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ADHD is associated with the delayed development of five brain regions and should be considered a brain disorder, according to a study published in The Lancet Psychiatry. The study involved more than 3,200 people. The authors say the findings could help improve understanding of the disorder, and might be important in challenging beliefs that ADHD is a label for difficult children or the result of poor parenting.

The disorder affects more than one in 20 (5.3 per cent) under-18 year olds, and two-thirds of those diagnosed continue to experience symptoms as adults. Previous studies have linked differences in brain volume with the disorder, but small sample sizes meant results were inconclusive.

Areas thought to be involved in ADHD are located in the basal ganglia – a part of the brain that controls emotion, voluntary movement and cognition – and research previously found that the caudate and putamen regions within the ganglia were smaller in people with ADHD. The new international study measured differences in the brain structure of 1,713 people with a diagnosis of ADHD and 1,529 people without, all aged between four and 63 years old. The study found 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.

The largest imaging study of Attention-Deficit Hyperactivity Disorder to date has identified differences in five regions of the brain, with greatest differences seen in children rather than adults.

“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, Radboud University Medical Center, Nijmegen, The Netherlands. The differences observed were most prominent in the brains of children with ADHD, but less obvious in adults with the disorder. Based on this, the researchers propose that ADHD is a disorder of the brain, and suggest that delays in the development of several brain regions are characteristic of ADHD.

Besides the caudate nucleus and putamen, for which previous studies have already shown links to ADHD, researchers were able to conclusively link the amygdala, nucleus accumbens and hippocampus to ADHD.

The researchers hypothesise that the amygdala is associated with ADHD through its role in regulating emotion, and the nucleus accumbens may be associated with the motivation and emotional problems in ADHD via its role in reward processing. The hippocampus’ role in the disorder might act through its involvement in motivation and emotion. At the time of their MRI scan, 455 people with ADHD were receiving psychostimulant medication, and looking back further, 637 had had the medication in their lifetime. The different volumes of the five brain regions involved in ADHD were present whether or not people had taken medication, suggesting the differences in brain volumes are not a result of psychostimulants.

http://www.thelancet.com/journals/lanpsy/article/PIIS2215-0366(17)30049-4/fulltext