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Attention deficit hyperactivity disorder (ADHD) is a developmental disorder primarily affecting children’s ability to pay attention and focus on tasks without becoming distracted. The hyperactivity component adds symptoms of restlessness, hyperactivity, and impulsivity. This article will address a few theories about ADHD pathophysiology that causes the disorder's symptoms.
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Definition of ADHD
ADHD typically manifests during childhood. Classic signs of a child with ADHD include inattention, daydreaming, distractibility, and lack of focus on the current task. A child with ADHD also often exhibits hyperactivity and impulsivity resulting in fidgeting, and creating problems with peers, and causing disruptions in class. 
In a literature overview of ADHD Drs. Montauk and Mayhall of The Affinity Center, Cincinnati, delve further into defining ADHD by the distortions caused to the child’s executive function tasks in six areas:
(1) Flexibility: unable to easily shift from one mindset or strategy to another;
(2) Organization: unable to anticipate both needs and problems;
(3) Planning, particularly goal setting;
(4) Working or short-term memory: difficulty in receiving, storing, and then retrieving information;
(5) Separating affect from cognition: difficulty detaching emotions from reasons; and
(6) Inhibiting and regulating verbal and motoric action: impulsive, anxious, impatient. 
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Over the course of studying the disruption of normal bodily functions due to ADHD, key theories have surfaced as to the underlying pathophysiology.
Neuropsychology: Brain Size and Neurotransmitters
Drs. Montauk and Mayhall maintained the primary pathophysiology for ADHD has a basis in neuropsychology, mentioning that numerous neuropsychological studies suggests that the brain’s frontal lobes are responsible for executive functions, and therefore to attention and exercising inhibition. They then cited a 10-year study from the National Institute of Mental Health (NIMH) revealing that children and adolescents with ADHD have brains up to 3 to 4 percent smaller than individuals without ADHD, which in turn inhibits the neurotransmitters responsible for attention-related processes. 
Genetics: Dopamine Receptors and Impaired Information Processing
In addition, Drs. Montauk and Mayhall cited studies from NIMH indicating a strong genetic component as an underlying cause of ADHD. Specifically, they stated the following:
“Molecular genetic studies have revealed several genes that appear to be associated with ADHD (ADD) because of their effect on dopamine receptors, dopamine transport, and dopamine beta-hydroxylase. Research by the NIMH has shown that variants of the gene for catecho-O-methyltransferase (COMT) are associated with different levels of prefrontal dopamine activity. COMT metabolizes dopamine. People with the val/val variant metabolize dopamine rapidly. Because fast metabolism of a substrate decreases the amount of substrate that is biologically available, these people have reduced prefrontal dopamine activity. This reduction, in turn, impairs prefrontal information processing."
A third potential factor that contributes to the pathophysiology of ADHD in children (although without substantial evidence) is environmental factors. Some of these may include maternal health and behavior during pregnancy (such as smoking or drug use), problems during pregnancy and delivery, head injury, exposure to toxins, family dysfunction, and lower socioeconomic class. 
Possible Additional Factors
NIMH further cited possible additional factors as contributing to ADHD pathophysiology, but cautioned that these factors are without valid proof, and advised further research. These additional factors may include traumatic brain injury, sugar, sugar substitutes, and food additives.
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The pathophysiology of attention deficit hyperactivity disorder includes a few key ADHD theories about this developmental disorder. Children with Attention Deficit Hyperactivity Disorder (AHD) experience a range of symptoms, from lack of focus and inattention, to hyperactivity and impulsivity. Underlying pathophysiology for ADHD potentially includes brain size, genetics, and environmental factors.
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References and Resources
Farone, S.V., and Biederman, J. (2005). [PDF]. Pathophysiology of Attention Deficit/Hyperactivity Disorder. Pediatric Psychopharmacology Unit of the Child Psychiatry Service, Massachusetts General Hospital, and Harvard Medical School, Boston, MA.
Montauk, S.L., and Mayhall, C.A. (2006). Attention Deficit Hyperactivity Disorder. Retrieved from http://emedicine.medscape.com/article/912633-overview
NIMH. Attention Deficit Hyperactivity Disorder (ADHD). Retrieved from http://www.nimh.nih.gov/health/publications/attention-deficit-hyperactivity-disorder/complete-index.shtml
Banks, K.L. (2010). How to Help a Child with ADD Learn at School. Retrieved from http://www.livestrong.com/article/101053-child-add-learn-school
LD Online. ADHD Basics. Retrieved from http://www.ldonline.org/adhdbasics
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