What are the differential diagnoses for syncope?

Differential Diagnoses for Syncope

Syncope has three major etiologic categories: reflex (neurally-mediated), orthostatic hypotension, and cardiac causes, with the critical distinction being that cardiac syncope carries significantly higher mortality risk and requires immediate evaluation. 1

Primary Classification Framework

The differential diagnosis must be organized systematically based on mechanism of cerebral hypoperfusion:

1. Reflex (Neurally-Mediated) Syncope - Most Common (~75% of cases)

This represents the largest and most benign category 1:

• Vasovagal syncope: Triggered by emotional stress, pain, prolonged standing, or crowded/hot environments
• Situational syncope: Cough, micturition, defecation, swallowing, post-exercise 2
• Carotid sinus syncope: Triggered by neck manipulation or tight collars (primarily in older adults)

Clinical pearl: These typically have prodromal symptoms (lightheadedness, nausea, diaphoresis, tunnel vision) and lack of post-event confusion 3.

2. Orthostatic Hypotension - Common in Elderly and Medicated Patients

Defined as sustained BP drop ≥20 mmHg systolic or ≥10 mmHg diastolic within 3 minutes of standing 1:

• Primary autonomic failure: Pure autonomic failure, Parkinson's disease, multiple system atrophy
• Secondary causes:
  • Volume depletion (hemorrhage, diarrhea, dehydration)
  • Medications (antihypertensives, diuretics, vasodilators, alcohol)
  • Prolonged bed rest/deconditioning
  • Addison's disease 2

Subtypes to recognize 1:

• Initial OH: BP drop within 15 seconds of standing
• Classic OH: Sustained drop within 3 minutes
• Delayed OH: Drop occurring >3 minutes after standing

3. Cardiac Syncope - Highest Mortality Risk (2-6% of cases)

This is the critical category requiring urgent evaluation 2, 4:

Arrhythmic Causes:

• Bradyarrhythmias: Sick sinus syndrome, high-grade AV block, pacemaker malfunction
• Tachyarrhythmias: Ventricular tachycardia, supraventricular tachycardia, inherited channelopathies (Long QT, Brugada, CPVT)

Structural/Obstructive Causes:

• Aortic stenosis, hypertrophic cardiomyopathy
• Acute myocardial infarction/ischemia
• Cardiac tamponade, constrictive pericarditis
• Pulmonary embolism, pulmonary hypertension
• Atrial myxoma or other cardiac tumors 2
• Congenital coronary anomalies

Red flags for cardiac syncope 4:

• Syncope during exertion or while supine
• No prodrome or very brief warning
• Family history of sudden cardiac death
• Known structural heart disease
• Abnormal ECG findings

Critical Differential Diagnoses (Non-Syncope TLOC)

These conditions mimic syncope but have different mechanisms 1, 3:

Neurological Causes:

• Seizures: Distinguished by prolonged post-ictal confusion, lateral tongue biting, gradual recovery 3
• Vertebrobasilar TIA: Associated with focal neurological symptoms
• Subclavian steal syndrome: Rare in children, presents with arm exercise 2

Metabolic/Toxic:

• Hypoglycemia
• Hypoxia
• Hyperventilation with hypocapnia
• Carbon monoxide intoxication 2
• Drug/alcohol intoxication 1

Psychiatric:

• Pseudosyncope (psychogenic): No actual loss of consciousness, eyes typically closed, prolonged duration, no injury despite frequent falls 1, 2

Key Diagnostic Discriminators

The absence of post-ictal confusion is the single most powerful factor distinguishing syncope from seizures 3. However, common pitfalls include:

• Myoclonic jerks occur in 90% of prolonged syncope and do NOT indicate seizure 3
• Incontinence and head injury occur in both syncope and seizures 3
• Brief tonic posturing can occur in syncope due to cerebral hypoxia 3

Risk Stratification Context

Syncope during exercise is high-risk and mandates cardiac evaluation, while post-exercise syncope is typically reflex-mediated 5. The emergency department population represents higher-risk patients compared to primary care settings 6, with most ED syncope patients meeting high-risk criteria requiring admission rather than direct discharge 6.

Age-Specific Considerations

• Children/adolescents: Neurocardiogenic causes account for ~15% of cases 2; cardiac causes are rare (2-6%) but must be excluded 2
• Older adults (>75 years): Higher prevalence of orthostatic hypotension, polypharmacy effects, and cardiac causes 1

Common pitfall: Attributing syncope to "seizure" based solely on witnessed jerking movements—this leads to inappropriate antiepileptic therapy when the actual mechanism is cerebral hypoperfusion 3.