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  • Sudden, unexpected, and transient loss of consciousness.

  • Spontaneous and full recovery.

  • Most common cause is neurally mediated reflex response.

  • Cardiac syncope has high-risk morbidity and mortality.

General Considerations

Syncope is defined as a transient loss of consciousness (TLOC) with a common pathophysiologic mechanism associated with reduction in cerebral blood flow leading to cerebral hypoperfusion. Syncope is common, with a similar incidence in men and women. The lifetime cumulative incidence of syncope is ≥ 35%, with peak prevalence of the first episode between ages 10–35. The incidence increases with age, especially after 70 years, and is bimodal with peaks at 20 and 80 years. It is estimated to account for 1–3% of emergency department (ED) annual visits and up to 6% of hospital admissions in North America and around the world.

Although most potential causes of syncope are benign and self-limited with a low rate of adverse events, some are associated with significant morbidity and mortality including cardiac arrhythmias and structural heart disease. Although many patients never experience a recurrence, others do, and such recurrences can be extremely unpredictable. Management strategies are variable and often inefficient and costly.

Pathophysiology & Etiology

Syncope may be classified as reflex mediated, orthostatic hypotension, and cardiac. Nonsyncopal causes of TLOC are always in the differential diagnosis because of obvious similarity in clinical presentation (Table 16–1). The most common cause, irrespective of age, sex, or comorbidity, is vasovagal syncope. The second most common cause is cardiac syncope. Carotid sinus hypersensitivity–associated syncope and orthostatic hypotension are rare causes of syncope in those under the age of 40. Unfortunately, as many as 50% of cases remain undiagnosed after clinical presentation.

Table 16–1.The Causes of Loss of Consciousness and Their Prevalence

A. Reflex or Neurally Mediated Syncope (NMS)

A heterogeneous group of disorders consisting of vasovagal syncope, situational syncope, carotid sinus hypersensitivity, and others is generally considered as reflex or neurally mediated syncope (NMS). Although the provoking stimuli may differ, they share a common final pathway characterized by hypotension and vasodilatation with relative or absolute bradycardia. This is thought to be related to an abrupt withdrawal of sympathetic tone and increase in parasympathetic ...

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