This article is reposted from the old Wordpress incarnation of Not Exactly Rocket Science.
Attention-deficit hyperactivity disorder is the most common developmental disorder in children, affecting anywhere between 3-5% of the world's school-going population. As the name suggests, kids with ADHD are hyperactive and easily distracted; they are also forgetful and find it difficult to control their own impulses.
While some evidence has suggested that ADHD brains develop in fundamentally different ways to typical ones, other results have argued that they are just the result of a delay in the normal timetable for development.
Now, Philip Shaw, Judith Rapaport and others from the National Institute of Mental Health have found new evidence to support the second theory. When some parts of the brain stick to their normal timetable for development, while others lag behind, ADHD is the result.
The idea isn't new; earlier studies have found that children with ADHD have similar brain activity to slightly younger children without the condition. Rapaport's own group had previously found that the brain's four lobes developed in very much the same way, regardless of whether children had ADHD or not.
But looking at the size of entire lobes is a blunt measure that, at best, provides a rough overview. To get an sharper picture, they used magnetic resonance imaging to measure the brains of 447 children of different ages, often at more than one point in time.
At over 40,000 parts of the brain, they noted the thickness of the child's cerebral cortex, the brain's outer layer, where its most complex functions like memory, language and consciousness are thought to lie. Half of the children had ADHD and using these measurements, Shaw could work out how their cortex differed from typical children as they grew up.
A child grows, their experiences manifest as connections between nerve cells and their cortex thickens. But during adolescence, the developing brain values efficiency over expansion and the cortex starts to thin, as unused connections are mercilessly trimmed. The growth of a child's brain into a teenager's is like the pouring of a block of clay that can then be sculpted away into the refined adult version.
In both groups of children, parts of the cortex peaked in terms of thickness in the same order, with waves of maturity spreading from the edges to the centre. The pattern was the same, but the timing wasn't.
On average, the brains of ADHD children matured about three years later than those of their peers. Half of their cortex has reached their maximum thickness at age 10 and a half, while those of children without ADHD did so at age 7 and a half; you can see an evocative Quicktime video of this happening online. According to these results, ADHD is a disorder of delay, not deviance.
These delays were most pronounced in the lateral prefrontal cortex, where the lag time was as high as 5 years (the diagram on the right shows parts of the brain that matured with the most delay).
These parts of the brain are responsible for suppressing inappropriate thoughts and actions, directing attention, short-term memory and controlling movement. All of these are tasks that children with ADHD can find difficult and other studies have found that as they try, their prefrontal cortex shows less activity than expected for a child of the same age.
The only part of the brain that matured faster in children with ADHD was the primary motor cortex, which helps to plan and control movements. It also takes orders form the prefrontal cortex and if one matures early and the other matures late, this might explain several hallmarks of ADHD, including restlessness, fidgeting and uncontrolled hyperactivity.
Like much good research, Shaw's study raises more questions than it answers. For the moment, the most pressing one is: what causes the delay? From his data, Shaw rules out Intelligence and gender, and thinks that prescribed drugs are unlikely to have an effect either.
Genes are almost certain to have an influence though and Shaw has his eye set on genes that produce a group of proteins called neurotrophins. These control the growth, division and survival of neurons, and changes in some of their genes have already been linked to ADHD.
Shaw's results should also be encouraging for many families, and they explain why so many children eventually grow out of the condition - as lagging brains catch up, the symptoms of the developmental lag might disappear.
But it will be interesting to see if the timing of development at specific parts of the brain relates to a child's chances of recovery. We'll only know that if scientists run larger studies where the brains of children with ADHD are regularly scanned over a long period of time.
Studying the speed at which a child's cortex matures could also shed light on other mental conditions. For example, autistic children show the opposite pattern to those with ADHD - their cortices mature much earlier than those of their peers. On the other hand, the cortices of children with exceptionally high IQ mature later, even though they thicken unusually quickly in early childhood.
Reference:Shaw, P., Eckstrand, K., Sharp, W., Blumenthal, J., Lerch, J., Greenstein, D., Clasen, L., Evans, A., Giedd, J., & Rapoport, J. (2007). From the Cover: Attention-deficit/hyperactivity disorder is characterized by a delay in cortical maturation Proceedings of the National Academy of Sciences, 104 (49), 19649-19654 DOI: 10.1073/pnas.0707741104
- Autistic children are less sensitive to the movements of living things
- Fishing expedition reveals unexpected link between Alzheimer's and prion diseases
- Child abuse permanently modifies stress genes in brains of suicide victims
- Beta-blocker drug erases the emotion of fearful memories
- Same gene underlies two language disorders
This is fascinating. Have there been any new developments along these lines or corroboration since you wrote this?
Good coverage, Ed! Another question: were these kids medicated, and what were the possible effects of long-term stimulant medication on cortical development? If these kids were medicated, could that be exacerbating the delays, or helping them catch up? How fast do they "catch-up" under normal conditions?
Any comments on patients with Adult ADD/ADHD in the article? The obvious (and probably wrong) extension of these observations would make them out to be "children who never grew up".
Comparisons with people who develop Attention Deficits after injury/illness would also be interesting.
Lilian and Sci - No info in the paper about medication, but the authors addressed this in a follow-up study. Among adolescents on psychostimulants, they found slower rates of cortical thinning in some parts of the brain (right motor strip, left middle/inferior frontal gyrus, and right parieto-occipital region). Rates were around 5 times slower. However, the authors found no differences in clinical outcomes.
Stagyar - the children in this study did eventually catch up with the standard level of development; it just took them longer. No data on adult ADHD.
Really interesting piece. Was there any correlation in the results here between the different typyes of ADHD? i.e. were children more likely to 'grow out' of ADHD if they had for instance innattentive type rather than hyperactive impulsive type or vice versa?
i was diagnosed as ADHD-I (predominantly inattentive type) in college in my mid-20s, and i can testify to the fact that, despite usually testing as "gifted" by one measure or another, i did peak physically, socially, intellectually and academically several years later than most of my peers. my mother recognized early on that i was, as she put it, 'Young for your age', though she never keyed in to my absent-mindedness as something out of the ordinary (not surprising, seeing as how my mother shares many of the same traits).
so this study certainly confirms some of my own non-expert observations and suspicions. thanks, Ed!
I know a lot of experts on dyspraxia (which is a developmental disorder that often co-occurs with ADHD) say that you do not grow out of dyspraxia. They point out that it's very hard to diagnose dyspraxia early as it is hard to distinguish between dyspraxia and a developmental delay. This therefore raises the interesting distinction between on the one hand showing dyspraxia-like, or indeed ADHD-like, symptoms at a younger age due to a developmental delay and on the other hand actually having dyspraxia or ADHD. The former would therefore 'grow out' of the condition whereas the latter would not. I'm not an expert in this field but I would be interested to see if this is a logical distinction.
I wish they did speak more about adult ADHD. The studies on adults are inadequate and when more studies come out on children and don't properly address these issues, it just perpetuates the myth that adults can't have ADHD. Though it is an interesting article.
Any research on what happens to kids with both ADD/ADHD and an autism spectrum disorder? If they have opposite effects on cortical maturation, I'm curious how kids with both disorders would look. Or kids who have very high IQs but one or both disorders.
Hey, nice article! One question though. If its a delay but not a deviance, how would this explain Adult ADHD? The other explanation by Dr. Thomase Brown about how ADHD is a result of an impairment of the Executive functions makes sense here.