CHAPTER 76 ■ SYNCOPE THOMAS B WELCH-HORAN, ROHIT SHENOI INTRODUCTION Syncope is a sudden, brief loss of consciousness and postural tone caused by transient global cerebral hypoperfusion and characterized by complete recovery Presyncope is a feeling of impending sensory and postural changes without loss of consciousness Syncope is a common condition in childhood In the United States, it accounts for about 3% of pediatric emergency department (ED) visits The incidence peaks during the second decade of life, and about 30% to 50% of children experience syncope by the end of adolescence Girls are more commonly affected than boys The most common cause of syncope in children is vasovagal syncope, which is related to a loss of vasomotor tone and is generally benign Occasionally, the etiology may be a life-threatening cardiac condition When evaluating a child who presents to the ED with syncope, the goal is to assess whether high-risk conditions are present, or whether the symptoms can be attributed to a more benign etiology When normal individuals assume an upright position, cardiac output and cerebral arterial blood pressure (BP) are maintained by a combination of mechanical pumping activity of the skeletal muscles on venous return to the right atrium, the presence of one-way valves in the veins that facilitate venous return, arterial vasoconstriction caused by the baroreceptor reflex, and cerebral blood flow autoregulation If stroke volume is not maintained, then reflex sinus tachycardia develops Vasovagal syncope (also known as neurocardiogenic syncope) is believed to begin with excessive peripheral venous pooling that leads to a sudden decrease in peripheral venous return This results in increased cardiac contractility and baroreceptor and left ventricular mechanoreceptor firing, followed by an efferent response consisting of peripheral α-adrenergic withdrawal and enhanced parasympathetic tone The hallmark is vasodilatation and bradycardia with hypotension Sudden activation of a large number of mechanoreceptors in the bladder, rectum, esophagus, and lungs may also provoke such a response In orthostatic hypotension, often caused by fluid depletion, the compensatory responses and ensuing sinus tachycardia are insufficient to maintain brain perfusion, and syncope develops when the patient stands Syncope on exertion suggests a cardiac or cardiopulmonary cause, such as obstruction to left or right ventricular outflow or pulmonary hypertension In