Expansion
Reflex, orthostatic and cardiac causes, all reducing cerebral perfusion
Expansion
All syncope results from a transient fall in cerebral perfusion; about 6 to 8 seconds of interrupted flow is enough.
Reflex (neurally mediated) - the commonest
- Vasovagal: emotion, pain, prolonged standing; preceded by nausea, sweating and pallor
- Situational: cough, micturition, defecation, swallowing
- Carotid sinus hypersensitivity
Orthostatic
- Hypovolaemia, drugs, autonomic failure
Cardiac - the most dangerous
- Arrhythmia: bradyarrhythmia, complete heart block, ventricular tachycardia
- Structural: aortic stenosis, hypertrophic obstructive cardiomyopathy, pulmonary embolism, tamponade
Red flags pointing to a cardiac cause are syncope on exertion, syncope while supine, absence of warning, associated chest pain or palpitations, an abnormal ECG, and a family history of sudden death.
Reflex syncope characteristically has a prodrome and occurs while upright, and rapid recovery on lying flat is typical because it restores venous return.