Boys are four to nine times more likely than girls to be diagnosed with attention-deficit/hyperactivity disorder (ADHD). But in a study involving almost 3,500 children, psychiatry researchers at Washington University School of Medicine in St. Louis found two distinct subtypes of ADHD, and at least one of those subtypes of ADHD affects boys and girls at practically the same rate.
"The hyperactivity of young boys is easy to spot, and a child who is excessively disruptive will come to the attention of a teacher more quickly than a little girl who may be sitting very quietly but having problems with inattention," says Rosalind J. Neuman, Ph.D., research assistant professor of mathematics in psychiatry and lead author of a study published in the Journal of the American Academy of Child and Adolescent Psychiatry.
Neuman and colleagues Andrew C. Heath, D.Phil., Gwendolyn G. Reich, Ph.D., Theodore Reich, M.D., and Richard D. Todd, Ph.D., M.D., found that while young boys are more likely than girls to receive treatment for hyperactivity, simply being female does not reduce the risk of ADHD.
"Inattention problems are just as common in girls as in boys, and just as impairing," says Todd, a co-investigator and the Blanche F. Ittelson Professor and director of the Division of Child Psychiatry at the School of Medicine. "For both sexes, attention problems affect grades, family functioning and functioning with peers."
Neuman, Todd and colleagues examined questionnaires from three groups for the study, which was funded by grants from the National Institutes of Health and the U.S. Public Health Service. They studied more than 2,600 women and girls who were part of a registry of twins born in Missouri between 1968 and 1994. Their parents participated in phone interviews to determine the prevalence of various ADHD symptoms, from fidgeting to difficulty with organization to distractibility.
The researchers also looked for symptoms of ADHD in groups of high-risk children, 430 girls and 425 boys from alcoholic families participating in the six-center Collaborative Study on the Genetics of Alcoholism (COGA).
Psychiatrists and psychologists have traditionally classified ADHD as belonging to one of three types: inattention or distractibility, hyperactivity and impulsivity, and a third form involving problems in both areas. But the children in this study provided the researchers with good evidence for only two of those subtypes.
"We found an inattentive subtype characterized by the inability to concentrate, and another subtype combining both inattention and hyperactivity," Neuman says. "It may be that what we once thought of as the strictly hyperactive and impulsive group is very rare or is really the combined type, and treatment for ADHD would improve both problems. But we found good evidence for only two subtypes."
By including twins in the study, the investigators were able to determine that genes play a role in the risk for ADHD. Identical twins suffered from the same class of symptoms 80 percent of the time. Non-identical, or fraternal twins, only matched up about half of the time.
"It's clear that ADHD runs in families," Todd explains. "That has important treatment implications because other children in a family are at higher risk, and the risk for parents to have similar problems is high. Obviously, if a parent is having a problem with attention, it is more difficult to successfully treat a child."
Todd says the finding that genetic factors influence risk may make it possible to create better treatments in the future. "On the basis of these twin studies, we believe ADHD has a genetic component, and if we could identify genes that predispose kids to ADHD, we might be able to develop better, more specific treatments."
Treatment for ADHD has been controversial. Child psychiatrists and pediatricians prescribe methylphenidate (Ritalin) and other stimulants. Todd agrees with critics that medication by itself is rarely sufficient. Children also need to learn organizational skills, social skills and how to compensate for problems that result from their disorder, but he bristles at the idea that medication is
not necessary. "There's a belief among some people that we're somehow using medication to artificially enhance a child's performance," he says. "We are simply trying to help these kids who have problem areas perform up to their potential, to bring them back to where they can be."
Identifying genetic roots
With this in mind, Neuman and Todd are working to further tease out the genetic
influences on ADHD. Eventually, they hope to identify genes that increase the
risk of developing the disorder, just as other researchers are looking for the
genetic roots of diseases such as high cholesterol, diabetes and hypertension.
Their current study shows that ADHD is similar to those disorders in that many
children suffer from some symptoms, but the severity of symptoms and their
impact can differ greatly among individuals.
"Using high blood pressure as an example, in terms of outcome or risk, a diastolic blood pressure reading of 90, where physicians usually prescribe medication, is not significantly different than a pressure of 88. At some point, however, a physician has to decide who to treat. Ninety is the usually accepted guideline," Todd says. "Our studies and others suggest a similar continuum in ADHD."
Some studies estimate that almost one in 10 school-age children is affected by ADHD. Why they have the disorder, who should be treated and whether girls are under diagnosed are some of the questions the investigators hope to answer as they look at the disorder's genetic and familial influences.
Note: For more information, refer to Neuman, RJ, Todd, RD, et al. "Evaluation of ADHD Typology in Three Contrasting Samples: A Latent Class Approach," Journal of the American Academy of Child & Adolescent Psychiatry, vol. 38 (1), pp. 25-33, January 1999.
Journal
Journal of the American Academy of Child & Adolescent Psychiatry