Attention deficit hyperactivity disorder (ADHD) is a developmental condition of inattention and distractibility, with or without accompanying hyperactivity. In the past, various terms were used to describe this condition, including hyperactive syndrome and, from the Diagnostic and Statistical Manual of Mental Disorders, Third Edition (DSM-III), “minimal brain dysfunction.” In the revised DSM-III, this condition was renamed ADHD. In the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV-TR), adults or children must have had an onset of symptoms before age 7 years that caused significant social or academic impairment. More recently, attention has focused on adult forms of ADHD, which probably have been underdiagnosed.
The pathology of ADHD is not clear. Psychostimulants (which facilitate dopamine release) and noradrenergic tricyclics used to treat this condition have led to speculation that certain brain areas related to attention are deficient in neural transmission. PET scan imaging indicates that methylphenidate acts to increase dopamine.1 The neurotransmitters dopamine and norepinephrine have been associated with ADHD.
The underlying brain regions predominantly thought to be involved are frontal and prefrontal; the parietal lobe and cerebellum may also be involved. In one functional MRI study, children with ADHD who performed response-inhibition tasks were reported to have differing activation in frontal-striatal areas compared to healthy controls. Adults with ADHD also have been reported to have deficits in anterior cingulate activation while performing similar tasks.
A PET scan study by Volkow et al revealed that in adults with ADHD, depressed dopamine activity in caudate and preliminary evidence in limbic regions was associated with inattention and enhanced reinforcing responses to intravenous methylphenidate. This concludes that dopamine dysfunction may be involved with symptoms of inattention but may also contribute to substance abuse comorbidity.2
Individuals with ADHD have inhibition impairment, which is difficulty stopping their responses.3
Incidence in school-age children is estimated to be 3-7%.
In Great Britain, incidence is reported to be less than 1%. The differences between the US and British reported frequencies may be cultural (“environmental expectations”) and due to the heterogeneity of ADHD (ie, the many etiological paths to get to inattention/distractibility/hyperactivity). Furthermore, the International Classification of Diseases, 10th Revision (ICD-10) criteria for ADHD used in Great Britain may be considered stricter than the DSM-IV-TR criteria. However, other studies suggest that the worldwide prevalence of ADHD is between 8% and 12%.
No clear correlation with mortality exists in ADHD. However, studies suggest that childhood ADHD is a risk factor for subsequent conduct and substance abuse problems, which can carry significant mortality and morbidity.
ADHD may lead to difficulties with academics or employment and social difficulties that can profoundly affect normal development. However, exact morbidity has not been established.
In children, ADHD is 3-5 times more common in boys than in girls. Some studies report an incidence ratio of as high as 5:1. The predominantly inattentive type of ADHD is found more commonly in girls than in boys.
In adults, the sex ratio is closer to even.
ADHD is a developmental disorder that requires an onset of symptoms before age 7 years. After childhood, symptoms may persist into adolescence and adulthood, or they may ameliorate or disappear.
The percentages in each group are not well established, but at least an estimated 15-20% of children with ADHD maintain the full diagnosis into adulthood. As many as 65% of these children will have ADHD or some residual symptoms of ADHD as adults.
The prevalence rate in adults has been estimated at 2-7%.
The 3 types of attention deficit hyperactivity disorder (ADHD) are (1) predominantly hyperactive, (2) predominantly inattentive, and (3) combined. The DSM-IV-TR criteria are as follows4:
Inattention – Must include at least 6 of the following symptoms of inattention that must have persisted for at least 6 months to a degree that is maladaptive and inconsistent with developmental level:
Often fails to give close attention to details or makes careless mistakes in schoolwork, work, or other activities
Often has difficulty sustaining attention in tasks or play activities
Often does not seem to listen to what is being said
Often does not follow through on instructions and fails to finish schoolwork, chores, or duties in the workplace (not due to oppositional behavior or failure to understand instructions)
Often has difficulties organizing tasks and activities
Often avoids or strongly dislikes tasks (such as schoolwork or homework) that require sustained mental effort
Often loses things necessary for tasks or activities (school assignments, pencils, books, tools, or toys)
Often is easily distracted by extraneous stimuli
Often forgetful in daily activities
Hyperactivity/impulsivity – Must include at least 4 of the following symptoms of hyperactivity-impulsivity that must have persisted for at least 6 months to a degree that is maladaptive and inconsistent with developmental level:
Hyperactivity evidenced by fidgeting with hands or feet, squirming in seat
Hyperactivity evidenced by leaving seat in classroom or in other situations in which remaining seated is expected
Hyperactivity evidenced by running about or climbing excessively in situations where this behavior is inappropriate (in adolescents or adults, this may be limited to subjective feelings of restlessness)
Hyperactivity evidenced by difficulty playing or engaging in leisure activities quietly
Impulsivity evidenced by blurting out answers to questions before the questions have been completed
Impulsivity evidenced by showing difficulty waiting in lines or awaiting turn in games or group situations
Onset is no later than age 7 years.
Symptoms must be present in 2 or more situations, such as school, work, or home.
The disturbance causes clinically significant distress or impairment in social, academic, or occupational functioning.
Disorder does not occur exclusively during the course of a pervasive developmental disorder, schizophrenia, or other psychotic disorder and is not better accounted for by mood, anxiety, dissociative, or personality disorder.
Numeric codes indicating type based on criteria (adapted from DSM-IV-TR) are as follows:
314.00 ADHD: Predominantly inattentive type if inattention criterion is met for the past 6 months, but hyperactivity/impulsivity criterion is not met
314.01 ADHD: Predominantly hyperactive/impulsive type if hyperactivity/impulsivity criterion is met for the past 6 months, but inattention criterion is not met
314.01 ADHD: Combined type if both inattention and hyperactivity/impulsivity criteria are met for past 6 months (Note that this code is the same as that used for the predominantly hyperactive type.)
314.9 ADHD not otherwise specified (NOS): Other disorders with prominent symptoms of attention-deficit or hyperactivity-impulsivity that do not meet DSM-IV-TR criteria
No physical findings have been well correlated with ADHD.
Mental Status Examination may note the following:
Appearance: Most often, appointments are difficult to structure and maintain due to hyperactivity and distractibility. Children with ADHD may present as fidgety, impulsive, and unable to sit still, or they may actively run around the office. Adults with ADHD may be distractible, fidgety, and forgetful.
Affect/mood: Affect usually is appropriate and may be elevated, but it should not be euphoric. Mood usually is euthymic, except for periods of low self-esteem and decreased (dysthymic) mood. Mood and affect are not primarily affected by ADHD, although irritability may frequently be associated with ADHD.
Speech/thought processes: Speech is of normal rate but may be louder due to impulsivity. Thought processes are goal-directed but may reflect difficulties staying on a topic or task. Evidence of racing thoughts or pressured speech should not be present. These symptoms are more consistent with a manic state (bipolar disorder).
Hallucinations or delusions: Not present.
Thought content/suicide: Content should be normal, with no evidence of suicidal/homicidal or psychotic symptoms.
Cognition: Concentration and storage into recent memory are affected. Patients with ADHD may have difficulty with calculation tasks and recent memory tasks. Orientation, remote memory, or abstraction should not be affected.
ADHD is associated with a number of other clinical diagnoses. Studies have a demonstrated that many individuals have both ADHD and antisocial personality disorder (ASD).5 These individual are at higher risk for self-injurious behaviors. ADHD is also linked to addictive behavior. The more severe the symptoms of ADHD, the greater the use of tobacco, alcohol, and marijuana.6 Some individuals have both ADHD and an autism spectrum disorder.7
Parents and siblings of children with ADHD are 2-8 times more likely to develop ADHD than the general population, suggesting that ADHD is a highly familial disease.
Concordance of ADHD in monozygotic twins is greater than in dizygotic twins, suggesting some contribution of genetics. Studies estimate the mean heritability of ADHD to be 76%, indicating that ADHD is one of the most heritable psychiatric disorders.
The involved genes or chromosomes are not definitively known. Vulnerability to ADHD may be due to many genes of small effect. For example, several genes that code for dopamine receptors or serotonin products, including DRD4, DRD5, DAT, DBH, 5-HTT, and 5-HTR1B, have been moderately associated with ADHD.
ADHD risk is significantly increased in the presence of one risk allele in genes DRD2 (OR=7,5), 5-HTT (OR=2,7), and DAT1 (OR=1,6). ADHD risk is significantly increased at homozygotes for risk alleles in genes DRD2 (OR=54,8), 5-HTT (OR=6,7), and DAT1 (OR=6,6).8
Studies of cognitive deficits reveal another facet to the genetic contributions to ADHD.9
Hypotheses exist that include in utero exposures to toxic substances, food additives or colorings, or allergic causes. However, diet, especially sugar, is not a cause of ADHD.
How much of a role family environment has in the pathogenesis of ADHD is unclear, but it certainly may exacerbate symptoms.