Children and adolescents with attention deficit hyperactivity disorder (ADHD) have smaller brains than children without the disorder—but medication treatment is not the reason, say researchers from the National Institute of Mental Health (NIMH). Moreover, although the brains of ADHD children were smaller, they developed in the same way as those of patients without ADHD. This suggests that whatever caused the disorder probably occurred before the medication was given; in fact, the researchers add, the medication may actually help to mature the brain.
The researchers used magnetic resonance imaging (MRI) to study 152 boys and girls. Most of the children were scanned at least twice, and some underwent up to four MRIs over a period of 10 years. Fifty of 594 scans had to be discarded because of blurring by motion in the scanner—which is not unexpected in children with ADHD.
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As a group, the children with ADHD had brain volumes that were 3% to 4% smaller in all regions. The more severe the symptoms, the smaller the frontal lobes, temporal gray matter, caudate nucleus, and cerebellum. Although the white matter (fibers that make long-distance connections between brain regions) of children taking medications did not differ from that in the children without ADHD, it was abnormally small in 49 children who had never received medication. The results were similar even when the researchers controlled for the fact that unmedicated children tended to be younger.
White matter, which normally thickens as a child grows, is one gauge of the brain’s maturation. Children with ADHD are often described as less mature than their peers; this may be related to the delays in the maturation of the white matter. Although the study did not show that medication accelerated the growth of white matter, children showed improved behavior during drug therapy.
The fundamental processes of late childhood and adolescence are essentially healthy in the children with ADHD, the researchers say. The symptoms may reflect fixed, earlier neurobiological insults or abnormalities.
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The evidence suggests that ADHD might run in families. The NIMH researchers also speculate that what is now called ADHD may ultimately prove to be a group of disorders with different causes. They caution that MRI is a research tool and that, because of normal genetic variations in brain structure, it cannot be used to diagnose ADHD in any given child.