What causes vasovagal syncope?

Causes of Vasovagal Syncope

Vasovagal syncope is primarily caused by an inappropriate reflex response that triggers vasodilation and bradycardia, leading to decreased cerebral blood flow and temporary loss of consciousness. 1

Primary Mechanisms

• Vasovagal syncope occurs when a triggered reflex causes inappropriate vasodilation and bradycardia, resulting in systemic hypotension and cerebral hypoperfusion 1
• The contribution of vasodilation (vasodepressor effect) and bradycardia (cardioinhibitory effect) varies between individuals and episodes 1, 2
• Cerebral blood flow reduction below critical levels (associated with systolic BP falling below 60 mmHg) leads to loss of consciousness 1
• The vasovagal reflex may serve as a protective "defense mechanism" against sympathetic overactivity to protect the heart 3

Trigger Factors

Emotional Triggers

• Fear, pain, instrumentation, and blood phobia are common emotional triggers 1
• Emotional distress activates neural pathways that initiate the vasovagal reflex 1
• Painful or emotionally stressful situations such as anxiety or fear can precipitate the reflex 4

Orthostatic Stress

• Prolonged standing or orthostatic stress is a major trigger for vasovagal syncope 1
• Orthostatic vasovagal syncope typically occurs after a period of stable cardiovascular control during upright posture, followed by sudden decrease in blood pressure and heart rate 4

Situational Triggers

• Specific situations that can trigger the vasovagal reflex include:
  • Coughing or sneezing 1
  • Gastrointestinal stimulation (swallowing, defecation, visceral pain) 1
  • Micturition (particularly post-micturition) 1
  • Post-exercise 1
  • Post-prandial (after eating) 1
  • Other activities (laughing, brass instrument playing, weightlifting) 1

Other Triggers

• Carotid sinus stimulation (spontaneous or during carotid sinus massage) 1
• Nicotine can trigger the vasovagal reflex through several pathways 2
• Trigeminal or glossopharyngeal neuralgia can be associated with neurally mediated syncope 4

Clinical Presentation

• Prodromal symptoms often precede loss of consciousness, including:
  • Lightheadedness, dizziness, and blurred vision 2, 4
  • Nausea and diaphoresis (sweating) 2, 4
  • Pallor 1, 2
  • Weakness 4
• Quick recovery is typical, though often followed by fatigue 2, 5
• Atypical presentations may occur, particularly in older adults, with short or absent prodrome 6

Pathophysiological Considerations

• The vasovagal reflex appears to be a non-pathological trait rather than a disease in typical cases 3
• Cerebral blood flow decrease sometimes precedes the drop in blood pressure and heart rate during tilt testing 3
• The final common pathway is global cerebral hypoperfusion, resulting in transient loss of consciousness 5
• Autonomic cardiovascular modulation is often reduced during episodes 4

Distinguishing Features

• Vasovagal syncope must be differentiated from other causes of syncope:
  • Cardiac arrhythmias (which cause decreased cardiac output irrespective of circulatory demands) 1
  • Structural heart disease (when circulatory demands exceed the heart's ability to increase output) 1
  • Orthostatic hypotension without reflex component 2
  • Non-syncopal conditions that mimic syncope (seizures, metabolic disorders, psychogenic pseudosyncope) 1

Understanding these mechanisms is crucial for proper diagnosis and management of vasovagal syncope, which despite being generally benign, can significantly impact quality of life when recurrent 6, 5.