Nejm 2005;352:1004; 2000;343:1856; Lancet 2001;357:348; Ann IM 1997;126:989; 1997;127:76

Cause: Neurocardiogenic syncope; orthostatic hypotension; decreased CO; neurologic disorders (uncommon)

Epidem: Framingham: The incidence of syncope is 6.2/1000 person-yr (Nejm 2002;347:878). 30% ofpts with obstructive hypertrophic cardiomyopathy, 30% ofpts with pulmonary HT, 10-15% ofpts with acute pulmonary embolus, and 25-70% ofpts with sick sinus syndrome have syncope. The overall prevalence of causes of syncope reported in different series is vasovagal, 8-37%; cough/micturation, 1-8%; orthostatic hypotension, 4-10%; medication side effect, 1-7%; neurologic causes, 3-32%; organic heart disease, 1-8%; arrhythmia, 4-38%; and unknown, 13-41%.

Pathophys: Neurocardiogenic syncope: Compensatory response to upright posture is replaced by a paradoxical decrease of sympathetic activity and an increase in parasympathetic (vagal) activity, producing vasodilatation and bradycardia. The hypothesized mechanism is excessive activation and/or hypersensitivity of cardiac mechanoreceptors.

Decreased cardiac output: Aortic, pulmonary, or mitral stenosis; obstructive hypertrophic cardiomyopathy; pulmonary embolus; atrial myxoma; arrhythmias; and tamponade can all produce impairment of cardiac flow and syncope.

Orthostatic hypotension may occur in Shy-Drager syndrome (primary autonomic failure), diabetes, amyloid, SLE, Guillain-Barré syndrome, porphyria, Fabry’s disease, vitamin B₁₂ deficiency, multiple sclerosis, and drugs (phenothizines, MAO inhibitors, TCAs, clonidine, hydralazine, prazosin, guanethidine, α-methyldopa, ACEIs).

Sx: Hx and physical exam identify possible cause of syncope in ˜45% ofpts (Ann IM 1997;126:989).

Si: Syncope occurring with use of an upper extremity raises the possibility of subclavian steal syndrome, as does a pulse pressure difference of > 20 mm Hg between arms. The occurrence of diplopia, dysarthria, or signs of brain stem ischemia suggest possible TIA or basilar artery migraine. Syncope with exertion is seen in AS, MS, pulmonary HT, HCM, and CAD.

Crs: Up to 45% ofpts with severe aortic stenosis will develop syncope. Life expectancy after onset of syncope is 2-3 yr without valve replacement. The overall 1-yr mortality forpts with syncope of cardiac origin is 18-33%.