Definition and Prevalence Long QT syndrome (LQTS) is a familial condition causing syncope and sudden death through rapid ventricular tachycardia (torsade de pointes), which can deteriorate to ventricular fibrillation, in otherwise fit and healthy young people. Prevalence is at least 1 in 5000 and may be up to 1 in 1000. Clinical diagnosis is made from a combination of suspicious history, family history and the 12 lead ECG, which typically reveals a heart-rate corrected QT interval (QT//R–R interval = QTc) of greater than 0.46 s in women and 0.45 s in men. Exercise testing is often helpful in making the diagnosis.

Clinical Presentation LQTS most commonly presents with syncope or sudden death during or following exercise or stress in a young person. Prolongation of QT can present on an incidental ECG, family screening or as sudden infant death. Misdiagnosis of LQTS as epilepsy, particularly “familial epilepsy”, is common. Seizures following exertion or arousal, even during sleep, must raise the suspicion of LQTS.