The largest brain-imaging study of ADHD to date identified differences in five areas of the brain that can be traced to the disorder. Furthermore, these differences were most pronounced in children rather than adults. The findings support ADHD’s recognition as a brain disorder.

Image credits Bob / Flickr.

Attention-deficit hyperactivity disorder (ADHD) symptoms include inattention, hyperactivity, and acting impulsively. The CDC estimates that this disorder affects roughly 5% of under-18-year-olds (although the exact figure is likely higher) and two-thirds of those diagnosed continue to experience symptoms as adults. But it often gets a bad rep as a smokescreen to hide difficult children or poor parenting, as a make-believe condition to excuse one’s behavior. Part of the problem stems from the fact that while investigations into the disorder have managed to link abnormalities in brain volume to ADHD, they’ve generally been performed on small samples — making some people refute their conclusions.

Now, the largest study to date of ADHD looked at the brain of more than 3,200 people to come up with clear, solid data on the condition. The authors say their findings further our understanding of ADHD and should offer solid footing for anyone who has to prove the validity of the disorder.

It’s all in the brain

The international study measured the differences in brain structure seen in 1,713 people diagnosed with ADHD and 1,529 who weren’t, between the ages of 4 and 63 years old. They each had an MRI scan performed to determine the overall brain volume and the size of seven areas previously linked to ADHD — the pallidum, thalamus, caudate nucleus, putamen, nucleus accumbens, amygdala, and hippocampus. The team also noted if those with ADHD had ever taken psychostimulant medication such as Ritalin.

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They report that overall brain volume and five of the regional volumes were smaller in people with ADHD — the caudate nucleus, putamen, nucleus accumbens, amygdala, and hippocampus.

“These differences are very small — in the range of a few percent — so the unprecedented size of our study was crucial to help identify these. Similar differences in brain volume are also seen in other psychiatric disorders, especially major depressive disorder.” said lead author Dr Martine Hoogman from the Radboud University Medical Center, Nijmegen, The Netherlands.

Small differences can stand out in a big way,
Image via Pixabay

These differences were most obvious in the brains of children but were harder to pick out in the brains of adults (in both cases, diagnosed with ADHD.) The team proposes that ADHD is a brain disorder caused by delayed development in several brain regions. Beyond the caudate nucleus and puntamen — which were linked by previous studies to the disorder — the team also showed that the the amygdala, nucleus accumbens, and hippocampus play a part in the onset of ADHD.

They believe that the slower development of the amygdala can explain the difficulty ADHD patients have in regulating their emotions, while that of the nucelus accumbens — which plays an important part in reward processing — explains their motivational and emotional difficulties. The hippocampus’ role in the disorder might act through its involvement in motivation and emotion.

“The results from our study confirm that people with ADHD have differences in their brain structure and therefore suggest that ADHD is a disorder of the brain,” added Dr Hoogman.

“We hope that this will help to reduce stigma that ADHD is ‘just a label’ for difficult children or caused by poor parenting. This is definitely not the case, and we hope that this work will contribute to a better understanding of the disorder.”

At the time the MRI scans were taken, 455 people with ADHD were receiving psychostimulant medication and a further 637 had taken it at one point in their life. The five brain regions showed different volumes whether or not people had received this treatment, suggesting the differences in brain volumes are not a result of psychostimulants.

Still, while this study addressed the main critiques of previous work and established a strong link between ADHD and brain development, it cannot determine how the disorder develops throughout life. Studies tracking people with ADHD from childhood to adulthood to see how brain differences change over time will be an important next step in the research.

The full paper “Subcortical brain volume differences in participants with attention deficit hyperactivity disorder in children and adults: a cross-sectional mega-analysis” has been published in the journal Lancet Psychiatry.