Long QT syndrome: A disorder of the heart's electrical system that predisposes individuals to irregular heartbeats, fainting spells, and sudden death. The irregular heartbeats are typically brought on by stress or vigorous activity. Abbreviated LQTS. LQTS is often symptomless and undiagnosed, but it is well known as a cause of sudden cardiac death in young, apparently healthy people, most notably competitive athletes. QT refers to an interval seen in an electrocardiogram (EKG) test of heart function. There are multiple genetic forms of LQTS. Romano-Ward syndrome is an autosomal dominant form of LQTS. The Jervell and Lange-Nielsen syndrome is an autosomal recessive form of LQTS and is characterized by congenital profound bilateral sensorineural hearing loss and long QT interval.