The congenital long QT syndrome Type 3: An update

Abstract

Congenital long QT syndrome type 3 (LQT3) is the third in frequency compared to the 15 forms known currently of congenital long QT syndrome (LQTS). Cardiac events are less frequent in LQT3 when compared with LQT1 and LQT2, but more likely to be lethal; the likelihood of dying during a cardiac event is 20% in families with an LQT3 mutation and 4% with either an LQT1 or an LQT2 mutation. LQT3 is consequence of mutation of gene SCN5A which codes for the Nav1.5 Na⁺ channel α-subunit and electrocardiographically characterized by a tendency to bradycardia related to age, prolonged QT/QTc interval (mean QTc value 478 ± 52 ms), accentuated QT dispersion consequence of prolonged ST segment, late onset of T wave and frequent prominent U wave because of longer repolarization of the M cell across left ventricular wall.

Keywords: Electrocardiogram; Long QT syndrome; Long QT syndrome-type-3; Torsade de Pointes.

Fig. 1

BrS1: Brugada Syndrome 1; CAS: Congenital Atrial Standstill; DCM: Dilated Cardiomyopathy; ERS: Early Repolarization Syndrome; FAF: Familial Atrial Fibrillation; IVF: Idiopathic Ventricular Fibrillation; LQT3: Long QT syndrome 3; MEPPC: Multifocal Ectopic Purkinje Premature Contraction; OSs: Overlapping Syndromes; PCCD: Progressive Cardiac Conduction Defect; PVT: Polymorphic Ventricular Tachycardia; SIDS: Sudden Infant Death Syndrome; SSS: Sick Sinus Syndrome; SUNDS: Sudden Unexplained Nocturnal Death Syndrome.

Fig. 2

Triggers for lethal events in LQT3.

Fig. 3

Normal (A) and LQT3 (B) ECG and action potential.

Fig. 4

ECG example of LQT3. This ECG belongs to a newborn baby with LQT3. Clear ST segment prolongation and delayed appearance of T wave. Affected gene: SCN5A, 3p21-24 mutation in chromosome 3, AP phase: plateau, dome or phase 2 by persistent sodium Na⁺ inflow (gain-of-function mutations in the SCN5A cardiac Na⁺ channel gene).

Fig. 5

Congenital LQT3 QTc: 670 ms Run of TdP after macro wave T alternans. T-wave alternans is a diagnostic feature of the LQT and reflects an enhanced electrical instability during repolarization .