Although children with ADHD have slightly smaller brains than children without the disorder, medications used to treat ADHD aren't causing this apparent difference in brain size and do not appear to be affecting normal brain development, according to the largest brain imaging study yet conducted on children with the disorder.
The new study confirms previous findings showing that the brains of children with ADHD tend to be smaller than the brains of children without the disorder, and it reveals for the first time that stimulant medications, such as Ritalin, aren't affecting brain size, says F. Xavier Castellanos, M.D., the Brooke and Daniel Neidich Professor of Child and Adolescent Psychiatry and Professor of Radiology at New York University School of Medicine, who led the new study.
"Our study should provide a certain amount of reassurance that medications aren't reducing brain size in children with ADHD," says Dr. Castellanos, who is also Director of the new Institute for Pediatric Neuroscience at the NYU Child Study Center at NYU School of Medicine. "Parents shouldn't be so concerned about the slight difference in brain volume among children with ADHD anyway, since this measurement doesn't have much meaning, " he says.
The study also shows that children with ADHD undergo normal brain development, although the data aren't considered definitive. Brain development appeared normal and healthy among all the children who were studied over the 10-year period of the study.
The study is published in the October issue of the Journal of the American Medical Association.
The researchers used MRI (magnetic resonance imaging) to study brain volume, comparing children with ADHD and those without the disorder. All the children had follow-up scans over a 10-year period. The study, which was conducted at the National Institute of Mental Health, a branch of the National Institutes of Health, also provided an analysis of the regions of the brain that tend to be smaller in children with ADHD.
Overall, it found that nearly all parts of the brain in children with ADHD were an average of 3 percent smaller, but brain size varied considerably, says Dr. Castellanos. For example, brain volumes were larger in some children with the disorder compared to those without the disorder. The cerebellum, however, a region in the back of the brain, was 6 percent smaller in the affected children compared to those without the disorder, and this average difference was consistently observed in the ADHD children.
It has long been known that the cerebellum is involved with motor coordination, but more recent studies indicate that the region may influence many activities, and may even be the brain's so-called metronome. "We still don't understand the essential role of the cerebellum, but this region is clearly affected in children with ADHD, and this area may be useful in providing timing information, that is, coordinating signals going from one region of the brain to another," says Dr. Castellanos.
ADHD is the most common mental disorder in children, affecting 3 percent to 5 percent of school-aged children, and is more common in boys than in girls, according to the National Institutes of Mental Health. Children with the disorder typically fidget, and are unable to sit still and pay attention in class. They are easily distracted, have trouble playing quietly, and may talk excessively, among other symptoms.
Healthcare practitioners typically diagnose the disorder by observing a child's behavior and by taking a family history. There are no independent tests to diagnose it, and researchers had hoped that MRI could provide diagnostic criteria. While the brain scans show a slight difference in brain volume, the difference was so slight that it could not be used to make a diagnosis. "Anatomic MRI studies remain appropriate only for research, as they cannot yet contribute to the diagnostic assessment of ADHD," the study says.
The study compared 152 children and adolescents with ADHD and 139 children without the disorder, matched for sex and age. Most of the children with ADHD were medicated, but 49 of the children had never been treated. The brains of the unmedicated group did not differ significantly from the medicated group, except for the white matter, which makes up about 50% of total brain tissue. The white matter was smaller in the unmedicated group.
"In fact, findings were generally as striking for the unmedicated patients with ADHD as for those who were being treated with medications, and were more pronounced for white matter volumes," the study says. "Thus, our analyses show that decreased brain volumes in ADHD in both white and gray matter compartments are not due to drug treatment. Conversely, we have no evidence that stimulant drugs cause abnormal brain development," it says.
The study was funded by the National Institute of Mental Health (NIMH). Dr. Castellanos was previously head of ADHD research at NIMH and is continuing his pioneering imaging studies at NYU Child Study Center. In one project, he will follow preschoolers with ADHD who take Ritalin in order to assess the effects of the drug on the developing brain. A second study, funded by NIMH, will use functional MRI to look at the working brains of adolescents with ADHD, part of a multidisciplinary study involving researchers throughout NYU.