What are the common causes of near syncope?

Common Causes of Near Syncope

Near syncope is most commonly caused by reflex syncope (vasovagal), orthostatic hypotension, or cardiac conditions that temporarily reduce cerebral blood flow without causing complete loss of consciousness. 1

Pathophysiology

Near syncope occurs when cerebral blood flow is temporarily reduced but not enough to cause complete loss of consciousness. This typically happens when cerebral blood flow decreases by less than 35% (compared to the >35% reduction needed for complete syncope) 2.

Major Categories of Near Syncope

1. Reflex (Neurally-Mediated) Causes

• Vasovagal near syncope - Most common form, triggered by:

  • Emotional stress, fear, pain, medical settings
  • Prolonged standing
  • Heat exposure
  • Characterized by prodromal symptoms: diaphoresis, warmth, nausea, pallor 1

• Situational near syncope - Associated with specific triggers:

  • Coughing, laughing, sneezing
  • Micturition (particularly post-micturition)
  • Defecation
  • Swallowing
  • Post-exercise
  • Post-prandial (after eating) 1

• Carotid sinus hypersensitivity - More common in older patients, can be triggered by:

  • Neck turning
  • Tight collars
  • Shaving 1

2. Orthostatic Causes

• Classical orthostatic hypotension - Drop in systolic BP ≥20 mmHg or diastolic BP ≥10 mmHg within 3 minutes of standing 1

  • Primary or secondary autonomic failure
  • Drug-induced (antihypertensives, vasodilators, diuretics)
  • Volume depletion (dehydration, hemorrhage)
  • Advanced age

• Initial orthostatic hypotension - Transient BP decrease within 15 seconds after standing 1

• Delayed (progressive) orthostatic hypotension - BP reduction developing after 3 minutes of standing 1

• Postural Orthostatic Tachycardia Syndrome (POTS) - Sustained increase in heart rate ≥30 bpm within 10 minutes of standing (≥40 bpm in ages 12-19) 1

3. Cardiac Causes

• Arrhythmias

  • Bradyarrhythmias (sick sinus syndrome, AV block)
  • Tachyarrhythmias (ventricular or supraventricular)

• Structural heart disease

  • Aortic stenosis or other valvular disease
  • Hypertrophic cardiomyopathy
  • Cardiac tumors
  • Pulmonary embolism
  • Acute myocardial infarction 1

4. Other Causes

• Cerebrovascular disorders - Vertebrobasilar insufficiency, subclavian steal syndrome 1

• Metabolic disorders - Hypoglycemia, hypoxia 3

• Medications - Antihypertensives, vasodilators, diuretics, antidepressants, antipsychotics 1, 4

• Psychogenic near syncope - Anxiety disorders, panic attacks, hyperventilation 1

Clinical Features of Near Syncope

Near syncope presents with prodromal symptoms without complete loss of consciousness:

• Lightheadedness or dizziness
• Blurry or tunnel vision
• Nausea
• Sensation of warmth
• Diaphoresis and pallor
• Weakness
• Palpitations 1, 3

Risk Stratification

High-risk features that warrant more urgent evaluation:

• Age >60 years with cardiovascular disease
• Known structural heart disease or heart failure
• Abnormal ECG
• Exertional symptoms
• Family history of sudden cardiac death
• No prodromal symptoms 1, 2

Low-risk features:

• Age <45 years without cardiovascular disease
• Clear vasovagal or situational trigger
• Prolonged prodrome with typical symptoms
• Recurrent episodes with similar presentation 1

Management Approach

For acute management of presyncope:

1. Have the person assume a safe position (sitting or lying down)
2. Use physical counterpressure maneuvers:
   • Lower-body maneuvers: leg crossing with muscle tensing, squatting
   • Upper-body maneuvers: arm tensing, isometric handgrip, neck flexion 1

Common Pitfalls

1. Failure to distinguish near syncope from other conditions - Seizures, hypoglycemia, and intoxication can present similarly but require different management

2. Overlooking cardiac causes - Cardiac causes of near syncope carry the highest mortality risk and should be ruled out, especially in older patients or those with known heart disease

3. Medication-induced near syncope - Always review medications, particularly in older adults on multiple drugs

4. Assuming benign cause in young patients - While reflex syncope is common in young people, potentially dangerous causes like long QT syndrome or hypertrophic cardiomyopathy should be considered, especially with exertional symptoms or family history of sudden death

5. Missing orthostatic hypotension - Check orthostatic vitals within the first 3 minutes of standing, as delayed orthostatic hypotension may be missed with earlier measurements 1, 2