Attention deficit hyperactivity disorder (ADHD) is a set of chronic conditions marked by an inability to pay attention, hyperactivity and a tendency to engage in impulsive acts. Children with ADHD often struggle academically and may have difficulty establishing friendships and other relationships. As a result, they may develop poor self-esteem.

There are three different forms of ADHD that affect children. They are:

• Predominantly inattentive ADHD. Marked by difficulty paying attention.
  
  
• Predominantly hyperactive/impulsive ADHD. Marked by difficulty controlling behavior.
  
  
• Combined ADHD. Combines symptoms of the other two forms of ADHD. It is the most common form of ADHD.

Between 3 and 5 percent of school-aged children – or about 2 million children in the United States – have ADHD, according to the National Institute of Mental Health (NIMH). Boys are significantly more likely to be diagnosed with ADHD, but the disorder affects girls as well.

The exact cause of ADHD remains unknown, although scientists now believe that changes in brain structure are a leading cause of the disease. Heredity and exposure to certain environmental chemicals also appear to play a role in ADHD. Poor parenting or disruptions at home or school cannot cause ADHD, although they may exacerbate the condition.

Diagnosis of ADHD usually involves a variety of tests and interviews with parents, teachers and other adults who can describe aspects of the child’s behavior that may indicate ADHD.