Causes of Syncope
Primary Classification
Syncope results from transient global cerebral hypoperfusion, with three main categories: neurally-mediated reflex syncope (most common and benign), cardiac syncope (highest mortality risk), and orthostatic hypotension. 1, 2
Neurally-Mediated (Reflex) Syncope
This represents the most common category with generally benign prognosis 1, 3:
- Vasovagal syncope (common faint): Triggered by emotional stress, pain, prolonged standing, or venipuncture, characterized by inappropriate vasodilation and bradycardia 1, 2
- Carotid sinus syncope: Occurs with mechanical manipulation of carotid sinuses 1, 2
- Situational syncope includes specific triggers 1, 2:
- Acute hemorrhage
- Cough or sneeze
- Gastrointestinal stimulation (swallowing, defecation, visceral pain)
- Micturition (post-micturition)
- Post-exercise
- Post-prandial
- Others (brass instrument playing, weightlifting)
- Glossopharyngeal and trigeminal neuralgia 1
Cardiac Causes (High-Risk Category)
Cardiac syncope carries a 24% one-year mortality rate, making it the most dangerous category requiring urgent evaluation. 2
Arrhythmias
- Sinus node dysfunction (including bradycardia/tachycardia syndrome) 1
- Atrioventricular conduction system disease 1
- Paroxysmal supraventricular and ventricular tachycardias 1
- Inherited syndromes: Long QT syndrome, Brugada syndrome 1
- Implanted device malfunction (pacemaker, ICD) 1
- Drug-induced proarrhythmias 1
Structural Cardiac/Cardiopulmonary Disease
- Obstructive cardiac valvular disease (note: syncope involves multiple mechanisms including restricted cardiac output, inappropriate neurally-mediated reflex vasodilation, and primary arrhythmias) 1
- Acute myocardial infarction/ischemia 1
- Obstructive cardiomyopathy 1
- Atrial myxoma 1
- Acute aortic dissection 1
- Pericardial disease/tamponade 1
- Pulmonary embolus/pulmonary hypertension 1
Orthostatic Hypotension
Autonomic Failure
- Primary autonomic failure syndromes: Pure autonomic failure, multiple system atrophy, Parkinson's disease with autonomic failure 1
- Secondary autonomic failure syndromes: Diabetic neuropathy, amyloid neuropathy 1
- Post-exercise and post-prandial 1
Other Orthostatic Causes
- Drug and alcohol-induced orthostatic syncope 1
- Volume depletion: Hemorrhage, diarrhea, Addison's disease 1
Cerebrovascular (Rare)
Important Pathophysiologic Considerations
A systolic blood pressure drop to 60 mmHg or a 20% decrease in cerebral oxygen delivery is sufficient to cause loss of consciousness. 1
Multiple mechanisms frequently contribute to syncope in individual patients, particularly with structural heart disease where restricted cardiac output may combine with inappropriate neurally-mediated reflex vasodilation and primary arrhythmias. 1, 2
Aging and comorbidities increase syncope risk through diminished cerebral blood flow, shifted autoregulatory ranges (hypertension), and altered chemoreceptor responsiveness (diabetes) 1.
Non-Syncopal Conditions (Must Differentiate)
These do NOT result from cerebral hypoperfusion 1:
- Disorders with partial/complete loss of consciousness: Seizure disorders 1
- Disorders resembling syncope without consciousness impairment: Psychogenic pseudosyncope, somatization disorders, falls 1
Treatment of Syncope
Risk Stratification (Critical First Step)
High-risk features requiring hospitalization include: 2
- Suspected or known significant heart disease
- ECG abnormalities suggesting arrhythmic syncope
- Syncope during exercise
- Syncope causing severe injury
- Family history of sudden death
Treatment by Category
Neurally-Mediated Syncope
Non-Pharmacological (First-Line)
- Avoid rapid positional changes from supine to standing 4
- Avoid high room temperatures and situations inducing peripheral vasodilation 4
- Increase sodium and fluid intake 4
- Mild physical exercise 4
- Postural counter-maneuvers 4
Pharmacological Options
- Beta-blockers: First-line for symptomatic suppression and arrhythmic risk reduction 2
- Mineralocorticoids (fludrocortisone) 4
- Vasoconstrictor agents (ephedrine, midodrine) 4
- Adenosine receptor blockers (theophylline) 4
- Anticholinergic agents (scopolamine, disopyramide) 4
- Serotonin reuptake inhibitors (fluoxetine, sertraline) for specific cases 4
Cardiac Syncope
Arrhythmia Management
- Beta-blockers for suppression of symptomatic ventricular extrasystoles 2
- Amiodarone for recurrent ventricular tachycardia/fibrillation when beta-blockers insufficient 2
- Verapamil for idiopathic left ventricular fascicular tachycardia 2
Catheter Ablation (Class I Indication)
Strongly recommended for: 2
- Ventricular extrasystoles triggering recurrent ventricular fibrillation with ICD discharges
- Electrical storm due to ventricular extrasystole triggers
- Left ventricular fascicular tachycardia in experienced centers
- Ventricular extrasystole-induced cardiomyopathy (burden >10,000-20,000/day with ventricular dysfunction)
ICD Implantation
Recommended for: 2
- Survivors of cardiac arrest due to ventricular fibrillation/tachycardia with LVEF ≤40%
- Secondary prevention in symptomatic sustained ventricular tachycardia
Structural Heart Disease Treatment
- Valve repair/replacement for valvular disease 2
- Revascularization for ischemic heart disease 2
- Surgical myectomy or alcohol septal ablation for obstructive cardiomyopathy 2
- Surgical removal of cardiac masses 2
Orthostatic Hypotension
Volume Expansion
- Desmopressin (V2-receptor specific vasopressin analogue) increases intravascular volume 4
- Erythropoietin improves anemia and augments blood pressure and cerebral oxygenation 4
Post-Prandial Hypotension Specific
- Octreotide (somatostatin analogue) 4
- Prostaglandin inhibitors (indomethacin, ibuprofen) 4
- Metoclopramide 4
- Two cups of coffee per day 4
Vascular Steal Syndromes
- Direct corrective angioplasty or surgery 2
Critical Pitfalls to Avoid
Cardiac pacemakers are often recommended without adequate indication—ensure proper diagnostic workup before placement. 4
Avoid unnecessary neurological testing (EEG, head CT/MRI) unless specific neurological signs are present or head trauma occurred. 2, 5
Laboratory testing and neuroimaging have low diagnostic yield and should only be ordered if clinically indicated. 3