|Brain Matures a Few Years Late in ADHD, But
Follows Normal Pattern
In youth with attention deficit hyperactivity disorder (ADHD)
the brain matures in a normal pattern but is delayed three years
in some regions, on average, compared to youth without the disorder,
an imaging study by researchers at the National Institutes of Healthís
(NIH) National Institute of Mental Health (NIMH) has revealed.
The delay in ADHD was most prominent in regions at the front of
the brainís outer mantle (cortex), important for the ability to
control thinking, attention and planning. Otherwise, both groups
showed a similar back-to-front wave of brain maturation with different
areas peaking in thickness at different times (see
"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," explained Philip Shaw, M.D., NIMH Child Psychiatry
Branch, who led research team.
Previous brain imaging studies failed to detect the developmental
lag because they focused on the size of the relatively large lobes
of the brain. The sharp differences emerged only after a new image
analysis technique allowed the researchers to pinpoint the thickening
and thinning of thousands of cortex sites in hundreds of children
and teens, with and without the disorder.
"If youíre just looking at the lobes, you have only four
measures instead of 40,000," explained Shaw. "You donít
pick up the focal, regional changes where this delay is most marked."
Among 223 youth with ADHD, half of 40,000 cortex sites attained
peak thickness at an average age of 10.5, compared to age 7.5 in
a matched group of youth without the disorder.
Shaw, Judith Rapoport, M.D., of the NIMH Child Psychiatry Branch,
Alan Evans, M.D., of McGill University, and colleagues report on
their magnetic resonance imaging (MRI) study during the week of
November 12, 2007, in the online edition of the Proceedings
of the National Academy of Sciences.
The researchers scanned most of the 446 participants — ranging
from preschoolers to young adults — at least twice at about
three-year intervals. They focused on the age when cortex thickening
during childhood gives way to thinning following puberty, as unused
neural connections are pruned for optimal efficiency (http://www.nimh.nih.gov/science-news/2004/imaging-study-shows-brain-maturing.shtml)
during the teen years.
In both ADHD and control groups, 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 — functions
often disturbed in people with ADHD.
Circuitry in the frontal and temporal (at the side of the brain)
areas that integrate information from the sensory areas with the
higher-order functions showed the greatest maturational delay in
youth with ADHD. For example, one of the last areas to mature,
the middle of the prefrontal cortex, lagged five years in those
with the disorder.
The motor cortex emerged as the only area that matured faster
than normal in the youth with ADHD, in contrast to the late-maturing
frontal cortex areas that direct it. This mismatch might account
for the restlessness and fidgety symptoms common among those with
the disorder, the researchers suggested.
They also noted 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. In future studies, the researchers hope to
find genetic underpinnings of the delay and ways of boosting processes
of recovery from the disorder.
"Brain imaging is still not ready for use as a diagnostic
tool in ADHD," noted Shaw. "Although the delay in cortex
development was marked, it could only be detected when a very large
number of children with the disorder were included. It is not yet
possible to detect such delay from the brain scans of just one
individual. The diagnosis of ADHD remains clinical, based on taking
a history from the child, the family and teachers."
Also participating in the research were: Kristen Eskstrand, Wendy
Sharp, Jonathan Blumenthal, Dede Greenstein, Liv Clasen, and Jay
Giedd, M.D., NIMH.
Maturation of the brain, as reflected in the age at
which a cortex area attains peak thickness, in ADHD (above)
and normal development (below). Lighter areas are thinner,
darker areas thicker. Light blue in the ADHD sequence corresponds
to the same thickness as light purple in the normal development
sequence. The darkest areas in the lower part of the brain,
which are not associated with ADHD, had either already
peaked in thickness by the start of the study, or, for
statistical reasons, were not amenable to defining an age
of peak cortex thickness. Movie of same data below.
Source: NIMH Child Psychiatry Branch
view this movie you will need Quicktime. To download Quicktime,
go to this URL: http://www.apple.com/quicktime/download/win.html
The National Institute of Mental Health (NIMH) mission is to reduce
the burden of mental and behavioral disorders through research
on mind, brain, and behavior. More information is available at
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