CHAPTER 73

Syncope

David Atkinson, MD, and Michael Nguyen, DO

Syncope, or fainting, is a transient loss of consciousness and tone; it is a common clinical problem in pediatric patients, particularly during puberty and adolescence. The most common causes of syncope in pediatric patients are benign neurocardiogenic events; however, in rare instances syncope is a harbinger of sudden death from arrhythmia, obstruction of aortic outflow, or other serious cardiovascular events.

The 3 general categories of syncope are neurocardiogenic (also called vasovagal syncope), cardiac syncope, and noncardiac syncope (Box 73.1). The workup for syncope can easily become expensive and time-consuming, and it may provide little information beyond that gleaned by the initial history and physical examination. It is the role of the pediatrician to appropriately direct the evaluation for syncope so that a cost-effective evaluation may occur without missing the patient who may be at risk for a sudden death event.

Epidemiology

Syncope is a temporary, transient loss of consciousness and muscle tone that usually is associated with rapid recovery. It is the result of decreased cerebral blood flow that can occur through many different mechanisms. Syncopal events are very common in the pediatric population; up to 50% of college undergraduates have reported experiencing syncope or near syncope, and it accounts for approximately 1% of all pediatric emergency department visits. Females are more commonly affected than males, and the mean age at presentation is 10 to 12 years. Syncope is uncommon in children younger than 5 years. Many cases of syncope quickly resolve and medical attention is not sought; thus, the true incidence of syncope is almost certainly underestimated.

Clinical Presentation

The clinical presentation of syncope varies with the etiology. Vasovagal syncope often is associated with a prodrome of symptoms, including lightheadedness, visual disturbances, nausea, and diaphoresis. The patient has usually been standing for a long period or has suddenly moved from the supine or sitting position to standing. Other forms of neurally mediated syncope include hair-grooming syncope, which occurs mostly in girls while combing, brushing, or blow-drying their hair. Micturition syncope, although most common in the elderly, may occur in individuals of any age. Younger patients with this type syncope tend to be male; predisposing factors may include reduced food intake, fatigue, alcohol ingestion, and recent respiratory infection. Micturition syncope often occurs at night when voiding after awakening from sleep (ie, while standing immediately after being recumbent). Recurrences of micturition syncope are rare in young patients. Breath-holding spells in toddlers brought on by anger, pain, fear, or frustration may be associated with syncope; this is an infantile form of cardioinhibitory neurally mediated syncope. Infants who experience syncopal breath-holding spells are more likely to grow up and have neurally mediated syncope (see Chapter 52).

Syncope of cardiac etiology often lacks the prodrome of vasovagal syncope. The main cardiac causes of syncope are arrhythmia and left ventricular outflow obstruction. Patients may report palpitations, chest pain, or chest tightness. Cardiac syncope commonly occurs during physical activity and may be accompanied by complete loss of body tone.

Syncope related to seizures generally has a longer recovery time associated with the postictal phase; witnesses may describe the patient as being dazed or “having a blank look on their face” before fainting. These episodes may occur whether the patient is recumbent or upright.

Pathophysiology

Autonomic

Autonomic causes of syncope are the most common etiology, accounting for up to 80% of cases of syncope that come to medical attention. They are also referred to as neurally mediated reflexive syncope, vasovagal syncope, neurocardiogenic syncope, and, in toddlers, pallid breath-holding spells. They have in common disturbances in autonomic control of heart rate and blood pressure in response to postural changes, bodily functions, pain, fear, or other strong emotional events. Vasovagal syncopal events usually occur when the patient is upright, resulting in decreased venous return, decreased arterial blood pressure, and decreased left ventricular volume. The resultant reflex stimulation of vagal fibers results in bradycardia, vasodilation, and worsening hypotension (ie, Bezold-Jarisch reflex). The 3 clinical types of neurally mediated syncope are vasodepressor, which starts and is primarily marked by profound hypotension; cardioinhibitory, which is marked by severe bradycardia or even brief asystole; and mixed response, which is a mixture of both vasodepressor and cardioinhibitory types.

Cardiac

Cardiac causes of syncope are more likely than noncardiac causes to be associated with sudden death than non-cardiac causes, so it is important to identify and treat these abnormalities. Cardiac mechanisms of syncope are mainly related to obstructive lesions or arrhythmias. Obstructive lesions cause decreased ventricular outflow, resulting in decreased cerebral perfusion; arrhythmia may result in decreased ejection volume, also causing cerebral hypoperfusion and syncope.

Left-sided obstructive lesions, including aortic stenosis and hypertrophic cardiomyopathy, are the most likely cause of obstructive syncope. Rarely, syncope is caused by pulmonary stenosis or severe primary pulmonary hypertension, mitral stenosis, atrial myx-oma, or cardiac tamponade. Cardiac syncope, unlike routine vasovagal syncope, often occurs during exertion, because of the inability of the heart to increase cardiac output to meet the demands placed on it by increased physical activity. Increased diastolic pressure caused by an obstructive lesion may also decrease myocardial perfusion, resulting in cardiac ischemia, dyskinesis, or ventricular arrhythmias, thereby further decreasing ventricular output.

Anomalous origin of the coronary arteries is not an obstructive lesion; however, it too may cause syncope with exercise. The left coronary artery may arise from the pulmonary artery, delivering deox-ygenated blood to the left coronary system; alternatively, the left coronary artery may arise from the right coronary cusp and course between the aorta and pulmonary arteries. When the patient is in a high cardiac output state, such as during exercise, the left coronary artery may be compressed between the great arteries, resulting in ischemia or arrhythmia.

Primary arrhythmias causing syncope are a rare but important cause of syncope. Typically, chest radiography, echocardiography, and other imaging modalities are normal, with no evidence of structural heart disease or pulmonary edema. Supraventricular tachycardia may cause syncope or near syncope. In most pediatric patients, the tachycardia is propagated through a concealed pathway, and the resting electrocardiogram (ECG) is normal if the tachycardia is not occurring while the ECG is being obtained. Supraventricular tachycardia may also be associated with Wolff-Parkinson-White syndrome, which itself is characterized by a short P-R interval followed by an abnormally wide QRS complex with an initial delta wave.

Ventricular tachycardia is rare in children with no underlying structural heart disease, but it may be brought on by infection (especially myocarditis or pericarditis), cardiomyopathies, drugs (eg, cocaine, amphetamines), drug interactions (eg, non-sedating antihistamines taken with erythromycin or ketoconazole), and long QT syndrome.

Patients with long QT syndrome have prolonged cardiac repolarization, which usually manifests on the resting ECG as a prolongation of the corrected QT interval. The prolongation of the repolarization period of the heart puts patients with long QT syndrome at risk for torsades de pointes, a malignant form of ventricular tachycardia. The genetic forms of long QT syndrome result from mutations in genes that code for ion transport channels or related proteins. Jervell and Lange-Nielsen syndrome is an autosomal-recessive form of long QT syndrome that is associated with congenital deafness. Autosomal-dominant long QT syndrome that is not associated with congenital deafness has been referred to as Romano-Ward syndrome. Although the clinical diagnosis of long QT syndrome is based on a QTc that is prolonged for the patient’s age, an estimated 20% of patients with a gene mutation associated with long QT syndrome have a normal resting ECG; thus, a critical part of the evaluation of the syncopal patient is obtaining a family history of long QT syndrome, sudden death or near sudden death, seizures, or a history of torsade de pointes. Long QT syndrome also may be brought on by electrolyte imbalance, increased intracranial pressure, or medications (Table 73.1).

Abbreviation: GI, gastrointestinal.

^a For a complete list of drugs to avoid in patients with long QT syndrome, visit www.crediblemeds.org.

Noncardiac

Noncardiac causes of syncope include neurologic etiologies (eg, seizures, migraines), metabolic disturbances (eg, hypoglycemia), hyperventilation (panic attacks or self-induced), and hysteria. Low iron stores have been associated with neurally mediated syncope in both children and adolescents and may provide a partial explanation for the higher incidence of neurocardiogenic syncopal events in adolescent girls than boys. Seizures may be difficult to distinguish from vasovagal events, because both can have tonic-clonic movements. Unlike vasovagal or cardiac syncope, seizures result in unconsciousness secondary to neurologic dysfunction. Syncope associated with hypoglycemia is caused by the insufficient delivery of substrate (glucose) to the brain. Cerebral vasoconstriction secondary to arterial hypocapnia produces syncope with hyperventilation that may be secondary to panic attacks or may be self-induced. The “choking game” or “fainting game” refers to intentional cutting off of oxygen to the brain, with the goal of inducing a “high” or euphoric feeling, usually resulting in syncope and occasionally in serious injury or death. Hysterical syncope lacks a true prodrome, and patients usually suffer no injury when they fall. This form of syncope is thought to be related to Munchausen syndrome.

Differential Diagnosis

The differential diagnosis of syncope is presented in Box 73.1. Events that precede and follow the syncopal event are key in determining the etiology of syncope.

Evaluation

History

A thorough history should be obtained from the family and patient, focusing on the patient’s symptoms, the situation surrounding the event, and the family history (Box 73.2). A primary goal of the history is to identify any underlying cardiac problems, because these patients are at the greatest risk for sudden death.

Physical Examination

The patient with syncope must undergo a thorough physical examination. Physicians should assess the blood pressure to screen for hypotension and hypovolemia. The cardiac examination should include palpation of the chest for the point of maximal intensity, thrills and lifts, and auscultation to assess the intensity of the heart sounds and detect the presence of murmurs or other adventitial sounds. Upper- and lower-extremity pulses should be palpated for their presence and quality. The remainder of the examination should focus on identifying any abnormal neurologic findings.

Laboratory Tests

The history and physical examination should guide which laboratory tests are necessary. Generally, few tests are needed. Serum glucose level shortly after the syncopal event may be abnormally low and reveal hypoglycemia. Fasting glucose or glucose tolerance tests are usually normal, and such testing is not indicated. In pubertal females, a pregnancy test should be obtained as well as a hemoglobin level. Iron levels may be low in patients with neurally mediated syncope.

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