What are the differential diagnoses for syncope?

Differential Diagnoses for Syncope

Syncope has three major categories of causes: reflex (neurally-mediated) syncope accounting for approximately 66% of cases, orthostatic hypotension (10%), and cardiac syncope (16% combined from arrhythmias and structural disease), with non-syncopal attacks representing the remaining 6%. 1

Primary Diagnostic Categories

1. Reflex (Neurally-Mediated) Syncope - Most Common (66%)

This is the predominant cause and includes:

• Vasovagal syncope (VVS) - the classic "fainting" response
• Situational syncope - triggered by specific circumstances (coughing, micturition, defecation)
• Carotid sinus hypersensitivity - particularly in elderly patients

Key distinguishing features: Autonomic activation symptoms including pallor, sweating, nausea, and a prodrome of light-headedness. The blood pressure drop in VVS starts several minutes after standing and accelerates until syncope occurs, making it short-lived compared to classical orthostatic hypotension. 2

2. Orthostatic Hypotension (10%)

This category has important subtypes that require different management:

Classical OH: Sustained BP reduction (≥20 mmHg systolic or ≥10 mmHg diastolic) within 3 minutes of standing. The BP drop starts immediately and may be sustained for many minutes. 2, 3

Initial (Immediate) OH: Transient BP decrease within 15 seconds of standing with presyncope or syncope. 3

Delayed OH: BP reduction taking >3 minutes to develop, with slow progressive decrease. The absence of bradycardia differentiates this from reflex syncope. Common in elderly patients with stiffer hearts and impaired compensatory reflexes. 2

Neurogenic OH: A subtype due to autonomic failure, often associated with Parkinsonism or diabetes. 2

Clinical clues: "Coat hanger pain" (neck/shoulder pain), low back pain, or precordial pain suggest classical OH with autonomic failure. Symptoms worsen in morning, with heat, after meals, or post-exertion. 2

3. Cardiac Syncope (16% Total)

A. Primary Arrhythmias (11%) - Most Common Cardiac Cause

• Bradyarrhythmias: Sick sinus syndrome, AV block
• Tachyarrhythmias: Supraventricular or ventricular tachycardia
• Conduction abnormalities: Second-degree heart block, complete heart block

Critical point: In pediatric populations, long QT syndrome, hypertrophic cardiomyopathy, AV nodal reentry tachycardia, and ventricular tachycardia can cause sudden death and require immediate identification. 4

B. Structural Cardiac/Cardiopulmonary Disease (5%)

• Myocardial infarction
• Hypertrophic cardiomyopathy
• Pulmonary embolism - frequently underdiagnosed in hospitalized syncope patients 2
• Aortic dissection

4. Postural Orthostatic Tachycardia Syndrome (POTS)

Primarily affects young women with:

• Marked orthostatic HR increase (>30 bpm or >120 bpm within 10 minutes of standing; >40 bpm in ages 12-19) 2, 3
• Severe orthostatic intolerance without OH
• Associated symptoms: light-headedness, palpitations, tremor, weakness, blurred vision, fatigue
• Often linked to deconditioning, recent infections, chronic fatigue syndrome, joint hypermobility 2

5. Non-Syncopal Attacks (6%)

Must differentiate from true syncope:

• Seizures - convulsive movements can occur in all syncope types due to cerebral hypoxia, leading to misdiagnosis 4
• Hypoglycemia
• Metabolic conditions
• Drug/alcohol intoxication
• Concussion from head trauma
• Pseudosyncope (apparent loss of consciousness)
• Vestibular diseases - can mimic presyncope in adults and elderly 5
• Psychiatric causes 1, 4

Critical Diagnostic Approach

The initial evaluation (history, physical examination, ECG) establishes diagnosis in 50% of cases and identifies potential causes in an additional 8% that require confirmatory testing. 1, 6

Red Flags Requiring Urgent Evaluation:

• Syncope during exertion or while supine
• Family history of sudden cardiac death
• Known structural heart disease
• Abnormal ECG findings
• Age >75 years with cardiac risk factors 7

Common Pitfalls:

1. Misdiagnosing seizures: Convulsive movements from cerebral hypoxia in syncope can mimic epilepsy 4
2. Over-testing: Routine blood tests rarely yield diagnostic information 6
3. Missing pulmonary embolism: Frequently underdiagnosed in hospitalized patients 2
4. Confusing delayed OH with reflex syncope: Look for absence of bradycardia in delayed OH 2

Testing Yield Reality:

• Cardiac monitoring (telemetry/Holter): 3-27% diagnostic yield, but symptomatic correlation during monitoring occurs in only ~4% 6
• TTE: 0-29% overall yield, 8-28% in high-risk groups 7
• Outpatient monitoring: 1-59% overall, 12-42% in high-risk patients 7

Despite extensive testing, syncope remains unexplained in 2-47% of patients, though guideline-based systematic evaluation reduces this to approximately 2%. 1, 6