New research has found that kids with attention-deficit hyperactivity disorder have several brain regions that are slightly smaller than usual.

Children with attention-deficit hyperactivity disorder have several brain regions that are slightly smaller than usual, more evidence that the disorder should be considered a neurological condition, a new study says.

The study, the largest review of ADHD patients' brain scans ever conducted, might also provide clues for developing new treatments.

"If you know what region of the brain is involved in ADHD, you could possibly target that part with medication," said Martine Hoogman of Radboud University in the Netherlands, the study's lead author.

ADHD causes inattention, hyperactivity and impulsivity, although a given person may not show all those traits.

Hoogman and colleagues analysed MRI scans for more than 3,200 people in nine countries aged four to 63, of whom 1713 who had ADHD. They found that the brains of children with the condition were slightly smaller in five regions, including those that control emotions, voluntary movement and understanding.

The scientists reviewed one scan per person and found no effect from ADHD medications.

Hoogman said the findings support previous theories that the brains of people with ADHD may develop more slowly but that those differences are mostly wiped out by the time children grow up.

"By the time they become adults, the differences in their brains are not significant anymore," she said.

The study was paid for by the National Institutes of Health and was published on Wednesday in the journal, Lancet Psychiatry.

Other experts described the findings as interesting but said there wasn't enough information to link the brain differences to behavioural problems seen in people with ADHD.

"The study confirms that there are structural differences in the brains of people with ADHD, but it doesn't tell us what they mean," said Graham Murray, a lecturer in psychiatry at Cambridge University, who was not part of the research.

"Having less brain in several regions sounds bad but it's not as simple as that," he said, pointing out that decreased brain matter can sometimes be beneficial - like in teenagers, when the outer cortex of their developing brains becomes thinner as their intellectual capacity grows.

"The brain is very good at adapting," Murray said.

"Just because you have less brain volume doesn't condemn the child to not being able to function well."

Jonathan Posner, an associate professor of psychiatry at Columbia University Medical Center in New York, also said the research should help families with children diagnosed with ADHD.

"To have a solid understanding that ADHD really does originate from brain systems and that it causes alterations in the way the brain is structured and functions, is important information for reducing stigma," said Posner, who co-authored an accompanying commentary. "It will hopefully create more empathy for children who have ADHD."