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.
Box 48-1 Differential Diagnosis of Presentation for “Syncope”
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.
Situational Syncope
The mechanism of situational syncope is similar to that of vasovagal syncope. Some physicians classify it separately, however, because it occurs only in the setting of a few classic triggers, most of which involve an exaggerated vagal stimulus, including micturition, defecation, coughing, swallowing, hair combing, or pain. In adolescents with new-onset situational syncope, pregnancy and acute or chronic blood loss should be considered in the differential diagnosis.
Orthostatic Syncope
As already described, the appropriate physiologic response to standing is systemic vasoconstriction. Orthostatic hypotension and syncope occur when there is a disruption in this response immediately upon sitting (from lying down) or standing, resulting in a decrease in systolic blood pressure by greater than 20 or 10 mm Hg within 3 minutes of changing position. This response can be idiopathic or exacerbated by medications, such as diuretics, other antihypertensives, and vasodilators.
Postural Orthostatic Tachycardia Syndrome
Postural orthostatic tachycardia syndrome (POTS) is defined as reproducible, exaggerated tachycardia (change >30 beats/min) within 10 minutes of standing associated with modest systemic hypotension. This is likely caused by a collection of disorders, all of which lead to a decrease in cerebral blood flow in the context of lower extremity or splanchnic blood pooling. Although much is still unknown about the cause of this condition, some studies suggest autoimmune or autonomic triggers.
Dysautonomia
Dysautonomia is a rare cause of syncope in children. When it does occur, it is typically part of a much larger presentation that includes severe orthostatic hypotension, temperature dysregulation, fatigue, bowel and bladder dysfunction, or pain disproportionate to the examination.
Breath-Holding Spells
Breath-holding spells typically occur in children age 6 to 24 months. They are classically triggered by emotional insults, such as anger, pain, or fear. Children cry, then forcibly exhale, and seemingly “forget” to inhale. Cyanosis ensues followed by transient loss of consciousness. Brief posturing or tonic-clonic movements may also occur. Children regain consciousness spontaneously. This condition is typically outgrown by the age of 5 years, and intervention is rarely necessary.
Cardiac Syncope
Although a cardiac cause is found in fewer than 2% of previously healthy children with syncope, it is important to recognize because it can be associated with an increased risk of sudden death. Cardiac syncope results from abrupt decline in cardiac output as the result of obstructive lesions (aortic stenosis, hypertrophic cardiomyopathy [HCM]), myocardial dysfunction (ischemia, cardiomyopathy), or primary arrhythmias (Figure 48-1). Many of these conditions can be asymptomatic until syncope or sudden death occurs. Arrhythmias can also occur secondary to structural heart disease, to myocardial dysfunction, or to postoperative changes in association with congenital heart disease. Any concern for a cardiac syncope warrants a referral to a pediatric cardiologist. Sudden cardiac death (SCD) in the young is estimated to affect between one in 50,000 and one in 200,000 children.
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