In the spectrum and continuum of developmental-behavioral diagnoses, attention-deficit/hyperactivity disorder (ADHD) represents a dissociation between cognitive abilities and the neurobehavioral domains of attention, impulse control, and motor activity. Individuals with ADHD have developmentally inappropriate levels of inattention and/or impulsivity/hyperactivity (ie, their levels of attention and impulse control/activity are discrepantly delayed or dissociated from their underlying cognitive abilities) that are impairing their functioning across settings.
ADHD is a frequently diagnosed but often misunderstood neurodevelopmental condition. The balance of the core features of inattention and/or hyperactivity and impulsivity combined with the complex array of socialization difficulties, impaired executive functions, and emotional dysregulation make ADHD a great masquerader for a variety of developmental/behavioral and psychiatric diagnoses. Therefore, understanding the many facets of this disorder can enhance diagnostic veracity and allow for a targeted approach to treatment.
ADHD is a disorder that begins in childhood, and symptoms are present from a young age. Several studies have been published on the prevalence of ADHD across populations and cultures, with most citing a prevalence between 4% and 12%. This relatively wide range reflects variations in diagnostic criteria, characteristics of the studied population, the number of sources used to establish the diagnosis, and the lack of biological markers for the disorder. Studies utilizing the diagnostic criteria of any edition of the Diagnostic and Statistical Manual of Mental Disorders have shown prevalence to be closer to 6% to 8%. ADHD is more prevalent in males than females, with a ratio of 3:1. Males are more likely to evidence the externalizing behaviors of hyperactivity and impulsivity (5:1 compared to females) and are therefore more likely to be identified early on, such as in preschool and early elementary school. Females tend to present with inattentive symptoms (2:1 male-to-female ratio) that may go undetected until later in childhood, such as middle to late elementary school. By early adolescence, hyperactivity mostly manifests as fidgetiness, restlessness, or impatience, while inattention persists.
Organic causes of ADHD can be found in 20% to 25% of cases and may include a history of very low birthweight (< 1500 grams), very preterm birth (gestational age < 32 weeks), fetal alcohol spectrum disorders, in utero drug exposure, environmental toxins (eg, lead, nicotine), and central nervous system trauma or infection. In the majority of cases, however, ADHD is thought to be multifactorial in etiology, drawing from genetic and environmental effects on neurobiology. Studies have found inherent differences in the structure and function of several areas of the brain, notably the basal ganglia (contributes to inhibition of automatic responses), the cerebellar vermis (contributes to regulation of motivation and coordination), and the prefrontal cortex of the frontal lobe. The latter in particular controls the executive functions of organization and regulation. Organization functions include attention, “filtering” (attending to relevant while blocking out irrelevant stimuli), planning, sequencing, rule acquisition, abstraction, and cognitive flexibility required for problem solving. Functions of regulation include initiation, inhibition, emotional regulation, monitoring (of internal and external stimuli), moral reasoning, and decision-making. Working memory and processing speed are 2 cognitive functions commonly measured on intelligence batteries that are used as surrogate indicators for executive function.
On a molecular basis, dopamine and norepinephrine imbalances have been implicated in the pathogenesis of ADHD. Both of these neurotransmitters may enhance the inhibitory effects of the frontal cortex on subcortical structures. Evidence also exists for a delay in cortical maturation (particularly in the lateral prefrontal cortex) in individuals with ADHD compared to the general population.
ADHD is polygenic with a significant component of heritability, as demonstrated in identical twin studies with > 50% concordance, and clinically by increased occurrence in first-degree relatives by 2- to 8-fold compared to the general population. Gene associations have been found with the dopamine transporter gene (DAT1), the D4 receptor gene (DRD4), and the human thyroid receptor-beta gene.