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  • Sudden, unexpected, and transient loss of consciousness and postural tone.
  • Spontaneous and full recovery.

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Syncope can be defined as a sudden, transient loss of consciousness and postural tone that fully resolves spontaneously without specific intervention (eg, cardiopulmonary resuscitation [CPR], electrical or chemical cardioversion). The common pathophysiologic mechanism responsible for most syncopal spells is a transient reduction in cerebral blood flow and cerebral hypoperfusion. Reduced cerebral blood flow from cardiovascular and neurocardiogenic causes accounts for most cases in which a diagnosis can be made. Even when cerebral blood flow is normal, a reduced delivery of such essential cerebral nutrients as oxygen and sugar can occasionally cause altered consciousness.

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Syncope is a common condition experienced by up to 50% of adults in a lifetime. It is responsible for about 3% of hospital admissions and 4% of emergency department visits. Physicians are frequently consulted to evaluate this symptom and—more commonly—presyncope, dizziness, or lightheadedness, which may have a similar pathogenesis.

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Syncope has many causes (Table 24–1), most of which have a benign prognosis. Because cardiac causes are associated with greater morbidity and mortality, early recognition of structural heart disease or other cardiogenic causes is important in order to prevent sudden death or injury.

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Table Graphic Jump Location
Table 24–1. Major Causes of Syncope. 
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Cardiac Causes

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Obstruction to Blood Flow

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Any obstructive structural lesion of the left or right side of the heart can critically reduce the cerebral blood flow. Exertional symptoms are common with obstructive lesions because cardiac output does not rise normally with exercise and cerebral perfusion is not maintained. Obstruction to left ventricular outflow occurs with aortic valve stenosis, mitral stenosis, left atrial myxoma, prosthetic aortic or mitral valve dysfunction, and hypertrophic cardiomyopathy. The ventricular arrhythmias that can occur with valvular heart disease may be responsible for both exertional and nonexertional syncope as well as sudden death.

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Lesions that obstruct flow through the right side of the heart include right atrial myxoma, pulmonary stenosis, tricuspid stenosis, pulmonary hypertension, and pulmonary emboli. Limitations to right ventricular outflow diminish the cardiac output and the ability to increase the output with exertion. Exertional syncope is common with severe pulmonary hypertension and severe pulmonic stenosis.

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Some congenital heart diseases, such as tetralogy of Fallot, can cause syncope ...

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