ADHD a result of delayed brain development

Scientists in the United States have discovered that youngsters with attention deficit hyperactivity disorder (ADHD) have a delay in the development of some parts of their brains.

They are suggesting that some regions of the brain reach maturity as much as three years later in children with ADHD even though there remains a normal a pattern of development.

The scientists at the National Institutes of Health (NIH) compared the brain scans of 446 children ranging from pre-schoolers to young adults.

Of the group 223 had been diagnosed with ADHD.

Magnetic resonance imaging (MRI) scans of the brain were carried out twice at around three-year intervals.

The researchers found that the delay in ADHD was most prominent in regions at the front of the brain’s outer mantle (cortex), which is important for the ability to control thinking, attention and planning.

Other than this both groups showed a similar back-to-front wave of brain maturation with different areas peaking in thickness at different times.

The imaging study revealed that in youngsters with ADHD, the brain matures in a normal pattern but is delayed on average three years in some regions.

Dr. Philip Shaw who led the research says that finding a normal pattern of cortex maturation, even though it is delayed, in children with ADHD should reassure parents and may also explain why many youngsters eventually appear to grow out of the disorder.

Dr. Shaw and colleagues at the National Institute of Mental Health (NIMH) Child Psychiatry Branch, were able to detect the thickening and thinning of thousands of cortex sites by using a new image analysis technique which picks up the focal and regional changes where the delay is most marked.

Previous brain imaging studies have failed to detect the developmental delay because they focused on the large lobes of the brain.

Of the 223 with ADHD, half of 40,000 cortex sites attained peak thickness at an average age of 10.5, compared to age 7.5 in those without the disorder.

The scans focused on the age when cortex thickening during childhood thins prior to puberty and unused neural connections are then pruned to provide optimal efficiency during the teenage years.

In both groups the sensory processing and motor control areas at the back and top of the brain peaked in thickness earlier in childhood, while the frontal cortex areas responsible for higher-order executive control functions peaked later, during the teen years.

These frontal areas support the ability to suppress inappropriate actions and thoughts, focus attention, remember things from moment to moment, work for reward, and control movement, all functions which are often disturbed in people with ADHD.

Circuitry in the frontal and temporal areas at the side of the brain that integrate information from the sensory areas with the higher-order functions showed the greatest maturational delay in those with ADHD and one of the last areas to mature, the middle of the prefrontal cortex, was delayed by five years in those with the disorder.

The motor cortex emerged as the only area that matured faster than normal in the youngsters with ADHD, in contrast to the late-maturing frontal cortex areas that direct it.

This mismatch say the scientists might explain the restlessness and fidgety symptoms common among those with the disorder.

They also say that the delayed pattern of maturation observed in ADHD is the opposite of that seen in other developmental brain disorders like autism, in which the volume of brain structures peak at a much earlier-than-normal age.

The findings support the theory that ADHD results from a delay in cortex maturation and in future the researchers hope to find genetic explanations for the delay and ways of boosting processes of recovery from the disorder.

Dr. Shaw says however that brain imaging is still not ready for use as a diagnostic tool in ADHD and the diagnosis of ADHD remains clinical, based on taking a history from the child, the family and teachers.

The research is published in the current online edition of the Proceedings of the National Academy of Sciences.