Dopamine transporter (DAT) density was measured by single photon emission computed tomography (SPECT) in six adult, unmedicated patients with attention deficit hyperactivity disorder (ADHD) and compared to 30 healthy controls, in a study at the Massachusetts General Hospital, Boston, MA. Striatal accumulation of iodine-123-labelled altropane was rapid, a maximum being reached in 10-15 minutes. Independent of age, DAT density was consistently elevated by 70% in the brain of patients with ADHD compared to healthy controls. Tracer accumulations were highest in the striatum and minimal in other areas of the brain. The use of SPECT could individualize treatment with psychostimulants, evaluate new drugs for ADHD, determine the pathophysiology of ADHD, and elucidate the mechanism of action of methylphenidate and other stimulants. [1]

COMMENT. Dopamine D4 receptor and dopamine transporter (DAT1) genes have been associated with ADHD, and DAT is the main target for psychostimulant medications. It was postulated that ADHD may be caused by an excess expression of DAT, since previous studies have shown a correlation between DAT and hyperactivity-impulsivity scores. The above SPECT study showing elevated DAT accumulation in the striatum of adults with ADHD appears to confirm this theory. See Ped Neur Briefs (Jan 2000; 14:4) for a report associating DAT1 with poor methylphenidate response in ADHD. Homozygosity of the 10-repeat allele characterized nonresponse to methylphenidate.

It should be emphasized to patients that SPECT studies are a research tool and are not generally available to monitor treatment of ADHD.