Children with the controversial "bad behaviour" syndrome ADHD are suffering from a genuine medical condition linked to abnormal development of the brain, scientists said today.

Brain scans of children diagnosed with attention deficit hyperactivity disorder, which some critics allege has been invented or exaggerated by drug companies seeking lucrative markets, show a common pattern of physical changes.

The condition, which involves restlessness, impulsive behaviour and a short attention span, is thought to affect up to one in 20 British children, with about a quarter of these suffering from a more severe form known as hyperkinetic disorder.

This involves a poor attention span, impulsive behaviour and restlessness even while asleep, impairing normal life both at school and at home. The condition is 2.5 times more common among boys than girls.

Increasing rates of diagnosis have alarmed some researchers and parents' groups, who accuse doctors and pharmaceutical companies of deliberately "medicalising" ordinary disruptive behaviour.

They fear that many children and teenagers are being treated with powerful drugs such as Ritalin that they do not really need, to control essentially normal childhood antics and acting up.

The latest research, however, suggests that these concerns are misplaced, and that if anything too few children with ADHD are getting the help they need, according to Professor Eric Taylor of the Institute of Psychiatry in London.

His review of 17 brain scan studies involving a total of about 350 children has identified common features that appear again and again in those with the disorder.

The regions involved are those affecting self-control and inhibiting impulsive behaviour - the right frontal lobes, the basal ganglia and the vermis of the cerebellum, which are all appreciably smaller in ADHD patients.

The scientists followed up several hundred boys diagnosed with ADHD at the age of seven, and found they were four times more likely than normal to have mental health problems such as bipolar disorder and personality disorders when they reached adulthood. The risks of later mental illness were higher if the child was not treated with drugs.

The researchers found that ADHD's origins were 80 per cent genetic, and it was not the result of poor parenting. Growing up in a "chaotic environment", to which parents with poor skills can contribute, may however trigger the condition among children who are already genetically susceptible.

"This work shows that there is a distinction between ordinary bad behaviour and ADHD," Professor Taylor said.

"Parents and families are right in saying that it starts with a physical influence, and it should be thought of as a long-term subtle disability. A proportion of the population are suffering from a real but somewhat invisible disability."

Up to 6 per cent of American children are now given drugs such as Ritalin to control ADHD, but there is no evidence for similar over-prescribing in Britain.

"It can be a severe problem that needs medication, but only about a third of those who could benefit are getting that in Britain," said Professor Taylor.

"I certainly don't think it's purely an effect of drug firms, though they might have contributed to the problem in America."

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