CHICAGO: Children with Attention Deficit Hyperactivity Disorder (ADHD) have delays in their development of about three years, U.S. researchers said.

Study’s lead author, Dr. Philip Shaw National Institutes of Health’s National Institute of Mental Health, said, “The sequence in which different parts of the brain matured in the kids with ADHD was exactly the same as in healthy kids. It’s just that everything was delayed by a couple of years.”

Most delays occur in brain’s region significant for controlling thought, attention and planning.

In study, scientists looked at 446 children (223 of these had ADHD), and found average delay of three years in development of cortex, significant in attention and planning.

Among 223 children with ADHD, half of 40,000 cortex sites examined reached peak thickness at 10.5, compared to age 7.5 in matched group without disorder. However, despite the delay, the brain follows a normal pattern of development.

Scientists used Magnetic Resonance Imaging (MRI) scans to examine brain structure at different ages, measuring the thickness of developing cortex.

Nearly 2 million US children suffer from ADHD.

Shaw said, “Finding a normal pattern of cortex maturation, albeit delayed, in children with ADHD should be reassuring to families and could help to explain why many youth eventually seem to grow out of the disorder.”

Studies in future will look into the cause of delay and probe ways of promoting recovery.

But, UK scientists warned that no findings show that children with ADHD “follow” after three year delay as brains of normal children.

Dr David Coghill of the University of Dundee, said, “During these later stages of development the cortex of the brain gets thinner due to a process called pruning which occurs as the brain refines its connections and becomes more organized, so, what will be happening is that whilst the children with ADHD's brains are still growing, the brains of the children without ADHD are starting this process of pruning.”

He added, meant “adolescents with ADHD remain behind on many of the important skills like memory, impulse control and planning.”

Professor Anita Thapar of Cardiff University School of Medicine said the study results were “interesting and useful, but it will be a long time before we see the implications of their findings for clinical practice.”

Treatment includes drugs like Ritalin, or methylphenidate, a stimulant intended to lower impulsiveness and hyperactivity and boost attention.