Syncope is a sudden loss of consciousness and postural tone caused by transient decreased cerebral blood flow; it is associated with spontaneous recovery. The occurrence of syncope in the general population, as reflected in the Framingham Study, is 3.0% in men and 3.5% in women in the general population.1 As a general rule, the incidence of syncope increases with age. In the United States, 1 to 2 million patients are evaluated for syncope annually, and 3% to 5% of emergency department visits and 1% to 6% of urgent hospital admissions are for syncope.2-5 As a result, management of syncope is associated with significant resource use and expense.6-8
Syncope can occur suddenly, without warning, or may be preceded by a prodrome of presyncope, including lightheadedness, dizziness but not true vertigo, nausea, a feeling of warmth, diaphoresis, and blurred or tunnel vision. Self-limited episodes of presyncope can occur in the absence of loss of consciousness.
The causes of syncope include cardiovascular disorders, disorders of vascular tone or blood volume, and cerebrovascular disorders. The relative incidence of these categories varies with the clinical site from which the patients are selected; in hospitalized patients, syncope is most often a result of a cardiovascular disorder, whereas in the emergency room, other causes of syncope predominate.4 In many cases, the cause of syncope may be multifactorial. Furthermore, in up to 50% of cases, the cause of syncope cannot be determined with certainty even after a rigorous evaluation.
Recent studies document the widely divergent mortality risks associated with an episode of syncope, ranging from episodes that are benign to cardiac arrhythmias that are potentially lethal.9 Syncope caused by cardiovascular disorders is associated with the highest risk for mortality, approaching 50% over 5 years and 30% in the first year after diagnosis.4 Furthermore, among patients with certain cardiac diseases, including hypertrophic cardiomyopathy, long QT syndrome, and others, those with syncope are at greater risk for mortality.10 The mortality rate is lower among patients with syncope from other causes (30% over 5 years and <10% in the first year) but still substantial. Syncope that is not associated with cardiac disease and is of undetermined cause is usually associated with the lowest mortality risk (6%-10% over 3 years and 24% over 5 years).2,4,7 Syncope can impact quality of life for patients and their families, particularly when it occurs abruptly without warning and is recurrent or when it is likely to occur in relationship to certain activities. In such cases, patients may need to adjust their lifestyle or change occupation.
For prognostic and therapeutic reasons, it is important to distinguish syncope from other causes of transient loss of consciousness, including seizures, psychogenic seizures, hypoglycemia, pharmacologic agents, and trauma. In some cases, this may prove difficult because reduced cerebral blood flow associated with syncope can cause tonic-clonic movements similar to those that occur with certain seizures. In one study, syncope had been misdiagnosed as seizures in 38% of patients who continued to have episodes despite adequate anticonvulsant therapy.11
Syncope can occur from ...