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Background to ADHD

All children have occasional trouble paying attention or suppressing their impulses. ADHD is a chronic condition, however, and its main symptoms have a larger effect on people’s lives.
The label "attention deficit/hyperactivity disorder" (ADHD) refers to a family of chronic neurobiological disorders that interfere with people’s capacity to attend to tasks, regulate their activity, and inhibit their behavior in ways appropriate to their ages and circumstances. There are actually three subtypes of ADHD: * Inattentive type—characterized principally by difficulty paying attention to tasks. * Hyperactive-impulsive type—characterized by being overly active and impulsive. * Combined type—the most common form, characterized by both inattentive and hyperactive-impulsive symptoms. All children have occasional trouble paying attention or suppressing their impulses. ADHD is a chronic condition, however, and its main symptoms have a larger effect on people’s lives. Inattention. People with ADHD often have a hard time keeping their minds on one thing and may get bored with a task after only a few minutes. Focusing conscious, deliberate attention to organizing and completing routine tasks may be difficult. However, the problem may not be uniform across all settings: a child or adult with ADHD may focus very well on some activities that require different forms of attention, such as video games or highly stimulating experiences. Hyperactivity. People with the full syndrome of ADHD (combined type), particularly boys in the elementary school years, usually seem to be in constant motion. They can’t sit still; they may dash around or talk incessantly. For them, sitting through a lesson can be impossible. They may roam around the room, squirm in their seats, wiggle their feet, touch everything, or noisily tap a pencil. The motor hyperactivity tends to decrease with age. Adolescents and adults with ADHD may report feeling intensely restless without actually having to move so much. Impulsivity. People with ADHD often seem unable to curb their immediate reactions or to think before they act. As a result, they may blurt out answers to questions, make inappropriate comments, or run into the street without looking. In the case of children, their impulsivity may make it hard for them to wait for things they want or to take turns in games. They may grab toys from other children or hit when they are upset. ADHD is the most commonly diagnosed disorder of childhood, estimated to affect 3 percent to 5 percent of school-age children. On average, about one child in every classroom in the United States needs help for this disorder. Parents or preschool teachers often notice the first manifestations of ADHD in a child who cannot sustain attention on a level comparable to his or her peers. Parents and teachers may be concerned that this child “can’t sit still,” “can’t pay attention,” daydreams, or needs repeated reminders to stay on task, finish household chores, or do schoolwork. At times, the child’s difficulties with attention may be more subtle or masked by other activities, such as acting the “class clown” or being overly social and talkative. ADHD occurs three times more often in boys than in girls. Furthermore, because girls tend to display fewer obvious problems with motor hyperactivity, they are often not diagnosed, or are diagnosed two to three years after boys. Likewise, very bright children with ADHD may be missed because they are “getting by,” but they often significantly underperform in comparison with their intellectual abilities. Life is often very difficult for children and adolescents with ADHD. They face challenges sitting still and paying attention in class, at home, and even in the playground. As a result, they often may be in trouble at school, cannot finish games, and have difficulties making friends. They may spend hours each night struggling to keep their minds on their homework, then forget to bring it to school. Complicating these problems, young people with ADHD often experience negative consequences of their behavior: being scolded, doing poorly on tests, or being shunned by classmates. Their difficulties can increase family conflict. Adolescents with ADHD are at increased risk for having automobile accidents, using tobacco, becoming pregnant, and doing poorly in school. Approximately 60 percent to 80 percent of children will continue to experience significant symptoms of ADHD in adulthood. As adults, people with ADHD face a higher risk of lower vocational attainment, marital problems, and injuries of all sorts and an increased likelihood of substance use or abuse. However, those poor outcomes are far from definite, and many people with ADHD have recognized their problems with inattention and impulsivity and found successful ways of coping. Mechanisms of ADHD Though we still do not know the exact brain mechanisms underpinning ADHD, imaging studies have linked the condition with specific structures of the brain. Using magnetic resonance imaging (MRI), a number of teams in the United States and abroad have examined the prefrontal cortex, basal ganglia, and cerebellum in children with ADHD. The results of these studies indicate that in children with ADHD, the size of these structures is 5 percent to 10 percent smaller than in ordinary children. These same structures, especially the prefrontal cortex and basal ganglia, are rich in dopamine receptors. And since such medications as methylphenidate (Ritalin) work on dopamine receptors, these findings support the hypothesis that one underlying problem for children with ADHD involves disturbances in the dopamine-signaling system. Researchers have used functional brain imaging to compare children with and without ADHD. These studies initially used single-photonemission computed tomography (SPECT) and positron-emission tomography (PET) to estimate where blood flows in a child’s brain. In general, these studies showed lower than normal blood flow into the brain areas already implicated in ADHD. The PET studies in particular have shown that the reduced blood flow is related to the severity of ADHD symptoms. In the past several years, a number of investigators have used functional magnetic resonance imaging (fMRI) to further document the relatively low blood flow in specific brain regions, and these findings have generally confirmed the earlier SPECT and PET studies of ADHD children and adults. Most recently, two PET studies have found that administering methylphenidate to ADHD children appears to increase blood flow to these brain areas. Similar studies have not always yielded consistent findings, but all together they are providing promising results, and the methods continue to improve. At this time, imaging is still not appropriate for diagnosing children with ADHD or screening them for it. Factors in ADHD One of the difficulties in researching the possible genetic aspect of ADHD is that the diagnosis can be overapplied. Studies must start by rigorously identifying people with the disorder using standard, reliable assessment procedures. When that is done, ADHD appears to be highly heritable. However, there have been few studies on the normal development and regulation of attention; since we do not know all the basics of this brain function, we may not know all that can go wrong with it. Furthermore, there may be nongenetic forces involved in turning supposedly “fixed” genes off and on. Therefore, we must be cautious about assigning an immutable role to genetic factors in the onset and development of ADHD. Geneticists are following two major approaches to identify genes associated with ADHD. The first, called whole-genome scanning, relies on obtaining DNA from large groups of affected family members (usually siblings). The research team tries to find links between specific chromosomal markers and ADHD disorder traits. The second method involves focusing on what is termed a candidate gene. Using this strategy, investigators typically examine people for the frequency of various forms of specific genes known to be of possible interest in understanding the source of the disorder. A number of candidate genes have been investigated by researchers in the last five years. A particular form of the dopamine transporter gene (DAT1) has been found to be associated with ADHD, and another candidate gene under study is dopamine D4 receptor (DRD4). Unlike similar studies involving other neuropsychiatric disorders, more times than not independent laboratories have been able to replicate the original findings of these studies, indicating that the work is on the right track. Investigators are also exploring other candidate genes involved in the dopamine and noradrenaline signaling systems, with promising results. There are also nongenetic factors in the development of ADHD. Mothers of affected children are more likely to have had complications during pregnancy, such as toxemia, lengthy labor and delivery, excessive nausea, and undue weight loss or gain. High-quality care before and just after birth may therefore forestall the development of ADHD in children who would otherwise be vulnerable. Some environmental toxins have been implicated in the development of ADHD: alcohol and nicotine in the fetal brain, and lead in growing children. Much more attention has been paid to the supposed role of food additives, sugar, and possible food allergens, but few rigorous studies of these substances have been replicated, and their possible effects remain controversial. Though studies of possible toxins and other chemical factors are intriguing, findings generally indicate that they account for relatively few cases of ADHD. Many children are exposed to comparable levels of these substances without developing the disorder. That fact indicates that combinations of trauma, toxic exposure, and subtle forms of brain injury, along with a certain pattern of susceptibility genes, are required for the full syndrome of ADHD to emerge. Diagnosis and Treatment Physicians and other health care practitioners can identify ADHD reliably using well-tested diagnostic interview methods. Checklists can be useful as screening devices, but doctors should not rely on them as diagnostic tools. Using checklists alone tends to identify far more children as having ADHD than careful interviews—up to two or three times more. The most reliable diagnoses are based on careful history and observable behaviors in the child’s or youth’s usual settings. Ideally, a health care practitioner should obtain information from parents and teachers. Children usually do not recognize their ADHD symptoms. Adults with ADHD appear to be more accurate in describing their behaviors and feelings, however. Treating ADHD usually involves medication, specific forms of psychotherapy or training in social skills, and often a combination of these strategies. They are effective in different ways. Medication Strategies By far the most widely researched and commonly prescribed treatments for ADHD are psychostimulant medications, including methylphenidate (for example, Ritalin, Concerta, Metadate), amphetamine (Dexedrine and Adderall), and pemoline (Cylert). Many studies have demonstrated their short-term efficacy compared with placebos in improving both core ADHD symptoms and associated features. These investigations have used a wide variety of assessment methods, including ratings from parents and teachers, direct observations of children in natural and laboratory settings, and performance on objective laboratory tests. Controlled studies of stimulants have shown them to be effective in reducing interruptions in class, activities not relevant to schoolwork, and overt and covert aggression and in improving performance on spelling and arithmetic tasks, sustained attention and compliance, making friends, short-term memory, and parent-child interactions. There have been studies showing that stimulants improve a child’s attention during baseball. These short-term benefits are limited by several important considerations. First, treating ADHD with medication alone does not always help a child attain normal behavior or improve in all domains of functioning. In addition, most investigations of stimulant efficacy have been quite brief, covering only weeks or months, though there have been recent clinical trials of one or two years’ duration.
ADHD, attention, deficit, hyperactivity, disorder, inattention, impulsivity, mechanism, diagnosis, treatment, medication
Creative Commons License This work by Cold Spring Harbor Laboratory is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.

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