Does size matter? In ADHD it might!

The brains of adults with ADHD might actually be smaller than neurotypical and that might explain many elements of the disorder.

ADHD is a disorder of how the brain works. It is caused by differences in how the brain develops as we grow (called neurodevelopment).  No-one knows exactly why this happens but it is highly genetic and it happens a lot. In fact, ADHD is thought to be diagnosable in around 150 million of us around the world and probably more. That still leaves nearly 7 billion other people but it is undeniably a big number.

ADHD is a lifelong neurodevelopmental disorder that manifests as an inability to choose what to focus on, feelings of mental or physical hyperactivity and impulsive behaviour. These factors can affect people with ADHD, either alone or in combination.

So what is it about an ADHD brain that makes it different? The short answer, everything. It is thought that the difficulties faced by people with ADHD are a combination of structural changes (anatomy) and the way brain cells communicate (physiology).

This means that the way we think, the way we respond emotionally, the way we act, are all affected. The main behavioural symptoms are:

  • Changes in the ability to suppress inappropriate or irrelevant actions. This includes things like checking your phone or chatting to friends when you should be studying or working (known as response inhibition).
  • Choosing small but immediate rewards over larger but delayed rewards (also known as delay aversion).
  • Relative problems with the mental skills that include working memory, flexible thinking, and self-control (known as executive function).
Brain scans can show which areas of the brain are normally sized and functional brain scans can show which parts of the brain are activated under different conditions.

Small and (im)perfectly formed.

A number of studies of the structure of the ADHD brain have shown that there may be overall reductions in total brain volume compared to non-ADHD brains. This means on average ADHD brains are smaller. Although this is thought to be involved in the problems with executive function, there is little evidence that intelligence is affected. One review of all the data suggested that intelligence/achievement might be lower but these effects were larger in children and adolescents than in adults. The variation from these studies also means that these differences cannot be used to diagnose ADHD.

Can you be more specific?

All the major areas (known as lobes) of the brain are affected. Lots of evidence points to the front of the brain as being the area with the biggest differences. Almost half of the reduced brain size is found here. More specifically, changes in areas called the pre-frontal cortex are thought to be largely responsible for the behaviours seen in ADHD.

The Prefrontal Cortex. Not only is the prefrontal cortex involved in manipulating raw information it receives into mental representations such as perceptual objects, episodic memories, emotional states, and motor actions, it also plays a critical role in the regulation of emotion. There is evidence from brain scans that this area of the brain is less active in ADHD. It also appears that this part of the brain develops more slowly in ADHD, especially in areas associated with attention. It is clear that the prefrontal cortex of the brain, especially on the right-hand side, is important in the development of ADHD.

Other brain areas have been reported to be involved in the cause of or manifestation of ADHD, including the bafflingly named basal ganglia, the default mode network and the anterior cingulate as well as the temporal, and parietal regions whatever that means. These will be covered in articles in the future.

Author: Alex Conner PhD.

Editor: James Brown PhD.

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