when the patient is at rest is a key to the diagnosis The upper limits of the corrected QT interval (QTc ) (using Bazett formula: QTc = (QT)/√RR interval) are 480 msec may be suggestive of LQTS in a patient presenting with syncope There are congenital and acquired causes for a prolonged QT interval The genetic arrhythmia syndromes consist of three subtypes: LQT1, LQT2, and LQT3 The disease occurs due to mutations in genes that encode for cardiac ion channels important in ventricular repolarization Syncope in these patients is related to polymorphic ventricular tachycardia (torsades de pointes) and death is due to ventricular fibrillation One form (LQT3) can also be associated with bradycardia and slow heart rates that may cause syncope Most cases are associated with an autosomal dominant form of the syndrome (i.e., Romano– Ward syndrome), which shows variable penetrance LQT1 is the most common genetic subtype, and the triggers for syncope or sudden death in affected patients include emotional or physical stress, such as diving and swimming One form of LQT1, the Jervell and Lange-Nielsen syndrome, is autosomal recessive and associated with a high risk of sudden death and congenital deafness In LQT2, syncope or sudden death can occur with stress or at rest Cardiac events triggered by sudden loud noises, such as the ringing of an alarm clock, are virtually pathognomonic for this condition Acquired causes for a prolonged QTc interval include hypocalcemia, hypokalemia, hypomagnesemia, hypothyroidism, eating disorders, and the use of drugs that prolong QT interval (e.g., haloperidol, methadone, pentamidine, and sotalol) TABLE 76.1 CAUSES OF PEDIATRIC SYNCOPE Autonomic Vasovagal a Breath-holding spell a Situational (micturition, defecation, hair grooming, coughing) Cardiac b Electrical disturbances a Pre-excitation syndromes (Wolff–Parkinson–White syndrome) Long QT syndromes (congenital and acquired) Brugada syndrome Polymorphic ventricular tachycardia (e.g., in arrhythmogenic right ventricular cardiomyopathy or catecholaminergic polymorphic ventricular tachycardia) Bradycardia (complete heart block) Structural heart disease (obstruction to blood flow) Hypertrophic obstructive cardiomyopathy Aortic valve stenosis Pulmonary hypertension Myocardial Dysfunction Dilated cardiomyopathy Ischemia (coronary artery anomalies, Kawasaki disease, and atherosclerotic disease) Myocarditis Pericarditis Postoperative cardiac repair Others Hypoglycemia a Anaphylaxis b Environmental Heat syncope b Orthostatic a Dehydration or hemorrhage b Toxins a Carbon monoxide poisoning b Inhalant abuse (volatile inhalants, nitrites) b Hypoxia b Drugs Proarrhythmic agents (Class 1A and 1C antiarrhythmics), vasodilators, depressants Conditions that mimic syncope Seizures Migraine Conversion disorder a Hyperventilation Pseudoseizures Intentional strangulation (e.g., “choking game”) Narcolepsy a Common causes of syncope life-threatening causes b Potentially It is crucial to obtain a full family history in patients suspected of having LQTS Some clinical features such as QT morphologic characteristics, the response of the QT interval to exercise, triggers of arrhythmia, and response to therapies vary according to the disease-associated gene In LQTS, recent and frequent syncopal episodes between the ages of 10 and 12 years, QTc prolongation >530 msec, and male gender are predictive of risk for aborted cardiac arrest and sudden cardiac death during adolescence Brugada Syndrome In this heritable disorder, there is an abnormality in the cardiac sodium channel that results in ST-segment elevation in anterior precordial leads (V1 and V2) with a susceptibility to polymorphic ventricular tachycardia The ECG pattern is diagnostic, but may be present only intermittently, and may change over time If the arrhythmia degenerates to ventricular fibrillation, it may lead to sudden death; if it terminates, the patient may have only syncope Structural Heart Disease Congenital heart conditions that interfere with cardiac output may result in syncope Such structural problems include hypertrophic obstructive cardiomyopathy (HOCM), aortic valve stenosis, and coronary anomalies that cause cardiac ischemia Functionally, pulmonary hypertension may cause similar results As with other cardiac causes of syncope, chest pain, dizziness, and dyspnea on exertion are concerning symptoms that should prompt further evaluation HOCM is a genetic disorder that affects the proteins of the cardiac sarcomere In this condition, the hypertrophied basal interventricular septum partially blocks ...TABLE 76.1 CAUSES OF PEDIATRIC SYNCOPE Autonomic Vasovagal a Breath-holding spell a Situational (micturition, defecation,