SCN5A and Down syndrome: SCN5A mutations are associated with various arrhythmic syndromes, including AF, sick sinus syndrome, long QT syndrome, Brugada syndrome, conduction disease, atrial arrest, mixed arrhythmia phenotype overlap syndrome, and drug arrhythmias.[6]SCN5A-H558R is a loss-of-function mutation associated with reduced sodium current density and a shortened refractory period, decreased atrial conduction velocity, and increased susceptibility to atrial arrhythmias.