Pediatric emergency medicine trisk 377

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 <450 msec, and a QTc >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