Smaller brains linked to ADHD diagnoses: Dutch researchers

A teacher and student at school.

The brains of people with ADHD are smaller in five regions than the brains of people without ADHD, according to a new international neuro-imaging study led by Dutch researcher Barbara Franke of the Radboud University Medical Center in Nijmegen. This is the largest such study for ADHD to date, Radboud UMC announced on its website.

The researchers looked at the brain volume of 1,713 people with ADHD and 1,529 people with out ADHD aged between 4 and 63 years. The images were obtained from data of 23 previous studies on the seven brain regions and were analyzed again. The researchers also looked at the effect of age, gender, medication and other psychiatric disorders.

The results showed that the brains of people with ADHD are smaller than those of healthy subjects. Specifically in the size of the amygdala, hippocampus and the three parts of the basal ganglia. The differences were bigger in children than in adults. The researchers therefore assume that a delay in the development of the brain is a characteristic of ADHD. 

The researchers believe that these results of delayed brain development may explain the characteristics of this attention disorder - inattention, impulsivity and hyperactivity, among other things. The biggest difference in size is found in the amygdala - which plays a big role in emotion regulation. The smaller areas in the basal ganglia may explain why people with ADHD prefer an small reward immediately, rather than waiting for a bigger reward. The role of the hippocampus is less clear, but the researchers think it may have to do with motivation and emotion regulation. 

The study also revealed that medication may suppress the symptoms of ADHD, but has no effect on the brain volumes. 

"The brain differences are very small, about a few percent smaller. This could only be found because we had a very large study population. International cooperation is therefore an absolute must", Martine Hoogman, researcher at Radboud UMC and lead author on the study paper, said. "Comparable differences in size are  also found in other psychiatric disorders, such as depression."

Hoogman hopes that these results will provide more understanding about ADHD and disprove certain stigmas about the disorder, such as that ADHD is caused by bad parenting or simply a label given to difficult children. 

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