A 17-year-old boy is brought to the emergency department after passing out. The patient was standing behind the dugout watching his friends play baseball when he suddenly felt warm and light-headed. Then he remembers awakening on the ground. Bystanders reported that the patient was unresponsive for approximately 2 minutes. He feels fatigued but does not have chest pain or shortness of breath. The patient has no significant medical history and does not use alcohol or drugs. Temperature is 36.7 C (98.1 F), supine blood pressure is 105/60 mm Hg and standing blood pressure is 110/65 mm Hg, pulse is 84/min, and respirations are 16/min. Pulse oximetry is 100% on room air. The patient is awake and alert. Pupils are equal and reactive. Heart and lung examinations are normal. The abdomen is soft and nontender. Neurologic examination is normal. ECG is unremarkable. Which of the following is the most likely cause of this patient's syncope?

Question

A 17-year-old boy is brought to the emergency department after passing out.  The patient was standing behind the dugout watching his friends play baseball when he suddenly felt warm and light-headed.  Then he remembers awakening on the ground.  Bystanders reported that the patient was unresponsive for approximately 2 minutes.  He feels fatigued but does not have chest pain or shortness of breath.  The patient has no significant medical history and does not use alcohol or drugs.  Temperature is 36.7 C (98.1 F), supine blood pressure is 105/60 mm Hg and standing blood pressure is 110/65 mm Hg, pulse is 84/min, and respirations are 16/min.  Pulse oximetry is 100% on room air.  The patient is awake and alert.  Pupils are equal and reactive.  Heart and lung examinations are normal.  The abdomen is soft and nontender.  Neurologic examination is normal.  ECG is unremarkable.  Which of the following is the most likely cause of this patient's syncope?

Quizplus · Accepted Answer

11ec160b_3ca5_25cb_9249_61fd9c00a2bb_MD0001_00 This patient has experienced vasovagal syncope, a type of reflex syncope. Patients with vasovagal syncope typically have a trigger (eg, prolonged standing) and experience a prodrome (eg, warmth, light-headedness) that immediately precedes loss of consciousness. The episodes usually have a short duration (eg, 1-2 min) and typically involve a rapid recovery. When vasovagal syncope is repeatedly triggered by a certain activity (eg, coughing, micturition), it is referred to as situational syncope. Reflex syncope is the result of inappropriate autonomic output from the brainstem in response to signals relayed by various baroreceptors or mechanoreceptors. Vasovagal syncope is often triggered by cortical emotions that increase sensitivity to afferent signals, but it can also be triggered situationally by increased afferent transmission from pulmonary, gastrointestinal, or genitourinary mechanoreceptors. Once these afferent signals are received by the brainstem cardioregulatory center, some combination of the following effector responses occurs: Cardioinhibitory response: Increased parasympathetic stimulation to the sinoatrial and atrioventricular (AV) nodes slows the heart rate. In addition, decreased sympathetic output to the myocardium decreases myocardial contractility. Vasodepressor response: Decreased sympathetic output to vascular smooth muscle causes vasodilation of venous capacitance veins (ie, decreased cardiac preload) and systemic arterioles (ie, decreased perfusion pressure) (Choice C). The result of these responses is a brief but profound decrease in cardiac output that leads to hypotension, decreased cerebral perfusion, and syncope. (Choices A and D) Cardiac syncope can occur due to the marked reduction in cardiac output that results from arrhythmia (eg, ventricular tachycardia, AV nodal reentrant tachycardia). Increased automaticity of the Purkinje fibers can trigger ventricular tachycardia, but this usually affects patients with ischemic or structural heart disease. AV nodal reentrant tachycardia usually affects young patients, but rarely reduces cardiac output enough to cause syncope. In addition, cardiac syncope of any etiology would not be associated with a vagal prodrome. (Choice E) Severe left ventricular outflow tract obstruction (eg, aortic stenosis, hypertrophic cardiomyopathy [HCM]) can cause cardiac syncope. Systolic anterior motion of the mitral valve can worsen outflow obstruction in patients with HCM, but syncope resulting from left ventricular outflow obstruction is typically exertional, and this patient has no systolic murmur to suggest HCM. Educational objective: Vasovagal syncope typically involves a trigger (eg, prolonged standing), a prodrome (eg, warmth, light-headedness), and rapid recovery (eg, within 1-2 min). It results from a transient, autonomic nervous system-mediated cardioinhibitory and vasodepressor response. __________ References: Syncope in children and adolescents: evaluation and treatment. (http://www.ncbi.nlm.nih.gov/pubmed/20084125) Reflex syncope in children and adolescents (http://www.ncbi.nlm.nih.gov/pubmed/15310717)

A 17-Year-Old Boy Is Brought to the Emergency Department After