In a patient with congenital long QT syndrome (LQTS), the 12-lead ECG may show prolongation of the corrected QT (QTc) interval. A normal baseline QTc interval of less than or equal to 0.44 seconds does not exclude LQTS; in fact, this may be observed in approximately 12 percent of genetic carriers of this condition.
Long QT syndrome can be identified in approximately 1 in 2,500 people, placing them at risk for sudden death. These patients often present with near-syncope and/or syncope due to polymorphic ventricular tachycardia (torsades de pointes).
The most frequent genetic substrate of LQTS are genes that encode cardiac ion channels. Abnormalities in the potassium ion channel genes KCNQ1 (Type I LQTS or LQT1) and KCNH2 (Type 2 LQTS or LQT2) result in the most common types of LQTS. Abnormalities in the sodium channel gene SCN5A (Type 3 LQTS or LQT3) is the third most common cause of LQTS. Commercial genetic testing is available for LQTS and has become an important part of the diagnostic process.