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Attention Deficit Hyperactivity Disorder: Overview and Risk Factors

Attention deficit hyperactivity disorder (ADHD) is characterized by inattentiveness, hyperactivity, and poor impulse control. It is estimated to affect 5% to 10% of children, and up to 70% of cases persist into adolescence and adulthood. It can affect cognitive, academic, behavioral, emotional, and social functioning and may be associated with comorbid psychiatric conditions, such as oppositional-defiant and conduct disorders, learning disabilities, anxiety, depression, and, later in life, substance use disorders.

The disorder is probably of multifactorial origin. That is, it may be caused by a combination of genetic and environmental factors. Neurotransmitter abnormalities have been postulated, focusing on catecholamine metabolism in the cerebral cortex and basal ganglia. An imbalance between norepinephrine and dopamine in the prefrontal cortex is suspected. Methylphenidate, a stimulant that is effective in treating ADHD symptoms, is known to increase synaptic dopamine concentrations.

Inattention may present as disorganization, forgetfulness, frequent misplacing of things, inability to follow instructions, academic underachievement, distractibility, inability to finish tasks, poor concentration, careless mistakes, or poor attention to detail. Hyperactivity is identified by fidgeting, restlessness, difficulty remaining seated, and talking excessively or inability to remain quiet when appropriate.

Impulsivity is noted by difficulty waiting turns, disruptive classroom behavior, interrupting others, peer rejection, and attempting risky activities without considering consequences. Affected adults may show inattention and impulsivity, rather than hyperactivity, and may have difficulty keeping a schedule, managing money, staying with a job, or maintaining a marriage.

Risk Factors

Male gender. ADHD is identified 2 to 3 times more frequently in boys than in girls.

Age. About half of cases present earlier than age 4. Although some will remit by adolescence, others persist into adulthood. At present, no "adult-onset" variety is recognized; symptoms must have been present before 7 years old to meet diagnostic criteria.

Genetics. Some studies indicate that as much as 75% to 80% of risk may be genetic. Studies of twins reveal a 90% concordance in monozygotic twins. Several genes have been identified as possible candidates, most notably dopamine receptor and transporter genes.

Environmental factors. Early lead exposure or head injury may increase risk.


Attention Deficit Hyperactivity Disorder: Diagnosis >>