ADHD symptoms usually arise in early childhood. Current diagnostic criteria indicate that the disorder is marked by behaviors that are long-lasting and evident for at least six months, with onset before age seven. Because everyone shows signs of these behaviors at one time or another, the guidelines for determining whether a person has ADHD are very specific.
In children, the symptoms must be more frequent or severe than in other children of the same age. In adults, the symptoms must be present since childhood and affect the person’s ability to function in daily life. For both children and adults, these symptoms must create significant difficulty in at least two areas of life, such as home, social settings, school, or work.
Increasingly, researchers are studying ADHD in the context of executive functions—the brain functions that activate, organize, integrate, and manage other functions. Impairment of these executive functions is considered highly interrelated to symptoms associated with ADHD.
There are three primary subtypes of ADHD, each associated with different symptoms.
ADHD—Primarily Inattentive Type:
ADHD—Primarily Hyperactive/Impulsive Type:
Research has demonstrated that ADHD has a very strong neurobiological basis. Although precise causes have not yet been identified, there is little question that heredity makes the largest contribution to the expression of the disorder in the population.
In instances where heredity does not seem to be a factor, difficulties during pregnancy, prenatal exposure to alcohol and tobacco, premature delivery, significantly low birth weight, excessively high body lead levels, and postnatal injury to the prefrontal regions of the brain have all been found to contribute to the risk for ADHD to varying degrees.
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