A rare group of genetic, cardiac rhythm diseases with characteristics of a prolongation of the QT interval at basal electrocardiography (ECG) and by a high risk of life-threatening arrhythmias. The two cardinal manifestations are syncopal episodes, which may lead to cardiac arrest and sudden cardiac death, and electrocardiographic abnormalities: prolongation of the QT interval and T wave abnormalities. Inheritance may be autosomal dominant or autosomal recessive and depends on the genes involved.