Vascular syncope occurs when a stimulus causes venous pooling in the legs, leading to a decrease in ventricular preload with a compensatory increase in heart rate and myocardial contractility. After fainting, return of consciousness occurs while lying on the floor because gravity no longer contributes to venous pooling of blood. Neurally mediated syncope (NMS) or reflex syncope occurs when receptors in the atria, ventricles, and pulmonary arteries sense a decrease in venous return, and an efferent brainstem response via the vagal nerve causes bradycardia, hypotension, or both. Because the pathophysiology of NMS is related to abnormal circulatory control, this form of syncope is often grouped with other forms of vascular syncope, such as orthostasis and hypovolemia.
Cardiac syncope occurs when there is an interruption of cardiac output from an intrinsic cardiac problem. These causes are divided into tachydysrhythmias, bradydysrhythmias, outflow obstruction, and myocardial dysfunction (Table..127-1).
Any event that causes sufficient cerebral hypoperfusion can lead to sudden death. The most common causes are seizures, cardiac diseases, and metabolic diseases. Little is known regarding the most common dysrhythmias that cause sudden death in children, because such cardiopulmonary arrests are unwitnessed. In children, bradycardic or asystolic arrests are thought to be most common, especially in children younger than 1 year of age, but in older children ventricular fibrillation is also seen.11 Overall in children, however, the incidence of ventricular fibrillation as the presenting cardiac dysrhythmia is much lower than that in adults. —i2
Syncope is characterized by a sudden onset of falling with a brief episode of loss of consciousness. Other associated symptoms or signs are usually related to the etiology for the syncopal event. Two-thirds of children experience a prodrome of light-headedness or dizziness prior to the event, 4 but vertigo is much less common. Involuntary motor movements occur with all types of syncopal events but are somewhat more common with seizures.4 Factors related to more serious causes of syncope are outlined in Table 121-2. Events that may mimic syncope in children are listed in T§bIe„„12Z,-3,.
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TABLE 127-2 Risk Factors for a Serious Cause of Syncope
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