What causes Attention Deficit Hyperactivity Disorder (ADHD)?

What Causes ADHD

ADHD has a multifactorial etiology with complex but high heritability, involving genetic factors, neurodevelopmental issues, and environmental influences that affect brain structure and function. 1, 2

Genetic Factors

• ADHD is highly heritable with multiple genetic variations contributing to risk
• Twin studies demonstrate 70-80% heritability, making genetics the strongest contributing factor 3
• No single gene is responsible; rather multiple genetic variations with small individual effects act together 2, 4
• Family history is a significant risk factor, with parents who have ADHD more likely to have children with the condition 1

Neurobiological Factors

Brain Structure and Function

• Structural imaging studies show that brains of children with ADHD are significantly smaller than unaffected controls 4
• Specific brain regions affected include:
  • Prefrontal cortex (executive function)
  • Basal ganglia (motor control and learning)
  • Cerebellum (coordination and regulation)
  • Reduced connectivity in white matter tracts 4
• Functional neuroimaging reveals abnormalities in:
  • Frontostriatal circuits
  • Frontoparietal networks
  • Ventral attention networks 1

Neurotransmitter Dysregulation

• Dopamine and norepinephrine dysregulation play central roles in ADHD pathophysiology 4, 5
• These neurotransmitters modulate brain areas involved in attention and behavior regulation 5
• Evidence for neurotransmitter involvement comes from:
  • Response to medications that target these systems
  • Genetic studies of dopamine-related genes
  • Neuroimaging studies showing altered dopamine activity 4, 6

Environmental Risk Factors

• Prenatal and perinatal factors:
  • Maternal smoking during pregnancy
  • Alcohol exposure in utero
  • Prematurity and low birth weight 6
  • Severe brain injuries 2
• Other environmental factors:
  • Exposure to environmental toxins
  • Neuroinflammation
  • Consanguineous marriages 2

Cognitive and Executive Function Deficits

• ADHD involves deficits in multiple cognitive domains:
  • Visuospatial and verbal working memory
  • Inhibitory control
  • Vigilance and sustained attention
  • Planning abilities
  • Reward regulation 1
• These deficits correspond to the observed structural and functional brain abnormalities

Clinical Implications

• Understanding the complex etiology helps explain:
  • The heterogeneity of ADHD presentations
  • Variable response to treatments
  • High rates of comorbidities with other psychiatric and developmental conditions 1
• The chronic nature of ADHD, with symptoms often persisting into adulthood, reflects its neurobiological basis 1, 5

Important Caveats

• ADHD is not caused by poor parenting, excessive sugar intake, or too much screen time, though these factors may exacerbate symptoms
• The diagnosis requires symptoms to be developmentally inappropriate and present in multiple settings 3
• Symptoms must cause significant functional impairment and not be better explained by another condition 3
• Cultural factors can affect perception and reporting of symptoms, requiring culturally sensitive assessment 3

Understanding ADHD as a neurobiological condition with genetic underpinnings helps reduce stigma and supports appropriate treatment approaches that address the underlying neurobiological mechanisms.