What is the initial approach to a patient presenting with syncope?

Differential Diagnosis of Syncope

Organizing Framework by Mechanism and Mortality Risk

The differential diagnosis for syncope must be organized into three primary categories with cardiac causes representing the highest mortality risk (20-30% one-year mortality) requiring immediate exclusion. 1

Cardiac Syncope (Highest Risk - Requires Immediate Evaluation)

Arrhythmic Causes:

• Bradyarrhythmias: Sinus bradycardia, sinoatrial blocks, 2nd or 3rd degree AV block, bundle branch blocks, and bifascicular block 1
• Tachyarrhythmias: Ventricular tachycardia, supraventricular tachycardia, and inherited channelopathies including Long QT syndrome and Brugada syndrome 1

Structural Heart Disease:

• Severe aortic stenosis, hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy 1
• Severe ostial left main stenosis, acute myocardial infarction 1
• Pulmonary embolism, cardiac tamponade, atrial myxoma 1

Reflex-Mediated (Neurally-Mediated) Syncope (Most Common Overall)

Vasovagal syncope is the most common overall cause, triggered by emotional stress, pain, fear, prolonged standing, and warm crowded environments 1

Situational syncope occurs with specific triggers: micturition, defecation, cough, swallowing, and post-exercise 1

Carotid sinus hypersensitivity particularly affects elderly patients, triggered by neck turning, tight collars, or pressure on the carotid sinus 1

Orthostatic Hypotension

Medication-induced (most common cause): antihypertensives, diuretics, vasodilators, phenothiazines, tricyclic antidepressants, and QT-prolonging agents 1

Volume depletion: hemorrhage, dehydration, anemia 1

Autonomic failure:

• Primary: Parkinson disease, multiple system atrophy 1
• Secondary: diabetic neuropathy, amyloidosis 1

Mandatory Initial Evaluation (All Patients)

Every patient with syncope requires three components that establish diagnosis in 23-50% of cases: 1, 2

History (Most Critical Component)

Circumstances before the event:

• Position: Supine suggests cardiac cause; standing suggests reflex or orthostatic 1
• Activity: Exertional syncope is high-risk and mandates cardiac evaluation 1
• Triggers: Warm crowded places/prolonged standing suggest vasovagal; urination/defecation/cough suggest situational 1

Prodromal symptoms:

• Presence of warning symptoms (nausea, diaphoresis, blurred vision, dizziness) favor vasovagal syncope 1
• Palpitations before syncope strongly suggest arrhythmic cause 1
• Brief or absent prodrome suggests cardiac etiology 1

Witness account:

• Duration of unconsciousness, skin color, movements during event help distinguish syncope from seizure 1
• Rapid, complete recovery without confusion confirms syncope 1

Past medical history:

• Known structural heart disease or heart failure (95% sensitivity for cardiac syncope) 1
• Family history of sudden cardiac death or inherited arrhythmia syndromes 1

Medications: Review antihypertensives, diuretics, QT-prolonging agents 1

Physical Examination

Orthostatic vital signs in lying, sitting, and standing positions (orthostatic hypotension defined as systolic BP drop ≥20 mmHg or to <90 mmHg) 1, 2

Complete cardiovascular examination with attention to heart rate, rhythm, murmurs, gallops, or rubs indicating structural heart disease 2

Carotid sinus massage in patients >40 years (positive if asystole >3 seconds or systolic BP drop >50 mmHg) 1

12-Lead ECG (Mandatory)

Look for specific abnormalities:

• QT prolongation (Long QT syndrome) 1
• Conduction abnormalities (bundle branch blocks, bifascicular block) 1
• Signs of ischemia or prior MI 1
• Bradycardia or tachycardia 1
• Any ECG abnormality is an independent predictor of cardiac syncope and increased mortality 1

Risk Stratification for Disposition

High-Risk Features (Require Hospital Admission)

• Age >60-65 years 1, 2
• Abnormal ECG findings 1, 2
• Known structural heart disease or heart failure 1, 2
• Syncope during exertion or while supine 1, 2
• Brief or absent prodrome 1
• Palpitations before syncope 1
• Family history of sudden cardiac death 1
• Systolic BP <90 mmHg 1

Low-Risk Features (Appropriate for Outpatient Management)

• Younger age with no cardiac disease 1
• Normal ECG 1
• Syncope only when standing 1
• Clear prodromal symptoms 1
• Specific situational triggers 1

Directed Testing Based on Initial Evaluation

Transthoracic echocardiography: When structural heart disease is suspected or abnormal cardiac examination/ECG 1, 2

Continuous cardiac telemetry monitoring: For suspected arrhythmic syncope, abnormal ECG, or palpitations before syncope 1

Exercise stress testing: Mandatory for syncope during or immediately after exertion 1

Tilt-table testing: Consider for young patients without heart disease with recurrent unexplained syncope when reflex mechanism is suspected 1

Tests NOT Routinely Recommended

Brain imaging (CT/MRI): Extremely low diagnostic yield (0.24% for MRI, 1% for CT); order only with focal neurological findings or head trauma 1

EEG: Low diagnostic yield (0.7%); order only with specific neurological features suggesting seizure 1

Carotid ultrasound: Low diagnostic yield (0.5%); not recommended routinely 1

Comprehensive laboratory panels: Low diagnostic yield; order targeted tests only based on clinical suspicion (e.g., hematocrit if hemorrhage suspected, electrolytes if dehydration suspected) 1, 2

Age-Dependent Considerations

Pediatric/Young patients: More likely neurocardiogenic syncope, conversion reactions, primary arrhythmic causes (Long QT, Wolff-Parkinson-White) 1

Middle-aged patients: More likely neurocardiogenic syncope, situational syncope, orthostasis, panic disorders 1

Elderly patients: More likely cardiac causes including obstructions to cardiac output (aortic stenosis, pulmonary embolus), arrhythmias from underlying heart disease, carotid sinus hypersensitivity 1

Common Pitfalls to Avoid

Failing to distinguish true syncope from seizure, stroke, or metabolic causes (syncope has rapid, complete recovery without post-event confusion) 1

Overlooking medication effects as contributors to syncope 1

Ordering brain imaging without focal neurological findings 1

Using Holter monitoring for infrequent events (use event monitors or implantable loop recorders instead) 1

Missing exertional syncope as high-risk requiring immediate cardiac evaluation 1