In a finding that offers the first concrete evidence that hyperactivity is caused by a biological abnormality, scientists have discovered that the brain activity of hyperactive people is quite different from that of those who do not have the disorder.
Using sophisticated imaging technology, researchers at the National Institute of Mental Health in Bethesda found that regions of the brain that regulate attention span and motor activity work less energetically in hyperactive people.
The finding is reported in today's New England Journal of Medicine.
Hyperactivity, technically known as attention deficit-hyperactivity disorder, affects 3 percent to 5 percent of U.S. children younger than 13 -- more than 1 million schoolchildren in all. Such children are restless, have trouble concentrating and often
are disruptive at school and at home.
While it used to be believed that most hyperactive children eventually outgrew the problem, medical experts now say 40 percent to 60 percent of hyperactive children continue to have symptoms in adulthood.
The study looked at the brain activity of 25 hyperactive adults. Each was also the parent of a hyperactive child.
The researchers found that metabolism -- the rate at which cells burn sugar to produce energy -- was 8 percent lower in the brains of the hyperactive patients than in a control group of 50 unaffected adults.
"Hyperactive parents of hyperactive children have low brain metabolism," said Alan Zametkin, the study's lead researcher. In particular, metabolism is low "in those regions of the brain that are important for the control of attention and motor output."
Because metabolism reflects the amount that cells are working, the finding that those parts of the brain were more sluggish in hyperactive people could explain why they are fidgety, easily distracted and susceptible to acting out before they think.
The researchers gauged the activity in the brain by injecting each of the participants with sugar labeled with a radioactive tracer. They then used a sophisticated imaging tool called PET, or positron-emission tomography, to measure the speed at which the sugar was metabolized in the brain.
The researchers found that metabolism was lower among the hyperactive group in 30 of 60 regions of the brain. Metabolism was particularly reduced in the premotor cortex and the superior prefrontal cortex, areas involved in controlling attention and motor activity.
Previous studies looking for a biological cause for hyperactivity focused on blood flow in the brain and the chemistry of nerve transmissions, but those studies were inconclusive.
Mr. Zametkin cautioned that it was too soon to say whether lower brain metabolism was itself to blame for hyperactivity. Instead, he said, the decreased activity could be a flag for something else.
His research team hopes that a study under way with hyperactive teen-agers will yield more insight into its causes.
Although sugar was used to study brain activity, Mr. Zametkin said his work had nothing to do with the question of how eating sweet food might affect hyperactivity. While there has been speculation over the past decade that food additives and candy can trigger hyperactivity, Mr. Zametkin said there was little evidence to support that theory.