Several subcortical brain structures are smaller in children and adults diagnosed with attention deficit hyperactivity disorder (ADHD), according to a study recently published in The Lancet.

ADHD is a neurological condition that is often diagnosed in childhood, but around two-thirds of patients continue to display symptoms into adulthood. Key symptoms of ADHD include hyperactivity, inability to sustain attention and impulsive behaviour.

Previous imaging studies report structural and functional brain abnormalities in patients with ADHD compared to healthy individuals. For example, studies have consistently found reduced basal ganglia volume in ADHD patients. But the number of participants in each of these studies has been too small to generalize the results to the population as a whole. Identifying brain differences in people with ADHD is crucial for increasing our understanding about the underlying mechanisms that cause the disorder and how to effectively treat it.

In the largest study of its kind to date, a team of scientists led by Martine Hoogman (Radbound University Medical Centre, The Netherlands) combined the results from multiple imaging studies in a meta-analysis consisting of 1,713 patients with ADHD and 1,529 healthy controls. The results revealed dramatic differences in the brain volume of patients with ADHD and healthy controls.

Areas of reduced brain volume included the amygdala, accumbens and hippocampus, which had not been identified in previous studies. The results also revealed similar findings to previous studies including reduced caudate and putamen volume.

ADHD is often treated with stimulants. The results of this study found that brain volume differences in ADHD patients were the same whether they had taken stimulant medication or not. The largest difference in brain volume was found in the amygdala which is important because it provides an explanation for the disruption of emotional regulation that is often seen in ADHD but is not listed in current diagnostic criteria. Hippocampus volume was reduced in ADHD patients which is interesting because there is little evidence of long-term memory deficits in ADHD. However, some studies suggest that the hippocampus plays a role in regulating emotion and motivation, which is compromised in ADHD.

Overall, the study confirms that patients with ADHD do have structural and functional brain alterations, and ADHD is therefore a disorder of the brain. The strongest differences in brain volume were found in childhood which supports current thinking that ADHD is produced by a delay in brain maturation. The results of this study will be important for improving our understanding and reducing the stigma of ADHD.

