Top Causes of Syncope
The three major categories of syncope are neurally-mediated reflex syncope (most common overall), cardiac syncope (arrhythmias and structural disease), and orthostatic hypotension, with vasovagal syncope being the single most prevalent cause in the general population. 1
Primary Categories and Their Relative Frequency
Neurally-Mediated (Reflex) Syncope - Most Common Overall
Vasovagal syncope is the most prevalent form of syncope, characterized by inappropriate vasodilation and bradycardia triggered by emotional stress, fear, pain, blood phobia, or prolonged standing. 1 This typically presents with prodromal symptoms including lightheadedness, dizziness, nausea, diaphoresis, and pallor before loss of consciousness. 1
Situational syncope occurs in specific scenarios: cough/sneeze-induced, gastrointestinal stimulation (swallowing, defecation), micturition (especially post-micturition), post-exercise, and post-prandial syncope. 2, 1
Carotid sinus syncope results from mechanical manipulation of the carotid sinuses triggering the vasovagal reflex, predominantly affecting older adults. 1
Cardiac Syncope - Highest Mortality Risk
Arrhythmias are the most common cardiac causes of syncope, inducing hemodynamic impairment through critical decreases in cardiac output and cerebral blood flow. 2 When arrhythmia is the primary cause, it requires specific treatment regardless of contributing factors. 2
Bradyarrhythmias causing syncope include:
- Sick sinus syndrome with long pauses from sinus arrest or sinoatrial block, most frequently in brady-tachy syndrome. 2
- Severe acquired AV blocks (Mobitz II, high-grade, and complete AV block) are most closely related to syncope. 2
Tachyarrhythmias (both supraventricular and ventricular) can cause syncope depending on heart rate, left ventricular function, posture, and vascular compensation adequacy. 2
Structural cardiac disease includes obstructive valvular disease, acute myocardial infarction/ischemia, obstructive cardiomyopathy, and pulmonary embolus/pulmonary hypertension. 1
Orthostatic Hypotension
Classic orthostatic hypotension is defined as sustained reduction of systolic BP ≥20 mmHg or diastolic BP ≥10 mmHg within 3 minutes of standing. 1
Variants include:
- Initial (immediate) orthostatic hypotension: transient BP decrease within 15 seconds after standing. 1
- Delayed orthostatic hypotension: sustained BP reduction taking >3 minutes of upright posture to develop. 1
Causes encompass:
- Primary autonomic failure syndromes (pure autonomic failure, multiple system atrophy). 2
- Secondary autonomic failure (diabetes, Parkinson's disease). 2
- Drug-induced (vasoactive drugs, diuretics). 2, 1
- Volume depletion. 1
Age-Dependent Patterns - Critical for Risk Stratification
Pediatric and young patients most commonly experience neurocardiogenic syncope, conversion reactions, and primary arrhythmic causes such as long QT syndrome and Wolff-Parkinson-White syndrome. 1
Middle-aged patients experience neurocardiogenic syncope, situational syncope, orthostasis, and panic disorders. 1
Elderly patients have significantly higher frequency of cardiac causes, including obstructions to cardiac output and arrhythmias from underlying heart disease. 1 The incidence increases dramatically after age 70, reaching 19.5 per thousand individuals after age 80. 3
Prognostic Implications
The presence of suspected or certain heart disease after initial evaluation is the strongest predictor of cardiac syncope with 95% sensitivity, while absence of heart disease excludes cardiac syncope in 97% of patients. 1
Annual mortality varies dramatically by etiology:
Common pitfall: The prognosis is determined by underlying etiology, specifically the presence and severity of cardiac disease, not by the syncope episode itself. 3 Therefore, identifying high-risk cardiac patients is the primary goal of evaluation, as these patients require immediate intervention to reduce mortality and prevent sudden cardiac death. 2, 4