eMedicine Specialties > Pediatrics: Cardiac Disease and Critical Care Medicine > Cardiology

Syncope

Author: M Silvana Horenstein, MD, Associate in Pediatric and Fetal Cardiac Diagnostic, Diagnostico Gineco-Obstetrico, PC; Associate Director, Legacy Department, Best Doctors, Inc
Coauthor(s): Robert Murray Hamilton, MD, MSc, FRCPC, Section Head, Electrophysiology, Director, High-Risk Hereditary Heart Conditions Clinic, Labatt Family Heart Centre; Professor, Department of Pediatrics, Associate Scientist, Physiology and Experimental Medicine, The Hospital for Sick Children and Research Institute, University of Toronto, Canada
Contributor Information and Disclosures

Updated: Oct 6, 2008

Introduction

Background

Syncope refers to loss of consciousness due to decreased cerebral perfusion to the areas of the brain necessary for consciousness, which include the brainstem reticular activating system and both cerebral cortices. This type of unconsciousness is not caused by electrical disorders of the brain (ie, seizures). Cerebral perfusion can be compromised in patients with decreased cardiac output, loss of vascular tone, or cerebrovascular disease. In children and adolescents syncope rarely indicates the presence of serious cardiovascular disease. In most instances, it simply reflects either an individual or familial predisposition to common faint.

Presyncope refers to feeling faint and lightheaded without losing consciousness.

Pathophysiology

Neuroregulatory syncope

Common faint was previously termed vasovagal syncope but is now usually known as neuroregulatory or neurogenic syncope. Syncope is caused by a sudden decrease in blood pressure, which deprives the brain of sufficient oxygen, temporarily causing dizziness (presyncope) or a brief loss of consciousness (syncope). The Bezold-Jarisch reflex, which is an extreme or overshoot of a normal response to hypotension, is the postulated cause.

Prolonged upright posture results in some degree of pooling of blood in the lower extremities that can lead to diminished intracardiac volume. This phenomenon is accentuated if the individual is dehydrated, as may occur following vigorous exercise, significant sweating, or prolonged restriction of fluid intake. The resultant arterial hypotension is sensed in the carotid body baroreceptors, and afferent fibers from these receptors trigger autonomic signals that increase cardiac rate and contractility.

However, pressure receptors in the wall and trabeculae of the underfilled left ventricle may then sense stimuli, indicating high-pressure C-fiber afferent nerves from these receptors. They may respond by sending signals that trigger paradoxical bradycardia and decreased contractility, resulting in additional and relatively sudden arterial hypotension. The relative autonomic responses are helpful in determining whether the faint is primarily hypotensive (vasodepressor), bradycardic (cardioinhibitory), or mixed (see Media file 1).

Syncope due to dysrhythmias

Abnormalities of cardiac rhythm that result in decreased cardiac output may cause syncope without warning. These dysrhythmias include supraventricular tachycardia (SVT), ventricular tachycardia (VT), ventricular fibrillation, and extreme forms of bradycardia (eg, heart block). An unusual etiology for syncope in pediatric patients with normal cardiac anatomy may be paroxysmal or transient advanced second-degree or third-degree atrioventricular block. In these patients, the sinus node rate does not decrease and even increases during syncopal episodes. These patients may benefit from permanent pacemaker implantation.¹

SVT usually produces some type of warning, such as palpitations, dizziness, or both, before causing syncope. Syncope in the patient with documented pre-excitation (ie, Wolff-Parkinson-White syndrome), with or without previously documented SVT, can be serious, suggesting a risk of sudden death. This can occur if the patient develops atrial fibrillation with resultant rapid conduction over an accessory pathway with a short refractory period. These patients should be treated with a pathway-specific medication (eg, Vaughan-Williams class IC or III antiarrhythmics) until curative radiofrequency catheter ablation therapy can be arranged.

Patients with VT may present with palpitations, dizziness, or both if the VT is monomorphic or nonsustained. Patients may also present with complete loss of consciousness if the VT is rapid or polymorphic. Polymorphic VT or ventricular fibrillation may be secondary to catecholamine-sensitive VT or the congenital long QT syndromes (LQTSs). Intermittent heart block with a slow escape rate can cause sudden syncope, termed Stokes-Adams attack. Treatment of these patients in the absence of irreversible VT consists of permanent pacing.

Hemodynamic causes of syncope

The presence of structural heart disease (subtle or otherwise) may initiate or influence syncopal episodes.

• Hypertrophic obstructive cardiomyopathy may reduce cardiac output because of subaortic muscular obstruction, intermittent coronary artery compression, or diastolic dysfunction (ie, poor ventricular filling) and predisposes patients to dysrhythmias (eg, atrial fibrillation, SVT, VT, heart block).
• Aortic stenosis may also initiate syncope because of diminished cardiac output or coronary ostial obstruction.
• Pulmonary hypertension may cause syncope when the right ventricle fails to pump against the high pulmonary pressure. As a result, cardiac output suddenly decreases, decreasing the blood and oxygen supply to the brain.
• Congenital or acquired coronary abnormalities may be susceptible to intermittent obstruction that results in sudden syncope. For example, ischemic heart disease due to anomalous coronary artery origin, hypercholesterolemia, or acute inflammatory diseases such as Kawasaki disease and myocarditis may predispose to syncope secondary to inadequate cardiac output, arrhythmias, or both.
• The interaction of hemodynamic residua of repaired congenital heart disease with atrial or ventricular dysrhythmias may result in syncope (eg, patients with tetralogy of Fallot or who have undergone Fontan or Mustard repair).
• Pericardial tamponade may cause syncope by decreasing cardial output secondary to atrial filling restriction.

Frequency

United States

According to the Rochester Epidemiologic Project, the incidence of syncope in children and adolescents who sought medical attention from 1987-1991 was 125.8 cases per 100,000 population.²

International

Syncope was reported in 8% of pediatric patients during SVT.³

Mortality/Morbidity

The morbidity and mortality rates of syncope depend on the underlying cause. Essentially, no morbidity is associated with neuroregulatory syncope (ie, common faint).

Sex

The incidence of neuroregulatory syncope was found to be higher in females than in males.²

Age

The incidence of syncope peaks in adolescents aged 15-19 years.²

Clinical

History

The description and circumstances associated with loss of consciousness are important in identifying the underlying cause. A detailed history of syncopal events allows for differentiation between neurocardiogenic and other causes of syncope.

• Common faint or neuroregulatory syncope is reportedly more likely to occur in the morning, particularly after rising, or upon prolonged standing during any time of day.
• A brief warning sensation usually occurs, sometimes accompanied by palpitations, as the individual senses the relative tachycardia that immediately precedes the Bezold-Jarisch reflex. Usually, the person can avoid injury by holding onto a support while collapsing.
• The episodes are often relatively mild and brief but can be more severe, particularly if the person has been held in an upright position during collapse. Secondary seizures are not uncommon and are typically generalized. Following such a secondary seizure, a brief postictal phase may occur.
• More malignant cardiac causes of syncope frequently occur without warning and are more likely to be associated with injury.
• Occasionally, syncope may occur following exercise, particularly if exercise is stopped suddenly rather than ended after a cool-down period. All of these circumstances produce some degree of dehydration.
• Syncope that occurs at the peak of exercise is considered to be a sign of more serious cardiac dysrhythmia or disease.

Physical

Patients with simple faint demonstrate no abnormal physical examination findings. Complete cardiovascular physical examination is warranted. Detection of an abnormality dictates need for further evaluation.

Causes

• Neuroregulatory causes
  • Three forms of neurally mediated syncope have been described, as follows:
    • The vasodepressor form, characterized by severe hypotension with minimal drop in cardiac frequency
    • The cardioinhibitory form, characterized by marked bradycardia that produces hypotension
    • The mixed form, which shares features of both
  • These distinctions may be helpful in guiding treatment. For example, most patients with the vasodepressor form of neurally mediated syncope benefit from increased water and salt intake. If these measures fail, a mineralocorticoid is also prescribed, in which case initial periodic measurements of blood pressure and serum electrolyte levels are recommended).
  • Beta-blockers may be most effective in patients with the cardioinhibitory form of neurally mediated syncope. However, if recurrent syncopal episodes persist, a pacemaker (to pace the heart during extreme bradycardia) may be indicated. Recognize that pacemaker efficacy is less when the vasodepressor component of the neurally mediated syncope is important, such as in the mixed type of neurally mediated syncope.
• Heat exposure: Heat syncope is characterized by dizziness or fainting while standing still in the heat for an extended period. It may also occur immediately following vigorous exercise. This results from cutaneous vasodilation, in which blood pools in the skin and lower extremities, and from volume depletion due to water loss, which results in decreased blood flow to the brain. Treatment consists of rest in a cooler environment. Prevention is based on acclimatization and avoidance of long periods of immobility or vigorous exercise in the heat.
• Micturition: This form of syncope represents 2.4-8.4% of cases of syncope in adult or combined series but is a less frequent cause of syncope in children. In the older patient, an association with orthostatic hypotension has been demonstrated.
• Cough: This form of syncope (ie, tussive syncope) occurs particularly in adolescents with asthma or cystic fibrosis. It accounts for 2-5% of syncope presentations in children.
• Swallowing: Similar to most neuroregulatory types of syncope, swallowing (ie, deglutition) syncope appears to be caused by vasovagal nervous modulation (of a GI stimulus) rather than by pre-existing intrinsic heart disease. However, tachycardias and bradycardias other than sinus bradycardia are occasionally induced by swallowing.
• Stretch: Syncope may be induced in adolescents who stretch with the neck hyperextended. Studies indicate that the mechanism is not simply the Valsalva maneuver but also involves a combination of vertebral and posterior cerebral artery compression (despite an intrinsically normal vessel) and a familial tendency to faint.
• Exercise: Exercise-related syncope may be related to multiple causes or pathophysiologies, not all of which are benign. Vasovagally mediated hypotension and bradycardia are believed to be a common but difficult-to-prove cause of this form of syncope. Exercise-related syncope from a neurally mediated mechanism is typically thought to occur immediately following the activity (rather than at the peak of exercise).
• Orthostatic cardiac isolated decrease in cardiac ejection: Orthostatic hypotension is defined as a drop in blood pressure related to a change to a more upright posture and may be the end result of different pathophysiologies. Dysautonomia may result in failure of peripheral vasoconstriction in response to hypotension or shifts in blood volume. In this setting, additional signs of dysautonomia are usually present (eg, sweating, GI distress with diarrhea or constipation), with abnormal cold pressor test results, handgrip response, and response to Valsalva maneuver.
• Aortic stenosis: In patients with severe aortic stenosis, ventricular baroreceptor reflex bradycardia and peripheral vasodilatation have been implicated as a mechanism for syncope. The occurrence of symptoms correlates well with end-systolic wall stress. The presence of syncope in patients with aortic valve stenosis is a serious symptom associated with a mean projected survival time of only 27 months after its occurrence if the stenosis is left untreated.
• Unrepaired tetralogy of Fallot
  • In infants and young children with unrepaired tetralogy of Fallot, symptomatic periods are characterized by episodes of markedly increased cyanosis that frequently culminate in syncope. They are considered to result from increasing stenosis of the muscular subpulmonary infundibulum. This is usually a progressive abnormality and can be exacerbated by increased heart rate and contractility, presumably through the initiation of pulmonary infundibular spasm. For this reason, digoxin and other inotropic medications, as well as chronotropic medications, are relatively contraindicated in these patients before repair of tetralogy of Fallot.
  • Beta-blockade can relieve these symptomatic periods. Additional immediate therapies of tetralogy symptoms include placement of the patient in a knee-chest position (to increase systemic venous return and elevate systemic vascular resistance), morphine infusion (to prevent agitation and resultant tachycardia), and, if necessary, phenylephrine administration (to elevate systemic vascular resistance). Patients with tetralogy symptoms may be transiently treated successfully with oral beta-blockade, but this should be a temporizing measure while complete surgical repair of the tetralogy of Fallot is being arranged.
• Pulmonary hypertension: The diagnosis of primary pulmonary hypertension is often difficult and, for that reason, may be significantly delayed following the onset of symptoms. The presence of syncope in either primary or secondary pulmonary hypertension is a poor prognostic sign.
• Hypertrophic cardiomyopathy: Syncope may affect as many as 15-25% of patients with hypertrophic cardiomyopathy. It may be secondary to atrial arrhythmias, ventricular arrhythmias, obstructed blood flow through the left ventricular outflow tract (especially during exercise), or diastolic dysfunction that causes impaired filling of the ventricles. All of these factors decrease blood flow to the brain by decreasing stroke volume. In this setting, syncope is an ominous sign, and such patients should receive a defibrillator.
• Dilated cardiomyopathy: Patients with dilated cardiomyopathy may have syncope secondary to atrial arrhythmias, ventricular arrhythmias, atrioventricular conduction defects, or poor cardiac output due to myocardial dysfunction. A history of syncope in this setting is associated with a high risk of sudden death. These patients should also receive a defibrillator.
• Intracardiac tumors: Atrial myxoma is the most common primary tumor of the heart and arises from the endocardium as a pedunculated mass. Approximately 80% of myxomas occur in the left atrium, and 20% occur in the right atrium. Atrial myxomas can cause syncope by decreasing atrial filling, thus decreasing ventricular filling with consequent decreased stroke volume. In this setting, syncope usually occurs during exercise. Myxomas may cause syncope when embolization occurs, either to the brain (causing decreased blood flow with decreased oxygenation) or to the coronary arteries. In this setting, syncope may be secondary to arrhythmias or myocardial infarction, both of which cause decreased stroke volume.
• Congenital coronary anomalies
  • Anomalous origin of coronary arteries is a congenital disease found in approximately 0.3% of all autopsies and in 0.5% of coronary angiograms.
  • Congenital coronary anomalies described in the literature include the following:
    • Anomalous left coronary artery originating from the pulmonary artery
    • Left main coronary artery takeoff from the right coronary sinus, with a course between the aorta and the right ventricular outflow tract
    • Anomalous origin of the right coronary artery from the left sinus of Valsalva, coursing between the aortic root and the right ventricular outflow tract
    • Single origin from the right sinus of Valsalva for both right and left coronary arteries in which the left coronary artery is compressed between the aorta and the pulmonary trunk
    • Anomalous circumflex artery originating from the right sinus of Valsalva and passing behind the aortic root
  • Most of these studies involved young athletes with a history of syncope who experienced sudden cardiac death. Pathological findings included myocardial fibrosis, necrosis, or both. Therefore, exertional syncope should be thoroughly evaluated, and surgery should be performed when coronary anomalies are present because of the high risk of sudden cardiac death. In these cases, syncope may be secondary to ischemia-induced tachyarrhythmias or bradyarrhythmias or myocardial dysfunction.
• Acquired coronary anomalies: Acquired coronary artery disease in childhood may be caused by inflammatory processes (eg, Kawasaki disease), atherosclerosis (eg, postorthotopic heart transplant), or, rarely, primary hypercholesterolemia. In patients with acquired coronary artery disease, syncope may also be related to ischemia-induced bradyarrhythmias or tachyarrhythmias or myocardial dysfunction, which is an ominous sign.
• Right ventricular outflow tract obstruction: Syncope has been described in patients with right ventricular outflow tract obstruction that results from a double-chambered right ventricle. A case report of an intracardiac tumor that caused third-degree heart block secondary to infiltration and right ventricular outflow tract obstruction was recently reported in a child who presented with syncope.
• Carditis
  • Transient myocarditis may be due to viral infections, such as coxsackievirus, adenovirus, and echovirus. The most frequent histological pattern is lymphocytic infiltration of the myocardium, followed by mixed inflammatory infiltration and a granulomatous infiltration. The clinical presentation of myocarditis may be asymptomatic or may involve heart failure, cardiac arrest, or, less frequently, syncope and chest pain.
  • Individuals with Lyme carditis may develop syncope secondary to transient complete atrioventricular block.
  • Rheumatic fever is uncommon in Western countries but is prevalent in nonindustrialized countries. It may be seen in immigrants from these poorer areas. When carditis is present, it is usually manifested by first-degree atrioventricular block. However, complete atrioventricular block may occur with syncope as its presenting symptom.
  • According to one study, patients with Brugada syndrome who presented with syncope were found to have carditis based on endomyocardial biopsy.⁴ Histology revealed prevalent or localized right ventricular myocarditis in most patients, with detectable viral genomes in some.
  • Giant cell myocarditis is a rare condition that usually affects young, previously healthy people. In these patients, syncope may be due to rapid development of complete atrioventricular block, ventricular arrhythmias, or heart failure. They often require cardiac transplantation.
  • Chagas myocarditis is one of the manifestations of a parasitic disease secondary to Trypanosoma cruzi infection, which is prevalent in Latin America. Syncope occurs secondary to varying degrees of atrioventricular and bundle block.
• Psychiatric conversion: Dizziness and syncope may be symptoms of depression, anxiety, panic disorder, somatization, and substance abuse. Unexplained syncope is likely to have a psychiatric etiology. These individuals tend to have multiple somatic symptoms and report frequent syncope. Treatment of these psychiatric illnesses results in lower rates of syncope recurrence.
• Hyperventilation syncope: Syncope is attributed to respiratory alkalosis, which induces cerebral vasoconstriction and, thus, hypoperfusion.
• Breath-holding spells (BHSs): These refer to involuntary episodes that occur in otherwise healthy children. After a few cries, the child becomes silent and apneic; this is quickly followed by pallor or cyanosis. Simple BHSs resolve with no associated syncope or postural change. However, severe BHSs are characterized by a loss of consciousness and a change in postural tone with occasional myoclonic jerks. The episodes last from seconds to a minute and end with a deep inspiration. In children with pallid BHSs, noxious stimuli may lead to cardiac inhibition through the vagus nerve, inducing bradycardia or brief asystole. In children with the cyanotic type of BHS, central inhibition of respiratory movements mediated through the vagus nerve is thought to occur. In both types of spells, cerebral hypoxia is the end result.
• Inappropriate pacemaker function: Patients who have pacemakers may develop syncope secondary to inappropriate pacemaker settings. For example, the ventricular lead may oversense atrial depolarizations and misinterpret them as ventricular depolarizations, which causes failure to pace the ventricle when needed. Other causes include inadequate device functioning (ie, when the battery is depleted due to the pacemaker reaching "end of life" or due to insulation defects in the lead with rapid current drain) and lead fracture, which causes failure to capture or make the ventricle contract when paced. Patients who have ventricular pacing may develop pacemaker syndrome in which presyncope or syncope are secondary to ventriculoatrial conduction of the paced beat, with consequent atrial distension, reflex vasodilation, and decreased stroke volume.
• Bradycardia: When the heart rate is slower than is required to maintain an adequate cardiac output, the brain becomes underperfused and the individual develops syncope. Examples include the following:
  
  • Sinus node disease: In these patients, syncope is generally due to marked sinus bradycardia, which causes inadequate cardiac output with cerebral hypoperfusion. Sinus node disease is frequently encountered in older individuals with congenital heart disease who have undergone Mustard or Senning surgery for D-transposition of the great arteries, which have been largely replaced by the arterial switch operation. Sinus node disease is also encountered in patients who had undergone the classic Fontan surgery for palliation of single ventricle and in some patients who have undergone other types of cardiac surgery, such as secundum and sinus venosus atrial septal defect repair. Sinus node disease may also be encountered in individuals with unrepaired atrial septal defects, polysplenia syndromes, and Ebstein anomaly of the tricuspid valve.
  • Congenital and acquired heart block: Syncope may develop in individuals with complete heart block, when the escape rhythm is too slow to maintain adequate cardiac output and, therefore, adequate cerebral perfusion.
• Tachycardia: Syncope may develop secondary to a rapid heart rate that decreases diastolic filling time enough to induce decreased stroke volume, myocardial ischemia, or both. This leads to cerebral hypoperfusion. Syncope secondary to tachycardia may occur during the following:
  • Atrial tachycardia: Syncope may occur secondary to decreased atrial filling because of loss of the "atrial kick," such as in atrial fibrillation or flutter, with subsequent decreased ventricular filling and cerebral hypoperfusion.
  • Supraventricular tachycardia (SVT): Syncope may be secondary to rapid conduction through the atrioventricular node and up through an accessory pathway. If the SVT is prolonged, syncope may be due to myocardial dysfunction, with a consequent inadequate cardiac output to maintain a normal cerebral perfusion.
  • Ventricular tachycardia (VT): VT is defined as 5 or more consecutive beats that arise below the atrioventricular node at a rate of more than 100 beats per minute. Syncope occurs because of ineffective stroke volume and myocardial ischemia, which aggravates the picture, with consequent cerebral hypoperfusion.
  • Ventricular fibrillation: This occurs when VT degenerates into chaotic electrical activity and the myocardium is rendered without synchronization and without effective myocardial contractions. Ventricular fibrillation leads to sudden cardiac death in minutes if it is not cardioverted or if it is unable to be cardioverted to a more stable rhythm.
  • Polymorphic VT: This type of VT is irregular in rate and rhythm and has varying shapes or morphologies on ECG. Ventricular fibrillation may occur. Long QT syndromes (LQTSs) are typically associated with a polymorphic VT often called torsades de pointes because of the original French description of the QRS complexes as twisting about its axis.
  • LQTS
    • LQTSs develop in young individuals with generally structurally normal hearts who carry certain mutations in the cardiac ionic channels that predispose them to ventricular fibrillation under emotional or physical stress, thereby putting the person at risk for sudden death.
    • In these patients, a prior history of syncope is a serious event compatible with an aborted episode of sudden death secondary to polymorphic VT or torsade de pointes, which may degenerate into ventricular fibrillation.
    • Mutations in certain genes that encode for certain ion channels produce a derangement in ionic flows across the cytoplasmic membranes of cardiac cells. This produces prolongation of the cardiac action potential and lengthening of the QT interval on the surface ECG. These genes encode for potassium channels, with the exception of SCN5A, which encodes for the sodium channel.
    • KCNQ1 mutations (in the LQT1 gene) are the most frequent, followed by HERG (in the LQT2 gene) and SCN5A (in the LQT3 gene) mutations. KCNE1 mutations (in the LQT4 and LQT5 genes) and KCNE2 mutations (in the LQT6 gene) are less frequent.
    • Because KCNQ1 mutations are the most frequent, homozygous mutations in the KCNQ1 gene are generally associated with Jervell and Lange-Nielsen syndrome. Heterozygous mutations of this gene cause Romano-Ward syndrome.
    • Jervell and Lange-Nielsen syndrome is the autosomal recessive inherited variant associated with sensorineural deafness and is mostly caused by mutations in the KCNQ1 gene or, less frequently, in the KCNE1 gene. These genes encode for the I(Ks) current. One study showed that this variant is the most severe variant of LQTS; most patients are symptomatic by age 3 years.⁵
    • In patients with Jervell and Lange-Nielsen syndrome, the QTc is markedly prolonged (557 ±65 ms) and beta-blockers have limited efficacy. Most arrhythmic events are triggered by emotions or exercise. Females are at lower risk for cardiac arrest and sudden death. Mutations in the KCNE1 gene have a more benign course. A QTc of more than 550 milliseconds and occurrence of syncope during the first year of life are independent predictors of subsequent cardiac arrest and sudden death. In these patients, early therapy with defibrillators must be considered.
    • Romano-Ward syndrome is the autosomal dominant inherited variant of LQTS and is the most frequent variant of inherited LQTS. When syncope occurs in these patients, ventricular fibrillation should be suspected as the cause and a defibrillator should be implanted.
    • Acquired LQTS may be caused by various drugs, such as quinidine, sotalol, and dofetilide, and numerous antihistamines, antibiotics, antipsychotics, and others. In addition, prolongation of the QTc may be secondary to electrolyte abnormalities such as hypokalemia and hypomagnesemia, CNS lesions, and significant bradyarrhythmias.
  • Short QT syndrome: This is a newly recognized congenital inherited channelopathy associated with familial atrial fibrillation and/or sudden death or syncope. Three different mutations in 3 genes (KCNH2, KCNQ1, KCNJ2) that encode for cardiac potassium channels have been identified. These mutations cause an increase in the net outward potassium current, leading to shortening of repolarization, a shorter QTc of less than 300-320 milliseconds, a lack of adaptation during increasing heart rates, and shorter atrial and ventricular refractory periods, with increased susceptibility to ventricular and atrial fibrillation.
  • Brugada syndrome: This is an inherited channelopathy caused by mutations in the SCN5A gene, which can cause idiopathic ventricular fibrillation.
  • Quinidine syncope: This refers to development of ventricular fibrillation with the use of quinidine. According to the Vaughan-Williams classification, quinidine is a class IA antiarrhythmic agent. Quinidine depresses the rapid inward depolarizing sodium current, with resulting prolongation of refractory periods in the atria, ventricles, and Purkinje tissues. It is used for treatment of reentrant SVTs, atrial fibrillation, and ventricular arrhythmias. Quinidine syncope is estimated to occur in 0.5-4.4% of treated patients. Quinidine syncope is dose-independent and occurs most frequently when elimination of atrial fibrillation or flutter is attempted. The ECG reveals a prolonged QTc and large U waves. It is thought to represent a reentry tachycardia caused by unequal recovery times in different parts of the ventricular myocardium.
  • Arrhythmogenic right ventricular dysplasia: This is an autosomal dominant type of cardiomyopathy seen primarily in children and young adults and is characterized by fibrofatty replacement of the right ventricular myocardium, with associated arrhythmias originating in the right ventricle. As many as 80% of individuals present with syncope or sudden cardiac death secondary to VT.
  • Catecholamine-sensitive monomorphic VT
    • This is an arrhythmogenic disease characterized by myocardial electrical instability that is exacerbated by activation of the adrenergic nervous system by acute emotions or during exercise.
    • VT may self-terminate or degenerate into ventricular fibrillation, causing syncope and sudden death.
    • Syncope occurs in 60-70% of individuals, and most events occur during childhood.
    • An alternating, 180-degree QRS axis on a beat-to-beat basis (also known as bidirectional VT) and irregular polymorphic VT without a QRS vector alternans may be seen on ECG.
    • RYR2 and CASQ2 are the 2 genes associated with this entity.
    • Beta-blockers have proven efficacy for approximately 60% of individuals, and the remaining 40% of individuals in whom exercise stress testing does not demonstrate adequate control of arrhythmias with the highest tolerated dose of beta-blockers may benefit from an ICD.
  • Repaired tetralogy of Fallot: Ventricular arrhythmias in patients after total surgical repair of tetralogy of Fallot have been associated with late sudden death. In a large study, spontaneous premature ventricular contractions and induced VT were related to delayed age at repair, longer follow-up interval, symptoms of syncope or presyncope, and right ventricular systolic pressure of more than 60 mm Hg.⁶ A large study in Japan showed that the risk factors for ventricular arrhythmias (in patients who developed syncope) included longer postoperative survival duration and QRS duration of longer than 120 milliseconds, whereas the risk factor for atrial fibrillation and flutter was older age at operation.⁷
  • Isolated Wolff-Parkinson-White syndrome: During atrial fibrillation, syncope may be secondary to rapid conduction through the accessory pathway, which becomes ventricular fibrillation because of ventricular pre-excitation.
  • Mixed or unclear hypertrophic cardiomyopathy: Syncope results from left ventricular outflow tract obstruction, mostly during exercise, or from the development of ventricular arrhythmias. See Hypertrophic cardiomyopathy.
  • Mitral valve prolapse: Patients with mitral valve prolapse are thought to be prone to endocardial ischemia and, thus, prone to development of ventricular arrhythmias. A study concluded that, in patients with mitral valve prolapse syncope, inferolateral repolarization changes, complex ventricular ectopy, and a markedly myxomatous valve are consistent with higher risk of sudden death, and, unfortunately, mitral valve surgery may not provide control of ventricular arrhythmias.⁸

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