What causes syncopal (fainting) episodes and when are they dangerous?

Understanding Syncope: Causes, Dangers, and Benign Presentations

Syncope occurs due to temporary global cerebral hypoperfusion and while many episodes are benign, those associated with structural heart disease or cardiac arrhythmias can be life-threatening and require immediate evaluation. 1

Mechanisms of Syncope

Syncope results from inadequate cerebral blood flow, which can occur through several mechanisms:

1. Reflex (Neurally-Mediated) Syncope 2, 1

   • Vasovagal syncope (most common form)
   • Situational syncope (coughing, micturition, defecation)
   • Carotid sinus hypersensitivity (more common in older adults)

2. Orthostatic Hypotension 2, 1

   • Classical: BP drop ≥20 mmHg systolic or ≥10 mmHg diastolic within 3 minutes of standing
   • Initial: Transient BP decrease within 15 seconds of standing
   • Delayed: Progressive BP reduction after 3 minutes of standing
   • Causes: Autonomic failure, medications, volume depletion, advanced age

3. Cardiac Syncope 2, 1

   • Arrhythmias (bradyarrhythmias, tachyarrhythmias)
   • Structural heart disease (aortic stenosis, hypertrophic cardiomyopathy)
   • Other cardiovascular conditions (pulmonary embolism, aortic dissection)

4. Cerebrovascular Causes 1

   • Vertebrobasilar insufficiency
   • Subclavian steal syndrome

When Syncope is Dangerous

Syncope is particularly concerning in the following circumstances:

1. Presence of Structural Heart Disease 2

   • Syncope with aortic stenosis carries an average survival of only 2 years without valve replacement
   • In hypertrophic cardiomyopathy, syncope is a predictor of sudden death
   • Arrhythmogenic right ventricular dysplasia with syncope has poor prognosis

2. Cardiac Arrhythmias 2, 1

   • Ventricular tachyarrhythmias with syncope indicate higher mortality risk
   • Bradyarrhythmias causing syncope may require pacemaker implantation

3. High-Risk Features 2, 1

   • Age >60 years with cardiovascular disease
   • Known heart failure or structural heart disease
   • Abnormal ECG
   • Exertional symptoms
   • Family history of sudden cardiac death
   • Syncope without warning or prodrome
   • Syncope causing severe injury

4. Specific Circumstances 2

   • Syncope during exercise
   • Syncope in supine position
   • Syncope with chest pain or palpitations
   • Syncope with abnormal vital signs

When Syncope is Benign

Syncope is generally benign in the following scenarios:

1. Reflex Syncope with Clear Trigger 2, 1

   • Vasovagal syncope with typical prodromal symptoms (pallor, sweating, nausea)
   • Situational syncope with clear precipitating factors
   • Presence of recognizable prodrome with gradual onset

2. Absence of Structural Heart Disease 2, 1

   • Normal cardiac examination
   • Normal ECG
   • No history of cardiac disease

3. Specific Patient Characteristics 2, 1

   • Age <45 years without cardiovascular disease
   • Recurrent episodes with similar presentation and benign cause
   • No injury from syncope

4. Orthostatic Hypotension with Clear Cause 2, 1

   • Medication-related with recent dose changes
   • Volume depletion with clear precipitating factor
   • Postural changes with clear temporal relationship

Risk Stratification and Management

The approach to syncope should be guided by risk assessment:

High-Risk Patients (Require Hospitalization) 2

• Suspected or known significant heart disease
• ECG abnormalities suggesting arrhythmic syncope
• Syncope during exercise
• Syncope causing severe injury
• Family history of sudden death
• Syncope with abnormal vital signs

Low-Risk Patients (Can Be Managed Outpatient) 2

• Isolated or rare syncopal episodes
• No evidence of structural heart disease
• Normal baseline ECG
• Clear vasovagal or situational trigger
• Typical prodromal symptoms

Prevention of Recurrent Benign Syncope

For patients with benign vasovagal or orthostatic syncope, physical counterpressure maneuvers can be effective:

1. Physical Counterpressure Maneuvers 2

   • Lower-body maneuvers (leg crossing with muscle tensing, squatting)
   • Upper-body maneuvers (arm tensing, isometric handgrip)
   • Should be initiated at first sign of presyncope

2. Preventive Measures 2, 3

   • Avoid rapid position changes
   • Maintain adequate hydration and salt intake
   • Avoid high room temperatures
   • Mild physical exercise
   • Compression stockings for orthostatic hypotension

Important Caveats and Pitfalls

1. Don't Underestimate Cardiac Causes 2, 1

   • Even a single syncopal episode in a patient with structural heart disease requires thorough evaluation
   • Cardiac syncope carries significantly higher mortality risk than reflex syncope

2. Consider Age-Specific Causes 2, 1

   • Older adults: Carotid sinus hypersensitivity, orthostatic hypotension, and cardiac causes are more common
   • Younger adults: Vasovagal syncope is most common

3. Watch for Injury Risk 2

   • Syncope can lead to fractures, intracranial hemorrhage, and other injuries in 30% of cases
   • Adolescents experiencing syncope may have myoclonic jerks in 90% of cases

4. Be Aware of Medication Effects 2, 1

   • Antihypertensives, vasodilators, diuretics, antidepressants, and antipsychotics can cause syncope
   • Review medication list carefully, especially in older adults

5. Monitor After Vaccination 2

   • Syncope can occur after vaccination, particularly in adolescents and young adults
   • 63% of post-vaccination syncope occurs within 5 minutes, 89% within 15 minutes
   • Observe patients for 15-20 minutes after vaccination to prevent injury from syncopal episodes