The difference would be even greater in hyperactive children, which would explain why the symptoms disappear in adulthood for almost a third of the patients.
The study examined 1,713 people with ADHD and 1,529 people without, ranging in age from 4 to 63 years old. Moreover, the findings propose that delays in the advancement of a few brain regions were normal for ADHD. After collecting some information, the research team applied a reliable technique for the investigation of every child and adult with ADHD. While the results point to possible places where doctors can look to diagnose ADHD, "unfortunately we don't have objective measures yet for many conditions in psychiatry, and that includes ADHD".
The newly developed study was published on February 15 in the Lancet Psychiatry magazine.
Attention deficit hyperactivity disorder (ADHD) is an issue that can affect a child's development.
Similar differences in brain volume are also seen in other psychiatric disorders, especially major depressive disorder, the researchers said.
Previous studies have found links between differences in brain volume and ADHD, but they were limited by small sample sizes, making it hard to draw any firm conclusions.
Of seven subcortical brain regions targeted in the study, five, including the amygdala, were found to be smaller in those with ADHD compared with a control group.
The five areas affected were the caudate nucleus, putamen, nucleus accumbens, amygdala and hippocampus.
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The differences in brain size were particularly prominent in the children and less obvious in the adults with ADHD, note the authors, who suggest that their findings show that ADHD is a brain disorder characterized by delayed development in several brain regions. In order to show that ADHD is a brain condition, researchers measured the overall brain volume and the volume of 7 regions that are linked to the disorder.
Martine Hoogman, a geneticist at Radboud University in the Netherlands and also the first author of the study, claimed that the amygdala is a brain structure which was not known to be involved in the evolution of ADHD.
Despite the large numbers of participants of all ages, the study was not created to investigate how ADHD might develop over a person's lifetime.
Prescriptions for drugs such as Ritalin for children diagnosed with ADHD are thought to have doubled in the last decade, despite concerns they can cause adverse reactions like weight loss, liver toxicity and suicidal thoughts.
The research was praised by Columbia University's Jonathan Posner as "an important contribution" to the study of the condition.
Diagnoses of ADHD have become increasingly common - at least one in 20 children in Canada are estimated to have the disorder.
To further observe how ADHD manifests throughout life, the researchers said longitudinal studies created to track subjects from childhood into adulthood will be needed and will be an important next step in their work.