What are the different types of syncope?

Types of Syncope

Syncope is classified into three major categories: neurally-mediated (reflex) syncope, cardiac syncope, and orthostatic hypotension, with neurally-mediated syncope being the most common type, accounting for approximately 60% of cases. 1, 2, 3

Neurally-Mediated (Reflex) Syncope

This is the most prevalent form of syncope and includes several distinct subtypes 1, 2:

Vasovagal Syncope

• The classic and most common form, triggered by emotional stress, fear, pain, blood phobia, or prolonged standing 1
• Results from an inappropriate autonomic reflex causing excessive vasodilation and bradycardia, leading to systemic hypotension and cerebral hypoperfusion 4, 1
• Typically preceded by prodromal symptoms including lightheadedness, dizziness, nausea, diaphoresis, and pallor 1, 5
• More common in younger adults compared to elderly patients 2

Situational Syncope

• Triggered by specific scenarios including 1:
  • Cough or sneeze-induced syncope (particularly in smokers with lung disease) 6
  • Micturition syncope (during or after urination) 4, 1
  • Defecation syncope (gastrointestinal stimulation) 4, 1, 5
  • Post-exercise syncope 4, 1
  • Post-prandial syncope (after eating) 4, 1
• Distinguished from classical vasovagal syncope by the specific triggering mechanism rather than emotional or orthostatic stress 6

Carotid Sinus Syncope

• Occurs when mechanical manipulation of the carotid sinuses triggers the vasovagal reflex 1
• More common in older adults 1
• Can be reproduced by carotid sinus massage during diagnostic evaluation 1, 5

Cardiac Syncope

Accounts for approximately 10-23% of syncope cases and carries increased morbidity and mortality risk 2, 7, 8:

Arrhythmic Causes

• Sinus node dysfunction (sick sinus syndrome) 1
• Atrioventricular conduction system disease (heart blocks) 1
• Paroxysmal supraventricular tachycardias 1
• Ventricular tachycardias 1
• More likely in patients with known cardiovascular disease or abnormal electrocardiography 2, 7

Structural Cardiac Disease

• Obstructive cardiac valvular disease (aortic stenosis, mitral stenosis) 1
• Acute myocardial infarction or ischemia 1
• Obstructive cardiomyopathy (hypertrophic cardiomyopathy) 1
• Pulmonary embolus or pulmonary hypertension 1
• Syncope occurring in the supine position or during exertion strongly suggests cardiac etiology 8

Orthostatic Hypotension

Represents approximately 15% of syncope cases 3:

Classic Orthostatic Hypotension

• Defined as sustained reduction of systolic BP ≥20 mmHg or diastolic BP ≥10 mmHg within 3 minutes of standing 1
• Symptoms typically occur just after rising from supine or sitting position 5
• Blood pressure decreases significantly without compensatory increase in heart rate 5

Subtypes by Timing

• 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

Etiologies

• Primary autonomic failure syndromes (pure autonomic failure, multiple system atrophy) 1
• Secondary autonomic failure (diabetes, Parkinson's disease, amyloidosis) 1
• Drug-induced (antihypertensives, diuretics, vasodilators) 1
• Volume depletion (dehydration, blood loss, anemia) 1, 5

Age-Related Patterns

The distribution of syncope types varies significantly by age 1:

• Pediatric and young patients: Predominantly neurocardiogenic syncope, with primary arrhythmic causes (long QT syndrome, Wolff-Parkinson-White syndrome) being important considerations 1
• Middle-aged patients: Neurocardiogenic syncope, situational syncope, orthostasis, and panic disorders 1
• Elderly patients: Higher frequency of cardiac causes (obstructions to cardiac output and arrhythmias from underlying heart disease), carotid sinus hypersensitivity, and orthostatic hypotension 1, 2

Unexplained Syncope

Approximately 10-18% of patients remain without a definitive diagnosis despite comprehensive evaluation 5, 3. Many of these cases likely represent neurally-mediated syncope with atypical presentations 5.

Critical Clinical Distinctions

The presence of suspected or certain heart disease after initial evaluation is the strongest predictor of cardiac syncope (odds ratio 16), with 95% sensitivity 8. Conversely, the absence of heart disease excludes cardiac syncope in 97% of patients 8.

Key Historical Features Distinguishing Cardiac vs. Neurally-Mediated Syncope:

Cardiac syncope indicators 8:

• Syncope in supine position or during exertion
• Blurred vision preceding event
• Convulsive movements during syncope
• Palpitations (especially in patients without heart disease)

Neurally-mediated syncope indicators 8:

• Time between first and last episode >4 years
• Abdominal discomfort before loss of consciousness
• Nausea and diaphoresis during recovery phase

Important Terminology Clarifications

The European Heart Journal guidelines provide specific recommendations on syncope terminology 9:

• "Neurally-mediated syncope" is recognized as an acceptable synonym for "reflex syncope" 9
• "Neurocardiogenic syncope" should be used strictly for reflex syncope where the trigger originates in the heart itself 9
• "Vasodepressor syncope" should be reserved for reflex syncope with documented vasodepressor reflex occurring without reflex bradycardia 9
• "Orthostatic intolerance" should be restricted to describing patient complaints, not as a specific syncope diagnosis 9