Attention-Deficit/Hyperactivity Disorder

ADHD is a neurodevelopmental disorder characterized by persistent patterns of inattention, hyperactivity, and impulsivity that interfere with functioning or development. Symptoms typically appear before age 12 and may persist into adulthood.

Definition

Epidemiology

Affects approximately 5–10% of children worldwide
Boys are more frequently diagnosed than girls (ratio ~2:1 to 3:1)
Symptoms often persist into adolescence and adulthood in 50–60% of cases
Diagnosis prevalence varies by region, assessment methods, and awareness
Comorbidities include learning disabilities, anxiety, depression, and conduct disorders

Etiology

Genetic predisposition: strong heritability (familial clustering, twin studies)
Neurobiological factors: abnormalities in prefrontal cortex, basal ganglia, and dopaminergic/noradrenergic pathways
Perinatal and prenatal factors: prematurity, low birth weight, maternal smoking or alcohol use
Environmental influences: psychosocial stressors, early exposure to toxins
Neurotransmitter dysregulation: dopamine and norepinephrine imbalance
No single causative factor; multifactorial origin involving genes and environment

Pathophysiology

Dysfunction of frontostriatal circuits affecting executive function, attention, and impulse control
Altered dopaminergic and noradrenergic neurotransmission
Delayed cortical maturation in certain brain regions
Impaired regulation of inhibitory control and working memory
Hyperactivity may result from compensatory mechanisms for attentional deficits
Functional imaging shows reduced activity in prefrontal cortex and basal ganglia
Neurodevelopmental delays contribute to behavioral and cognitive symptoms