What causes vasovagal syncope?

Causes of Vasovagal Syncope

Vasovagal syncope is primarily caused by a reflex response that triggers vasodilation and/or bradycardia, leading to systemic hypotension and cerebral hypoperfusion. 1

Pathophysiological Mechanism

Vasovagal syncope involves a complex reflex pathway that includes:

1. Triggering factors:

   • Emotional triggers: Fear, pain, emotional distress, sight of blood, medical procedures 1
   • Orthostatic stress: Prolonged standing, especially in warm or crowded environments 1
   • Situational factors: Coughing, laughing, swallowing, micturition, defecation 1

2. Reflex pathway activation:

   • Initial sympathetic activation followed by sudden withdrawal
   • Paradoxical activation of vagal (parasympathetic) response
   • Results in inappropriate vasodilation and/or bradycardia 1

3. Hemodynamic consequences:

   • Peripheral vasodilation causing blood pooling
   • Decreased venous return to the heart
   • Reduced cardiac output
   • Systemic hypotension 1, 2

4. Cerebral effects:

   • Decreased systemic arterial pressure leads to cerebral hypoperfusion
   • A drop in cerebral oxygen delivery of as little as 20% can cause lightheadedness
   • Further reduction leads to loss of consciousness 2

Key Physiological Components

• Vasodepressor component: Widespread vasodilation causing decreased peripheral vascular resistance and blood pooling
• Cardioinhibitory component: Bradycardia due to increased vagal tone
• Mixed type: Combination of both vasodilation and bradycardia 1, 3

Predisposing Factors

• Age: Classical vasovagal syncope is more common in younger adults, while atypical presentations are more common in older adults 4
• Hypovolemia: Dehydration, blood loss, diuretic use 1, 2
• Medications: Vasodilators, diuretics, antihypertensives 1
• Alcohol consumption: Causes diuretic effect and peripheral vasodilation 2

Clinical Presentation

Vasovagal syncope typically presents with:

• Prodromal symptoms: Diaphoresis, warmth, nausea, pallor (may be absent in older patients) 1
• Positional relationship: Often occurs with upright posture (standing or seated) 1
• Recovery phase: Often followed by fatigue 1

Important Distinctions

• Classical vs. Non-classical VVS: Classical VVS has clear emotional or orthostatic triggers and typical prodromal symptoms, while non-classical forms may have less obvious presentations 1

• VVS vs. Cardiac Syncope: VVS is neurally mediated, while cardiac syncope is due to arrhythmias or structural heart disease causing decreased cardiac output 1

• VVS vs. Orthostatic Hypotension: Both can occur with standing, but OH is defined by specific BP drops within 3 minutes of standing without reflex bradycardia 1

Clinical Implications

While vasovagal syncope is generally benign in terms of mortality, it can lead to:

• Serious injuries from falls during loss of consciousness 5
• Reduced quality of life in patients with recurrent episodes 6, 4
• Psychological impact including anxiety and depression 6

Understanding the underlying mechanisms of vasovagal syncope is essential for appropriate diagnosis and management, particularly in distinguishing it from more serious causes of syncope that may require specific interventions.