What is Syncope?
Syncope is defined as a transient loss of consciousness due to global cerebral hypoperfusion characterized by rapid onset, short duration, and spontaneous complete recovery. 1
Pathophysiology
Syncope occurs when the brain is temporarily deprived of blood flow. A 35% reduction in cerebral blood flow or complete disruption of cerebral perfusion for 5-10 seconds is sufficient to cause syncope 2. The underlying mechanisms can be classified into three basic categories:
- Vasomotor instability - decreased vascular resistance, reduced venous return, or both
- Reduced cardiac output - from pump failure, mechanical obstruction, or arrhythmias
- Neurologically reduced cerebral perfusion - from cerebrovascular disease or vasospasm 2
Classification of Syncope
Syncope can be categorized into several types based on pathophysiology:
1. Neurally-Mediated (Reflex) Syncope
- Vasovagal syncope (common faint) - classical or non-classical
- Carotid sinus syncope
- Situational syncope - triggered by specific scenarios:
- Acute hemorrhage
- Cough, sneeze
- Gastrointestinal stimulation (swallow, defecation, visceral pain)
- Micturition (post-micturition)
- Post-exercise
- Post-prandial
- Others (e.g., brass instrument playing, weightlifting) 2
2. Orthostatic Hypotension
- Primary autonomic failure syndromes
- Secondary autonomic failure syndromes
- Drug-induced orthostatic syncope
- Volume depletion (hemorrhage, diarrhea, Addison's disease) 2
3. Cardiac Syncope
- Arrhythmias (sinus node dysfunction, AV conduction disorders, tachyarrhythmias)
- Structural heart disease (valvular disease, cardiomyopathy, myocardial infarction)
- Other cardiovascular conditions (pulmonary embolism, aortic dissection) 2, 1
Clinical Presentation
Syncope has distinctive features that help differentiate it from other causes of loss of consciousness:
- Prodromal symptoms may include lightheadedness, weakness, headache, blurred vision, diaphoresis, nausea, and vomiting 3
- Duration is typically brief (10-30 seconds) 1
- Recovery is spontaneous and complete, without prolonged confusion 2
- Post-syncopal symptoms may include persisting drowsiness, headache, dizziness, and nausea 3
Epidemiology
Syncope is a common medical problem that:
- Accounts for up to 3% of emergency department visits
- Represents approximately 6% of hospital admissions
- Shows a trimodal distribution peaking around ages 20,60, and 80
- Affects approximately one-third of the population at least once during their lifetime 2, 1, 3
Distinguishing Features by Type
Cardiac Syncope
- Often occurs during exertion or in supine position
- May be preceded by palpitations or chest pain
- Associated with structural heart disease or arrhythmias
- Carries higher mortality risk 1
Reflex (Neurally-Mediated) Syncope
- Often has clear prodromal symptoms
- May be triggered by emotional distress, pain, or blood phobia
- Generally has favorable prognosis 1, 4
Orthostatic Syncope
- Occurs after rising from supine or sitting position
- Blood pressure decreases without compensatory heart rate increase
- Associated with autonomic dysfunction 3
Important Distinctions
It's crucial to distinguish syncope from other conditions that may cause transient loss of consciousness but are not due to cerebral hypoperfusion:
- Seizures
- Metabolic disorders (hypoglycemia, hypoxia)
- Intoxication
- Transient ischemic attacks
- Psychogenic pseudosyncope 2, 1
Syncope is specifically characterized by its transient nature with complete recovery, distinguishing it from prolonged states of altered consciousness like coma 2.
The prognosis and management approach vary significantly based on the underlying cause, with cardiac syncope carrying the highest risk of adverse outcomes 1, 4.