What is reflex syncope?

Reflex Syncope: Definition and Mechanisms

Reflex syncope refers to a heterogeneous group of conditions in which cardiovascular reflexes that normally control circulation become intermittently inappropriate in response to a trigger, resulting in vasodilation and/or bradycardia, leading to a fall in arterial blood pressure and global cerebral hypoperfusion. 1

Types and Classification

Reflex syncope can be classified based on:

Efferent Pathway Involvement

• Vasodepressor type: Hypotension predominates due to loss of upright vasoconstrictor tone 1
• Cardioinhibitory type: Bradycardia or asystole predominates 1
• Mixed type: Both vasodilation and bradycardia mechanisms are present 1

Trigger Mechanisms (Afferent Pathway)

• Vasovagal syncope (VVS): The most common form of reflex syncope 1

  • Mediated by emotional distress (fear, pain, blood phobia) 1, 2
  • Mediated by orthostatic stress (prolonged standing) 1, 2
  • Typically preceded by prodromal symptoms including diaphoresis, warmth, nausea, and pallor 1, 2

• Situational syncope: Associated with specific circumstances 1

  • Cough or sneeze-triggered 1, 3
  • Gastrointestinal stimulation (swallowing, defecation, visceral pain) 1
  • Micturition (post-micturition) 1
  • Post-exercise 1
  • Post-prandial 1
  • Others (laughing, brass instrument playing, weightlifting) 1

• Carotid sinus syncope: 1

  • Triggered by mechanical manipulation of carotid sinuses 1
  • Often diagnosed through carotid sinus massage when no mechanical trigger is found 1

• Atypical forms: Occur with uncertain or apparently absent triggers 1

  • Diagnosis relies less on history and more on exclusion of other causes 1

Pathophysiology

• The fundamental mechanism involves inappropriate reflex activity causing vasodilation and/or bradycardia 1, 4
• These reflexes, normally useful in controlling circulation, become intermittently inappropriate 1
• The result is a fall in arterial blood pressure and global cerebral perfusion 1, 4
• Cerebral blood flow reduction below critical levels leads to loss of consciousness 2
• The contribution of vasodilation and bradycardia varies between individuals and episodes 2, 4

Clinical Presentation

• Prodromal symptoms often precede loss of consciousness: 1, 2

  • Lightheadedness/dizziness
  • Visual disturbances
  • Nausea
  • Diaphoresis (sweating)
  • Pallor
  • Warmth sensation

• Post-syncope features: 1

  • Often followed by fatigue
  • Quick recovery of consciousness

Distinguishing Features from Other Syncope Types

• Unlike orthostatic hypotension, where sympathetic efferent activity is chronically impaired, reflex syncope involves intermittent inappropriate reflex activity 1
• Reflex syncope is classified as noncardiac syncope, distinguishing it from cardiac causes of syncope 1
• Classical vasovagal syncope typically starts in young subjects as an isolated episode 1
• Atypical presentations more commonly occur in older adults with cardiovascular or neurological disorders 1

Clinical Implications

• While generally benign in prognosis, reflex syncope can cause substantial changes in lifestyle and psychological distress 5
• Falls during syncope can result in injury, highlighting the importance of proper management 6
• Education about the benign nature of the condition and avoidance of triggers are cornerstone management strategies 5, 4