Causes of Attention Deficit Hyperactivity Disorder (ADHD)
ADHD has a multifactorial etiology with complex genetic and environmental risk factors that act together to increase susceptibility, with no single factor being necessary or sufficient to cause the disorder. 1, 2
Genetic Factors
- ADHD has high heritability, with genetic factors playing a significant role in its development 2, 1
- Multiple genetic variants with small individual effects contribute to ADHD susceptibility rather than a single gene 1, 3
- Specific candidate genes associated with ADHD include those involved in dopamine regulation (DRD2, DBH, COMT), serotonin pathways (HTR1E, HTR2A, TPH2), and synaptic function (SNAP25) 4
- Novel genetic associations have been identified in genes such as CDH13 and GFOD1, with CDH13 also implicated in substance use disorders 4
- ADHD symptoms are commonly observed in several genetic syndromes including Fragile X Syndrome, Neurofibromatosis 1, DiGeorge Syndrome, Tuberous Sclerosis Complex, Turner Syndrome, Williams Syndrome, and Klinefelter Syndrome 5
Neurobiological Factors
- Structural brain differences are evident in ADHD, with affected children showing significantly smaller brain volumes compared to unaffected controls 1, 3
- The prefrontal cortex, basal ganglia, and cerebellum are differentially affected in ADHD 3, 6
- Reduced connectivity in white matter tracts in key brain areas has been observed 3
- Dopamine dysregulation plays a central role in ADHD neurobiology, as evidenced by genetic, pharmacological, imaging, and animal models 3, 6
- Patients with ADHD demonstrate executive function deficits in multiple cognitive domains including:
- Visuospatial and verbal working memory
- Inhibitory control
- Vigilance
- Planning
- Reward regulation 2
Environmental Risk Factors
- Prenatal and perinatal risk factors have been identified as contributing to ADHD development 6
- Environmental factors interact with genetic predisposition to influence ADHD expression 1, 3
- No single environmental factor is sufficient to cause ADHD, but multiple factors may increase risk 1
Clinical Implications
- ADHD is a chronic neurodevelopmental condition that persists into adulthood in many cases, with an estimated prevalence of 2.5% in adults 2, 1
- The disorder affects approximately 5% of children and adolescents worldwide 1
- ADHD increases the risk of other psychiatric disorders, educational and occupational failure, accidents, criminality, social disability, and substance use disorders throughout an individual's lifetime 1
- Comorbid conditions are common in ADHD patients, including:
- Emotional or behavioral conditions (anxiety, depression, oppositional defiant disorder, conduct disorders, substance use)
- Developmental conditions (learning and language disorders, autism spectrum disorders)
- Physical conditions (tics, sleep disorders) 2
- The presence of comorbid conditions may alter the treatment approach for ADHD 2
Diagnostic Considerations
- ADHD diagnosis requires symptoms to be persistent, more severe than typically observed at comparable developmental levels, and cause clinically significant impairment 7
- Symptoms must be present in multiple settings (e.g., school/work and home) 7
- The specific etiology of ADHD is unknown, and there is no single diagnostic test 7
- Adequate diagnosis requires comprehensive evaluation using medical, psychological, educational, and social resources 7
Treatment Implications
- Understanding the complex etiology of ADHD helps inform treatment approaches 3, 6
- While there are no curative treatments for ADHD, evidence-based interventions can significantly reduce symptoms and associated impairments 1
- Medications, particularly stimulants, are highly effective in addressing the neurobiological aspects of ADHD 1, 3
- Drug treatment should always be part of a comprehensive plan that includes psychosocial, behavioral, and educational interventions 3