Research has shown that children with attention deficit hyperactivity disorder (ADHD) have smaller brains than average.

As this ScienCentral news video reports, now scientists have looked into whether medication is to blame.

A small but significant difference

Researchers have known that kids with ADHD have brains that are smaller than those of other children. But the details of the size difference were unknown until recently, when Dr. Xavier Castellanos, Director of Pediatric Neuroscience at the NYU Child Study Center, published the results of his 10-year study in the Journal of the American Medical Association.
“Previously most people had assumed that the differences were only going to be found in the frontal part of the brain,” he says. “And in fact we found that there are differences throughout the entire brain.”

The overall difference is about 3 percent on average, a small but significant difference. When looking at more specific parts of the brain, Castellanos saw differences of about 4 percent in the caudate nucleus (which he says regulates attention and activity levels) and 6 percent in the cerebellum (which helps control coordination and fine motor control).

Scientists had debated whether the brains of kids with ADHD were simply smaller from the start or whether they actually shrank as result of something else; indeed, says Castellanos, those opposed to the use of medication wondered if drugs were shrinking the kids’ brains. This study—which was funded by and conducted at the National Institute of Mental Health—used magnetic resonance imaging (MRI) to look at the brains of almost 300 children at different times over a period of ten years. Since even the youngest children in the study (age 5) showed a decreased size from the start, Castellanos is fairly confident that “After birth, the brain is probably developing relatively normally in children with ADHD, but … they may have a slightly smaller brain to begin with.”

To look more specifically at the issue of the effects of medication, Castellanos looked at the 152 kids in the study who had been diagnosed with ADHD, and split them up into those who had been on an ADHD medication, like Ritalin, and those who had not.

“We wanted to make sure that the differences in brain size were not because of medication treatment,” he says. “And that’s what we confirmed.”

In fact, the brains of the children who never used medication had 10 percent less white matter than those who had used medication. White matter is what determines “how quickly a nerve impulse can get from one point to another,” says Castellanos. “And that presumably then has effects on how efficiently and how effectively the brain can operate.”

This finding suggests that medication may even be promoting brain development in kids with ADHD. “But we can’t conclude that that’s the case yet,” Castellanos says. “Because we need to do a different kind of study in order to really be certain that that’s true.”

Beyond that, it’s important to note that brain researchers still don’t understand the significance of the difference in brain size for kids with ADHD. But now scientists can take the specific findings of Castellanos’s study, and design a more detailed study that might help tease that out.

“That’s one of the joys of science,” says Castellanos. “It gives us a clue that we can then use to find other clues.”