Attention Deficit Hyperactivity Disorder (ADHD) is a neurobehavioral disorder that is characterized by attention-related problems and hyperactivity. ADHD is the most commonly diagnosed mental disorder that affects about 3 to 5 percent of children globally. Up to 50 percent of children who are diagnosed with ADHD continue to have ADHD-related symptoms when they are adults, and the ones who don't, are likely to have learned how to cope with their problems, as there is no known cure for ADHD. There are three different sub-types of ADHD: Some patients are predominantly hyperactive-impulsive, some are mainly inattentive and most patients with ADHD are a combination of both of these types. Although it is not quite clear why some people develop ADHD, we do know that their brains show differences in both structure and function as compared with healthy individuals.
Reduction in Brain Volume
A study published in 2002 in "JAMA" reported that children with ADHD had a smaller brain overall, and that most brain subregions were also smaller than normal. Approximately 3 percent smaller brain volume in many different regions was reported, but the small size was especially easy to detect in the prefrontal cortex. This area is responsible for planning, decision making, and other higher-order cognitive functions. Magnetic resonance imaging (MRI) studies have reported that ADHD patients have smaller volumes of white matter, but only if they have never been treated with ADHD medication.
Reduced Blood Circulation
Dr. Castellanos reported in a study published in 1997 in "Clinical Pediatrics Journal" that ADHD patients had reduced blood circulation, indicating low neural activity in the right frontal lobe, and in the basal ganglia parts such as the caudate nucleus. The basal ganglia region is connected with the frontal lobe, and processes movements. Reduced blood circulation has also been reported in the striatum, an area responsible for planning ahead.
Brain Maturation
National Institute for Mental Health reported in 2007 that in middle school-age ADHD patients, the brain maturation of the frontal and temporal cortices are approximately three years behind that of healthy children. Interestingly, they stated that the motor cortex of ADHD children seems to develop more rapidly than that of healthy children. It was speculated that this might be due to the ADHD medication.
Reversed Caudate Asymmetry
Caudate nucleus, which is part of the basal ganglia, is larger in left side than in the right in normal, healthy people, but this asymmetry is reversed in ADHD patients, as shown by Dr. Semrud-Clikeman in a study published in 2000. It is believed that the reversal of caudate asymmetry is related to deficits in response execution tasks in ADHD patients.
Neurotransmitters and ADHD
The neurotransmitter that has mostly been linked with ADHD is called dopamine. Unfortunately, the results of dopamine studies so far have been quite contradictory, with some reporting that ADHD patients have higher than normal levels of dopamine transporters in an area called striatum, while others have reported lower than normal levels of dopamine.
References
- "Jama" Journal; Developmental trajectories of brain volume abnormalities in children and adolescents with attention-deficit/hyperactivity disorder.; Castellanos, Lee, Sharp; vol. 288, 1740-1748, 2002.
- "Clinical Pediatrics" Journal; Toward a pathophysiology of attention-deficit/hyperactivity disorder.; Castellanos; vol. 7, 381-393, 1997.
- "National Institute of Mental Health" Press Release; Brain Matures a Few Years Late in ADHD, But Follows Normal Pattern.; Shaw, Rapoport, Evans; Nov. 2007.
- "J AM ACAD CHILD PSY" Journal; Using MRI to examine brain.; Semrud; vol. 39, 477-84, 2000.
