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Syncope is a symptom complex that is composed of a brief loss of consciousness associated with an inability to maintain postural tone that spontaneously and completely resolves without medical intervention. It is distinct from vertigo, seizures, coma, and states of altered consciousness.

Syncope accounts for approximately 1% to 2% of ED visits each year and up to 6% of hospital admissions.1,2 It is estimated that one in four people will faint during their lifetime, it is often recurrent and can affect the young and the old, with the elderly having the highest incidence and risk for morbidity.3,4 Near-syncope, a premonition of syncope without loss of consciousness, shares the same basic pathophysiologic process as syncope and likely carries the same risks.

The final common pathway of syncope is the same regardless of mechanism. A lack of blood flow or vital nutrient delivery to both cerebral cortices or to the brainstem reticular activating system for 10 to 15 seconds will lead to loss of consciousness and postural tone. A reduction of cerebral perfusion by 35% or complete disruption for 5 to 10 seconds will cause symptoms. Most commonly, an inciting event causes a drop in cardiac output, which decreases oxygen and substrate delivery to the brain.5 Less commonly, vasospasm or other alterations in flow reduce central nervous system blood flow. The reclined posture assumed after syncope, the response of autonomic autoregulatory centers, or reversion to a perfusing cardiac rhythm will reestablish cerebral perfusion, leading to a spontaneous return of consciousness.

The causes of syncope are numerous, from common benign disorders to life-threatening processes (Table 56-1). In the Framingham Heart Study, 7814 patients were followed for 17 years, and 822 reported syncope. The causes determined in the study were: vasovagal (often called reflex mediated) (21%), cardiac (10%), orthostatic (9%), medication related (7%), seizure (5%), neurologic (4.1%), and unknown (37%).6 In most studies even with exhaustive evaluation the unknown cause remains around 40%.7,8 After limited ED investigation the unknown proportion may be 50% to 60% at the time of discharge.9,10 If a diagnosis can be made, it is important, as each diagnostic classification carries with it prognostic risk. For example, in the Framingham study those with documented heart disease and syncope had twice the rate of death of patients without syncope, and those with syncope with a neurologic cause were 50% more likely to die. Those with an unknown cause also had a significant increased risk of death of 30%, whereas those with neurally mediated or vasovagal syncope had a lower risk of death than the general population cohort of the study.

Table 56-1 Causes of Syncope

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