Zametkin's Research on ADHD

In its search for a physical cause, the ADHD movement reached a milestone with the publication in the New England Journal of Medicine of a study by Alan Zametkin and his colleagues at the National Institute of Mental Health. This study appeared to link hyperactivity in adults with reduced metabolism of glucose (a prime energy source) in the premotor cortex and the superior prefrontal cortex — areas of the brain involved in the control of attention, planning, and motor activity. In other words, these areas of the brain were not working as hard as they should have been.

PET scans of glucose metabolism in the brains of a normal adult (left) compared to an adult diagnosed with ADHD (right).

The PET scan study found that global cerebral glucose metabolism was 8.1% lower in medication-naive adults who had been diagnosed as ADHD while children. The image on the left illustrates glucose metabolism in the brain of a 'normal' adult while doing an assigned auditory attention task; the image on the right illustrates the areas of activity in the brain of an adult who had been diagnosed with ADHD as a child when given that same task.

Dr. Thomas Armstrong continues the story: "The media picked up on Zametkin's research and reported it nationally. ADHD proponents latched on to this as 'proof' of the medical basis for ADD. Pictures depicting the spread of glucose through a 'normal' brain compared to a 'hyperactive' brain began showing up in CH.A.D.D. (Children and Adults with Attention Deficit Disorder) literature and at the organization's conventions and meetings. One ADHD advocate seemed to speak for many in the ADHD movement when she wrote: 'In November 1990, parents of children with ADHD heaved a collective sigh of relief when Dr. Alan Zametkin released a report that hyperactivity results from an insufficient rate of glucose metabolism in the brain. Finally, commented a supporter, we have an answer to skeptics who pass this off as bratty behavior caused by poor parenting.'

"What was not reported by the media…was the study by Zametkin and others that came out three years later in the Archives of General Psychiatry. In an attempt to repeat the 1990 study with adolescents, the researchers found no significant differences between the brains of so-called hyperactive subjects and those of so-called normal subjects. And in retrospect, the results of the first study didn't look good either. When the original 1990 study was controlled for sex (there were more men in the hyperactive group than in the control group), there was no significant difference between groups."

A recent critique of Zametkin's research by faculty members of the University of Nebraska also pointed out that the study did not make clear whether the lower glucose rates found in "hyperactive brains" were a cause or a result of attention problems. The critics pointed out that if subjects were startled and then had their levels of adrenaline monitored, adrenaline levels would probably be quite high. We would not say, however, that these individuals had an adrenaline disorder. Rather, we'd look at the underlying conditions that led to abnormal adrenaline levels. Similarly, even if biochemical differences did exist in the so-called hyperactive brain, we ought to be looking at the non-biological factors that could account for some of these differences, including stress, learning style, and temperament.