ADHD is defined by the 'core' signs of inattention, hyperactivity and impulsiveness, according to the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (4th edn, 1994) and is known as 'hyperkinetic disorder' in the 10th revision of the International Classification of Diseases (WHO, Geneva, 1992).
There are three subtypes of ADHD:
1.combined type, with signs of inattention and hyperactivity/ impulsivity;
2. predominantly inattentive type, with inattention, but not hyperactivity/impulsivity;
3. predominantly hyperactive impulsive type, with hyperactivity/impulsivity, but not inattention.
The diagnostic criteria further require that:
- the signs have persisted for at least six months to a degree that is maladaptive and inconsistent with the developmental level of the child;
- there must be clear evidence of clinically significant impairment in social or academic functioning;
- some impairment is present in two or more settings (usually at home and at school);
- some of the signs that caused impairment were present before the age of seven;
- the signs do not occur exclusively during the course of a pervasive developmental disorder, schizophrenia or other psychotic disorder, and are not better accounted for by other mental disorders (such as depression or anxiety).
The diagnosis of hyperkinetic disorder (HKD), sometimes used by UK clinicians, defines a sub-group of ADHD. HKD requires the presence of all three core signs of inattention, hyperactivity and impulsiveness. It also requires that all of the core symptoms were present before the age of seven, are pervasive (present in two or more settings) and cause impairment. HKD is broadly similar to severe combined type ADHD.