Clinical meaning
Syncope is transient loss of consciousness (TLOC) due to transient global cerebral hypoperfusion, characterized by rapid onset, short duration, and spontaneous complete recovery. Cerebral blood flow below 25 mL/100g/min (normal ~50) for 6-10 seconds causes LOC. Classification: (1) Reflex (neurally-mediated) syncope (most common, ~60%): vasovagal (emotional stress, prolonged standing, pain → cardioinhibitory and/or vasodepressor response via Bezold-Jarisch reflex), situational (cough, micturition, defecation, deglutition syncope), and carotid sinus hypersensitivity (head turning, tight collar → excessive baroreceptor response). (2) Orthostatic hypotension (~15%): autonomic failure (primary: Parkinson's, MSA, pure autonomic failure; secondary: diabetic neuropathy, amyloidosis), drug-induced (antihypertensives, diuretics, alpha-blockers, nitrates, TCAs), or volume depletion (hemorrhage, dehydration). Defined as SBP drop ≥20 mmHg or DBP drop ≥10 mmHg within 3 minutes of standing. (3) Cardiac syncope (~15-20%): arrhythmic (bradycardia: sick sinus syndrome, high-grade AV block; tachycardia: VT, SVT, torsades de pointes, WPW with AF) or structural (aortic stenosis, HOCM, PE, aortic dissection, cardiac tamponade, atrial myxoma). Cardiac syncope has the highest mortality and must be identified. (4) Neurological (rare cause of TRUE syncope): subclavian steal, vertebrobasilar TIA — most 'neurological' causes (seizure, stroke) are...