Scientists are divided on what causes attention deficit hyperactivity disorder, which affects 3 to 5 percent of school-age children. Some say it's a developmental delay, while others argue that the brains of children with ADHD are abnormal from the start. One puzzle: Some, but not all, kids seem to "grow out" of the disorder, which can cause restlessness, inattention, and difficulty focusing. An intriguing new finding by researchers at the National Institute of Mental Health, reported in today's Proceedings of the National Academy of Sciences, suggests that for about half of kids with ADHD, the troubling symptoms they experience in childhood could be a result simply of slower—but otherwise normal—maturation of the brain.
Rapoport cautions that the research is in the early stages. 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.
You've been studying children's brains for almost 20 years to figure out how they develop, and have done MRI brain scans of healthy children, hyperactive children, and some children with mental illnesses like schizophrenia. You say you're very excited about this latest finding. How come?
In our earlier studies, we just looked at brain development, measuring the change in volume in gray matter in children with ADHD and in typically developing children. The results were similar, but they focused only on the size of brain's lobes, which are relatively large. In this latest study, we were able to use new technology and measure the thickness of 40,000 different points in the cortex, each at a different point in the brain. We were then able to follow how that thickness changed over time in 446 children and adolescents [223 with ADHD and 223 controls]. The brain matures from back to front, with very basic things like vision developing first, at the back. When you plot how those brain areas change over time, you end up with a movie that looks like a wave, moving from back to front. We looked at these movies and said, "Oh, my God!" This is what we'd missed in [studying the] lobes, this ripple with a three-year lag. The hyperactivity group was delayed anywhere between two and three years. The lateral prefrontal cortex is in the front of the brain, and that's what you saw developing most slowly in the ADHD kids. What does that area of the brain do?
These are some of the last brain functions to mature. There are some intellectual functions, such as the ability to suppress inappropriate actions and thoughts, focus attention, remember things from moment to moment, and control movement. It also involves the ability to shift flexibly, to adapt how you're thinking about things. It may have to do with overall organization. Deficits in all these functions have been associated with ADHD. What does this mean for children who are diagnosed with ADHD?
For some kids, it looks like they're "growing out of it," which is what the grandmothers of kids with ADHD have been saying all along. Very soft data does suggest that the kids who have later brain maturation are these "good outcome" kids. The other half are different. It makes sense for two reasons. Some of the kids really do grow out of this. That's part of why ADHD is such a controversial diagnosis. Every clinician knows somebody who in third grade was always in the principal's office and always in trouble. The kid ends up going to Harvard. All of our biological studies suggest there are subgroups of kids with ADHD who have different-than-normal processes going on in their brains. This may be the most benign one. With other subgroups, the kids may get better because certain parts of the brain get larger to create an alternative intelligence system. They're not the ones who grow out of it.