A 10-year US study has confirmed that the brains of children and teenagers with Attention Deficit Hyperactivity Disorder (ADHD) are 3% to 4% smaller than those of other children.

However, the scientists, from the National Institute of Mental Health (NINH) found no evidence that medication was the cause.

We still don't understand the essential role of the cerebellum, but this region is clearly affected in children with ADHD

Dr Xavier Castellanos

In fact, ADHD children who had taken no stimulant drugs had the smallest brains of all.

The study found that the brains of ADHD children tended to develop in the same way as those of children without problems.

This suggests that whatever brain changes may be linked to the disorder take place at an earlier stage of development which predates the use of drugs.

Controversial condition

Children with ADHD tend to be over-active, easily distracted and impulsive. They fidget, are unable to sit still, and often talk excessively.

Some scientists believe the condition is widespread, affecting up to 5% of the school-age population. However, sceptics believe the condition is often wrongly applied to children who are simply exhibiting normal boisterous behaviour.

The controversy has been fuelled by the fact that the use of stimulant drugs such as Ritalin to treat the condition is widespread, particularly in the US, where they are given to as many as one in five boys in some school systems.

Some critics are concerned that the use of drugs had directly triggered changes to the structure of children's brains.

But the new research appears to have allayed these fears.

Reassurance

Researcher Dr Xavier Castellanos said: "Our study should provide a certain amount of reassurance that medications aren't reducing brain size in children with ADHD.

"Parents shouldn't be so concerned about the slight difference in brain volume among children with ADHD anyway, since this measurement doesn't have much meaning."

The researchers used MRI scans to compare the brains of 152 ADHD children with those of 139 children who had not been diagnosed with behavioural problems.

Over the course of 10 years, most children were scanned at least twice, and some up to four times.

On average, nearly all parts of the brain of ADHD children were an average of 3% smaller.

Children with the most severe ADHD symptoms were most likely to have the smallest brain structures.

Key brain structure

The most striking size difference was found in an area known as the cerebellum, which was on average 6% smaller in ADHD children.

The cerebellum is known to be involved in motor coordination, but recent studies suggest it may also play a role in controlling the speed at which the brain works.

Dr Castellanos said: "We still don't understand the essential role of the cerebellum, but this region is clearly affected in children with ADHD, and this area may be useful in providing timing information, that is, coordinating signals going from one region of the brain to another."

Professor Julio Licinio, the editor of the journal Molecular Psychiatry, told BBC News Online: "I think this is very significant.

"ADHD treatment is widely available in the United States; therefore, it becomes very hard to separate the effects of treatment from those of the disease itself.

"This study has two great strengths: First it shows a small change in brain volume in ADHD and second, it shows that this is not due to the effects of medications used."

The research is published in the Journal of the American Medical Association.