Long QT Syndrome

KEY FEATURES

ESSENTIALS OF DIAGNOSIS

• Clinical constellation of congenital deafness, prolongation of the QT interval (QTc > 480 ms), syncope, and sudden death

• Family history of sudden death

• Characteristic arrhythmia is torsades de pointes

• The fundamental abnormality is prolongation of action potential secondary to abnormality of transmembrane ion transport

• Emotion seems to trigger cardiac events, particularly in patients with KvLQT1

GENERAL CONSIDERATIONS

• Family history of sudden cardiac death (SCD), QT prolongation on the ECG, and bilateral neural deafness with autosomal recessive pattern of inheritance affecting several children in a family was first reported by Jervell and Lange-Nielsen

• Family history of SCD, QT prolongation, and autosomal dominant pattern of inheritance was subsequently identified (Romano-Ward syndrome)

• Romano-Ward syndrome is more common than Jervell and Lange-Nielsen syndrome

• Deafness is not a manifestation of Romano-Ward syndrome

• Mutations in 7 LQTS genes have thus far been identified

• Repolarization (potassium) currents are affected in most of the disorders except in LQT3 and LQT4, where the sodium current is affected

• Family members with identical gene mutations may experience wide variations in clinical symptoms (variable penetrance) from asymptomatic to recurrent syncope

• SCDLQT1 (43%) and LQT2 (45%) account for most of the recognized patients

CLINICAL PRESENTATION

SYMPTOMS AND SIGNS

• Resuscitated SCD or syncope

• Incidental finding of prolonged QT interval or during family screening

• Syncope with stress

• Congenital deafness

• Symptoms usually present during adolescence

• Males are affected during adolescence and females during adulthood

• LQT1 patients experience symptoms during exercise, and LQT2 patients during acute arousal

• Once clinical symptoms occur, risk of recurrence is high

PHYSICAL EXAM FINDINGS

• Bilateral nerve deafness

• Otherwise, clinical examination is normal

DIFFERENTIAL DIAGNOSIS

• Metabolic abnormalities with prolongation of QT

• Drug-induced long QT

• Idiopathic ventricular arrhythmia

• Central nervous system disorder that prolongs QT

DIAGNOSTIC EVALUATION

LABORATORY TESTS

• Metabolic panel to exclude hypokalemia and hypomagnesemia

• Genetic testing is helpful and indicated, but the test results may take too long to influence clinical decision making and are commonly only performed in the outpatient clinic

  • – A negative test is not helpful unless testing is performed to identify a mutation that is already known within the family

ELECTROCARDIOGRAPHY

• QTc > 480 ms

• Outpatient cardiac monitoring may uncover asymptomatic torsades de pointes

• Exercise ECG may be helpful if activity precipitates arrhythmia, or QT interval prolongation occurs during recovery if there is no prior diagnosis of long QT syndrome

• Usefulness of outpatient cardiac monitoring and exercise stress test, although frequently used, is supported by very few studies

IMAGING STUDIES

• Echocardiogram may be used to establish structurally normal heart

• Depending on clinical situation, exclude central ...