Study Shows Boys With ADHD Have Smaller Volume in Frontal Lobes of Brain

July 23, 2003
Abnormality likely related to key behavior problems associated with the disorder

Baltimore - Scientists at Kennedy Krieger Institute have confirmed that children with attention deficit hyperactivity disorder (ADHD) have smaller frontal lobes of the brain, a region that another Kennedy Krieger study has shown controls a key behavior, called "response inhibition," which generally is lacking in individuals with ADHD.

Using anatomic magnetic resonance imaging, the study showed that the volume of the frontal lobes in boys with ADHD was an average 11 percent smaller than that in a control group of boys. In addition, significant reductions in volume were found in specific regions within the frontal lobes - the prefrontal, premotor and deep white matter regions - suggesting that the abnormalities are not localized, but may be distributed among several frontal lobe neurocircuits.

Response inhibition, the suppression of responses that are inappropriate or unwanted, is central to the difficulties faced by individuals with ADHD, contributing to the impulsiveness, hyperactivity, inattentiveness and other problems that are characteristic of the disorder. Scientists are now testing the theory that these characteristics are the result of the differences in the frontal lobe areas of the brain.

Previous anatomic imaging studies, which used crude methods to divide the brain into a grid pattern, have reported general differences in volumes in brains of individuals with ADHD. This is the first study to conclusively measure differences in volumes of specific regions of the brain known for specific functions.

The precise measurement of individual brain regions means scientists can more accurately interpret the implications of the abnormalities, and better define and subtype the condition, which will lead to better diagnosis and treatment interventions.

"Because we were able to actually isolate regions of the brain that are known for specific functions, such as motor control and response inhibition, we are in a much better position to interpret the functional relevance of the findings," said Stewart Mostofsky, M.D., a research scientist in the Institute's Department of Developmental Cognitive Neurology and principal investigator of the studies.

"This is going to lead to a greater understanding of what frontal circuits are involved in ADHD. Understanding how each circuit is related to behavioral and cognitive functions of individuals with ADHD will help us better understand, subtype and define treatment for the disorder," he said.

The study, published last October in the journal Biological Psychiatry, involved 12 boys with ADHD and 12 controls.

A second Kennedy Krieger study, published last Friday in the journal Cognitive Brain Research, employs functional magnetic resonance imaging technology to investigate the neurologic basis of response inhibition, which is an important factor contributing to the behavioral, cognitive and motor problems associated with ADHD. Preliminary findings of this study involving adults suggest that the supplementary motor area, which is within the premotor region of the frontal lobe, seems to be critical in selecting the motor response, which includes response inhibition.

But when applied to a new study involving children with ADHD, scientists already have noticed some interesting differences.

"Adults recruit the supplementary motor area to a greater extent than children do, suggesting that they are utilizing these regions to a greater extent in response inhibition," Dr. Mostofsky said. "Children seem to use a more distributed circuit.

"These studies have real potential to allow us to understand and subtype ADHD based on more biologically meaningful variables. Right now, the condition is subtyped based on observations of children's behavior. If we can detect dysfunction in specific neurocircuits, then we will be better able to define effective treatments."

Magnetic resonance imaging allows scientists to view the most intricate structures and the subtlest details of the functioning brain. The technology displays everything from how the brain is wired to the chemicals it uses and produces. The magnet reveals important regions in minute detail and even discerns the speed and location of a thought process.

Kennedy Krieger Institute is dedicated to helping children and adolescents with disabilities resulting from disorders of the brain achieve their potential and participate as fully as possible in family, community and school life. For more information about Kennedy Krieger Institute, visit www.kennedykrieger.org.

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