News from SNM: Controversy surrounds PET diagnosis of ADHD
When nuclear imagers reported two years ago that adults with attention-deficit/hyperactivity disorder (ADHD) had a greater than usual concentration of dopamine transporters (DAT), it painted a tidy picture. The high concentration helped explain dopamine's link to ADHD as well as the effectiveness of psychostimulant drugs such as Ritalin in treating the condition.
That assumption was called into question at this year's Society of Nuclear Medicine meeting, however, when researchers at the Brookhaven National Laboratory in Upton, NY, were unable to replicate previous findings. They cited several possible reasons for the discrepancy: differences in medication histories, patient heterogeneity, or radiotracers utilized.
"These results do not corroborate previous studies documenting increase in DAT in adults with ADHD," said lead author Dr. Nora Volkow.
Volkow was referring to the 1999 research by Harvard's Dr. Darin Dougherty and a 2000 study by University of Munich researchers.
Although ADHD is most frequently seen in children, some patients continue to exhibit the disorder into adulthood, according to Volkow. For both children and adults, diagnosis remains elusive and difficult.
"Diagnosis has remained very controversial, in part due to a lack of understanding of the biological basis underlying ADHD," she said.
Volkow and colleagues used PET to evaluate the dopamine (DA) system in five adult patients who had child-onset ADHD but had never been medicated, and in five healthy control patients. It was important to make sure that the patients had never been treated with medication -- Volkow noted that this could be a serious confounding factor in the study. The researchers performed the scans using [carbon-11]cocaine to measure DAT and [C-11]raclopride to measure DA D2 receptor availability.
The researchers had been expecting to find increased DAT levels, but they were surprised to find that DAT levels in subjects with ADHD were significantly lower than in controls. They also found that DA D2 receptor availability did not differ between subjects with ADHD and controls.
Volkow acknowledged that the study population was small. She said the researchers wanted to get the information out to show that it may be premature to associate increased DAT levels with ADHD.
During the question and answer period that followed her talk on Sunday, Volkow fielded questions about whether looking at such static measurements as DAT and receptor levels would ever give reproducible results. Volkow said that she believed information gleaned from both receptor levels and function is needed to provide a more accurate diagnosis of ADHD.
"We need to solve this. We cannot promote ligands with these discrepancies in results," she said.
