Congenital long QT syndrome has a tendency to develop the following condition:

The congenital long-QT syndrome is a family of disorders characterized by prolongation of cardiac repolarization with a prolonged QT interval on the scalar ECG and a tendency to develop polymorphic ventricular tachycardia that may degenerate to ventricular fibrillation. The most common types of the long QT syndrome are caused by mutations in genes that encode ion channel proteins. The resultant ion channel dysfunction causes a prolonged repolarization phase of the ventricular action potential. This promotes polymorphic ventricular tachycardia triggered by oscillations in the action potential called early after-depolarizations. Ref: DiMarco J.P. (2009). Chapter 25. Sudden Cardiac Death. In M.H. Crawford (Ed), CURRENT Diagnosis & Treatment: Cardiology, 3e.