Attention deficit-hyperactivity disorder (ADHD) is a neurobehavioral disorder, characterized by impulsive behavior, inattentiveness, lack of concentration and hyperactivity. Some ADHD patients are predominantly inattentive, some predominantly hyperactive-impulsive, and some are a combination of these two. It is not entirely clear why some people develop ADHD, but it has been shown to have strong genetic components, and certain environmental toxics, such as lead, have been linked to ADHD. Mothers who consume alcohol or tobacco during pregnancy are at much higher risk for having a child with ADHD. People will ADHD show abnormalities in both brain structures and functions.
Frontal Lobe and Executive Functions
A study published in 2002 in the Journal of the American Medical Association reported that children with ADHD had approximately 3 percent smaller brain volume in the prefrontal cortex than children without ADHD. The brain maturation of the frontal cortex has also been shown to be approximately three years behind in middle school ADHD patients as compared to healthy children, says the National Institute for Mental Health in 2007. This area is responsible for so-called executive functions such as planning ahead, making good decisions, paying attention to tasks, and other higher-order cognitive functions. This area also helps a person to behave appropriately in a particular situation. Not surprisingly, these are also the functions that are most impacted in people with ADHD.
Inhibitory Mechanisms of the Brain
Executive functions that allow a person to withhold a response are called inhibitory mechanisms. These mechanisms help a person to inhibit certain behaviors, so that he does not lose his temper, become hyperactive, make poor decisions or act in an impulsive way. Needless to say, these are the exact behaviors that many people ADHD struggle with. Due to the lack of inhibitory mechanisms, people with ADHD get into arguments, make careless mistakes and are easily distracted from a task. In a study published in the American Journal of Psychiatry in November 2001, Jennifer Crosbie, M.A. and colleagues asked ADHD patients to perform a task that required them to make a quick ongoing motor response and the sudden cessation of that response following a signal. As expected, most ADHD patients had great difficulties stopping the motor response. Interestingly, ADHD participants who exhibited poor inhibition were significantly more likely to have relatives with ADHD than participants with ADHD who exhibited good inhibition.
The Limbic System
The limbic system is responsible for processing emotion, but it also helps the person to regulate her energy levels and assists in coping with stress. A person with ADHD often has huge mood-swings, and a quick temper. In a study published in the Archives of General Psychiatry Journal in May 2008, Kerstin J. Plessen, M.D. and colleagues used magnetic resonance imaging to study the size of the amygdala and hippocampus, regions of the limbic system in ADHP patients and their healthy controls. The study demonstrated that the amygdala was significantly smaller in size in ADHD subjects. Abnormal connectivity between the amygdala and prefrontal cortex was also noticed in the ADHD group. The authors concluded that the abnormal size and connectivity of the amygdala is likely to contribute to the increased risk for affective disorders in children with ADHD. Interestingly, the authors also reported that the hippocampus was enlarged in children and adolescents with ADHD, as compared with the healthy controls, and suggested that this may be a compensatory response to the disturbances in the perception of time and stimulus-seeking behaviors that are commonly found in ADHD patients.
References
- "Journal of the American Medical Association"; Developmental trajectories of brain volume abnormalities in children and adolescents with attention-deficit/hyperactivity disorder.; Castellanos, Lee, Sharp; vol. 288, 1740-1748, 2002.
- "The American Journal of Psychiatry": Deficient Inhibition as a Marker for Familial ADHD
- Internet Special Education Resources: The Biology of ADD
- "National Institute of Mental Health" Press Release; Brain Matures a Few Years Late in ADHD, But Follows Normal Pattern.; Shaw, Rapoport, Evans; Nov. 2007.
- "Archives of General Psychiatry": Hippocampus and Amygdala Morphology in Attention-Deficit/Hyperactivity Disorder
