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48 Syncope

Syncope is defined as a brief, sudden loss of consciousness and postural tone as the result of a decrease in cerebral blood flow. Syncope is reported to occur at least once in 15% to 25% of children by age 18 years. The vast majority of cases are benign in etiology. Syncope, however, can be a result of more ominous conditions and should be evaluated critically.

Etiology And Pathogenesis

Syncope occurs when cerebral blood flow decreases below 30% to 50% of baseline. This decrease may be the result of systemic vasodilation, decreased cardiac output, or both. This results in transient ischemia, causing temporary loss of consciousness and motor tone. Syncope may also be accompanied by brief autonomic movements. Syncope should be distinguished from presyncope (dizziness or lightheadedness without loss of consciousness), vertigo (the sensation of spinning), and syncopal-like events (e.g., seizures, migraines, or conversion disorder). The causes of syncope can be divided into three major categories: neurally mediated, cardiac, and metabolic (Box 48-1). Syncopal-like events are discussed briefly in the following section.

Neurally Mediated Syncope

Neurally mediated syncope (NMS) accounts for more than 80% of cases that present to the emergency department (ED) or primary care physician. Although subtle differences exist among the following subtypes, abnormal regulation of the autonomic nervous system is common to all.

Vasovagal Syncope

The most common cause of NMS is vasovagal syncope, otherwise known as the vasodepressor, neurocardiogenic, or reflex syncope. It is believed to be initiated by an exaggerated response to a sudden decrease in ventricular filling pressure. Standing without movement causes a decrease in venous return because of stasis of blood in the lower extremities. In unaffected individuals, the subsequent reduction in left ventricular filling causes a decrease in signaling from ventricular mechanoreceptors to the brainstem, which stimulates sympathetic activity, causing an increase in heart rate and systemic vasoconstriction. In patients with vasovagal syncope, however, stimulation of ventricular mechanoreceptors results in paradoxic central inhibition of peripheral sympathetic tone, and hence, vasodilation and relative bradycardia. The individual experiences a sudden decrease in cardiac output and cerebral blood flow.

Children typically report a prodrome characterized by lightheadedness, dizziness, nausea, pallor, diaphoresis, and visual and auditory changes. Loss of consciousness typically lasts less than 1 minute. The recovery period extends for 5 to 30 minutes, during which time children report fatigue, dizziness, nausea, and occasionally vomiting. Vasovagal syncope occurs more often in girls. It is most frequently the result of standing for prolonged periods of time, particularly in warm temperatures; rising rapidly from supine or sitting positions; taking hot showers; or emotional stresses such as venipuncture or viewing disturbing images.

Cardiac Syncope

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