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Results of a US study in this week's issue of THE LANCET provide details of the underlying physical causes of attention-deficit hyperactivity syndrome, with reductions in size of some brain areas and an increase in grey matter proportions being characteristic of children with the disorder.

Attention-deficit hyperactivity disorder (ADHD) is a serious neuropsychiatric problem in schoolchildren (an estimated 3-6% of US schoolchildren are affected, for example). The disorder is characterised by poor attention span, impulsivity, and high motor activity. Its nature and cause are poorly understood, although previous research has suggested that structural changes in areas of the brain controlling attention are responsible for the disorder.

Elizabeth R Sowell, Assistant Professor of Neurology from the Laboratory of Neuro Imaging at the University of California Los Angeles, USA, and colleagues undertook the first detailed morphological study using high-resolution magnetic resonance imaging (MRI) and sophisticated computational systems to more accurately determine the specific areas of the brain underlying ADHD. Brain assessment of 27 children (11 girls, 16 boys) and adolescents with ADHD was compared with that of 46 control children without ADHD who were matched for age and sex.

Abnormal brain structure was observed in the frontal cortices (on both sides of the brain) of children with ADHD, with reduced regional brain size localised mainly to small areas of the dorsal prefrontal cortices. Children with ADHD also had reduced brain size in anterior temporal areas, also on both sides of the brain. Substantial increases in grey matter were recorded in large portions of the posterior temporal and inferior parietal cortices in children with ADHD.

Elizabeth R Sowell comments: "Our morphometric procedures allow more precise localisation of group differences than do the methods used in previous studies. Our results therefore suggest that the disturbances in prefrontal cortices are localised to more inferior aspects of prefrontal regions than was previously appreciated. Our findings also indicate that prefrontal abnormalities are represented bilaterally, by contrast to the predominantly right-sided findings that were emphasised in other reports."

Bradley Peterson, the Suzanne Crosby Murphy Associate Professor of Psychiatry at Columbia University & the New York State Psychiatric Institute, a child psychiatrist and co-author on the study comments: "The findings are not only in brain regions controlling attention, but also in regions that subserve impulse control. Disordered impulse control is often the most clinically debilitating symptom in children with ADHD. These findings may help us understand the sites of action of the medications used to treat ADHD, particularly stimulant medications. In conjunction with other imaging techniques, the findings may help us to develop new therapeutic agents given our knowledge of the cellular and neurochemical make-up of brain regions where we detected the greatest abnormalities."

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