What causes pre-syncope?

Causes of Pre-syncope

Pre-syncope is primarily caused by transient cerebral hypoperfusion, resulting from decreased cardiac output or reduced peripheral vascular resistance, which can lead to diminished systemic arterial pressure and cerebral perfusion. 1

Definition and Presentation

• Pre-syncope (near-syncope) refers to a condition in which patients feel as though syncope (fainting) is imminent, but without complete loss of consciousness 1
• Common symptoms include lightheadedness, dizziness, blurred or tunnel vision, nausea, sensation of warmth, weakness, and sweating 1
• Signs may include pallor, diaphoresis, diminished postural tone, and confusion 1

Main Pathophysiological Mechanisms

Cerebral Hypoperfusion

• Cerebral blood flow normally ranges from 50-60 ml/100g tissue/min (12-15% of resting cardiac output) 1
• Pre-syncope occurs when cerebral perfusion decreases but not enough to cause complete loss of consciousness 1
• A decrease in systolic blood pressure to around 60 mmHg is associated with syncope 1

Factors Affecting Cerebral Perfusion

1. Cardiac Output Reduction

   • Decreased venous filling (most important physiological determinant) 1
   • Excessive blood pooling in dependent body parts 1
   • Diminished blood volume/dehydration 1
   • Bradyarrhythmias or tachyarrhythmias 1
   • Valvular heart disease 1
   • Structural heart disease 1

2. Peripheral Vascular Resistance Decrease

   • Widespread excessive vasodilatation (main cause in reflex syncopal syndromes) 1
   • Thermal stress-induced vasodilatation 1
   • Impaired capacity to increase vascular resistance during standing 1
   • Vasoactive medications 1
   • Autonomic neuropathies 1

3. Increased Cerebral Vascular Resistance

   • Low carbon dioxide tension (primary cause) 1
   • Unknown mechanisms in some cases 1

Major Etiological Categories

1. Neurally Mediated (Reflex) Syncope

• Most common type of syncope (21.2% of cases) 1
• Involves sudden decrease in blood pressure and heart rate after period of stable cardiovascular control 2
• Triggers include:
  • Painful or emotionally stressful situations (anxiety, fear) 2
  • Prolonged standing 2
  • Specific situations (micturition, defecation, coughing, sneezing) 2
  • Visceral or carotid sinus stimulation 2
  • Trigeminal or glossopharyngeal neuralgia 2

2. Orthostatic Hypotension

• Accounts for 9.4% of syncopal episodes 1
• Types include:
  • Initial (immediate) - BP decrease within 15 seconds of standing 1
  • Classic - sustained BP reduction within 3 minutes of standing 1
  • Delayed - BP reduction taking >3 minutes to develop 1
  • Neurogenic - due to autonomic nervous system dysfunction 1
• Risk factors include:
  • Use of vasoactive drugs 1
  • Autonomic neuropathies 1
  • Advanced age 1
  • Postprandial state 1

3. Cardiac Causes

• Associated with increased morbidity and mortality 3
• Includes:
  • Arrhythmias (bradyarrhythmias, tachyarrhythmias) 1
  • Structural heart disease 1
  • Coronary artery disease 1
  • Cardiomyopathies 1
  • Aortic disease 1

4. Other Contributing Factors

• Age-related factors (more common in elderly) 1
• Medications (cardiovascular medications responsible for nearly half of episodes in elderly) 1
• Dehydration or diminished blood volume 4
• Post-meal state (postprandial hypotension) 1, 4
• Warm environments 4
• Emotional stress 4

High-Risk Features

• Age >60 years 1
• Male gender 1
• Known underlying cardiac disease 1
• Palpitations prior to episode 1
• Occurrence during exertion 1
• Occurrence in supine position 1
• Low number of prior episodes 1
• Family history of sudden cardiac death 1

Clinical Implications

• Presyncope patients have similar prognosis to syncope patients and should undergo similar evaluation 3, 5
• Among patients with presyncope, approximately 5.1% experience serious outcomes within 30 days 5
• Serious outcomes include death (0.3%), cardiovascular events (3.1%), and non-cardiac events (1.8%) 5
• Emergency physicians often have difficulty predicting which presyncope patients are at risk for serious outcomes 5

Understanding the underlying cause of pre-syncope is essential for appropriate management and prevention of progression to complete syncope, which can result in falls and injuries 1.