What causes vasovagal syncope?

Causes of Vasovagal Syncope

Vasovagal syncope is primarily caused by a triggered reflex that leads to inappropriate vasodilation and bradycardia, resulting in systemic hypotension and cerebral hypoperfusion. 1, 2

Primary Mechanisms

• Vasovagal syncope occurs when the autonomic nervous system is inappropriately activated, causing both vasodilation (vasodepressor effect) and bradycardia (cardioinhibitory effect), though the contribution of each varies between individuals 1, 2
• The resulting systemic hypotension leads to cerebral hypoperfusion, which when blood flow drops below critical levels causes loss of consciousness 1, 2
• The final common pathway in all forms of syncope is global cerebral hypoperfusion that results from the circulatory system's inability to maintain adequate blood pressure for brain perfusion 3

Common Triggers

• Emotional triggers:

  • Fear, severe pain, emotional distress 1, 2
  • Blood phobia and instrumentation (e.g., blood draws, medical procedures) 1, 2
  • Sudden unexpected unpleasant sights, sounds, or smells 4

• Orthostatic stress:

  • Prolonged standing, especially in warm or crowded environments 1, 2
  • Standing at attention for extended periods 4

• Situational triggers:

  • Coughing or sneezing (cough syncope) 1, 2
  • Gastrointestinal stimulation 1, 2
  • Micturition (urination) 1, 2
  • Post-exercise 1, 2
  • Post-prandial (after eating) 1, 2
  • Defecation 4
  • Swallowing 4

• Other physiological triggers:

  • Carotid sinus stimulation (especially in older adults) 1, 2
  • Nicotine exposure 5

Clinical Presentation

• Prodromal symptoms often precede loss of consciousness:

  • Lightheadedness and dizziness 1, 2
  • Nausea and sometimes vomiting 1
  • Diaphoresis (sweating) 1, 2
  • Pallor (paleness) 1, 2
  • Feeling warm/hot or cold 1
  • Abdominal discomfort 1
  • Blurred vision 1

• Important clinical patterns:

  • Typical vasovagal syncope (more common in young adults) usually has clear triggers and prodromal symptoms 6
  • Atypical vasovagal syncope (more common in older adults) often has short or absent prodrome and amnesia for loss of consciousness 6
  • Quick recovery is characteristic, though often followed by fatigue 5

Age-Related Patterns

• Pediatric and young patients most commonly experience neurocardiogenic (vasovagal) syncope 2
• Middle-aged patients experience neurocardiogenic syncope, situational syncope, and orthostasis 2
• Elderly patients have higher frequency of cardiac causes but can still experience vasovagal syncope, often with atypical presentation 2, 6

Clinical Implications

• Although vasovagal syncope is not associated with mortality, it can cause injuries from falls, reduce quality of life, and is associated with anxiety and depression 7
• Recognition of prodromal symptoms allows for early intervention with physical counterpressure maneuvers to prevent progression to syncope 1
• The multifaceted nature of vasovagal syncope presents challenges in understanding the condition and developing preventative strategies 3

Diagnostic Considerations

• Differentiating vasovagal syncope from other causes requires careful attention to prodromal symptoms, circumstances surrounding the event, and post-event symptoms 2
• Ruling out cardiac arrhythmias or structural heart disease is essential in diagnosing vasovagal syncope 5
• Tilt-table testing is a widely used diagnostic tool, but results should be interpreted in the context of patients' clinical presentations 8