind schedule by an average of three years, were most evident in areas at the front of the brain's outer covering, or cortex, that house the ability to control thinking, attention, and planning. In some of the children and teens with ADHD, the brain regions reached peak thickness when the children averaged 10.5 years old, compared with 7.5 in children without ADHD, she revealed.
But Rapoport says it's not clear from the scans why some kids with ADHD continue to have problems. Researchers suspect it's because their brains remain immature.
Rapoport says teachers, doctors and investigators could do more for these children if they were able to identify them at the earliest possible age.
'If we could know earlier on in the game which are the children who are going to grow out of it one could reassure the parents,' she says. 'We could take all those resources that are being wasted, apply them to the ones that are not going to do as well.'
The clues may come from studying the brains of infants — something this study didn't do. Nearly all of the children in the study were at least 5 when they had their first MRI. Younger kids are hard to diagnose and less likely to lie still for a brain scan.
Stewart Mostofsky of Kennedy Krieger Institute in Baltimore says researchers need to find a way to look at younger brains.
'The real important question is what is happening earlier in brain development and brain maturation that is contributing to the onset of the disorder,' he says.
Mostofsky says that may involve events that occur before birth.
Still Rapoport cautions that the research is in the early stages. More research is certainly called for, but what has been unlocked now holds interesting possibilities.