Long QT syndrome (LQTS) is an inherited or acquired heart rhythm disorder in which the electrical recovery phase of the heartbeat (measured as the QT interval on an ECG) is prolonged. This abnormal prolongation creates a window of vulnerability during which a dangerous arrhythmia called torsades de pointes can occur, potentially deteriorating into ventricular fibrillation and sudden cardiac arrest.
Congenital LQTS is caused by mutations in genes encoding ion channels in heart muscle cells, affecting the flow of potassium, sodium, or calcium ions during the cardiac cycle. The most common subtypes are LQT1 (mutations in KCNQ1, with arrhythmias often triggered by exercise or swimming), LQT2 (mutations in KCNH2, with arrhythmias often triggered by sudden auditory stimuli such as a loud noise or alarm), and LQT3 (mutations in SCN5A, with arrhythmias often occurring at rest or during sleep). The condition is inherited in an autosomal dominant pattern in most cases, and family screening following diagnosis is essential.
The QT interval can also be prolonged by many common medications, including certain antibiotics, antipsychotics, antihistamines, and antiarrhythmics, as well as by electrolyte abnormalities such as low potassium or magnesium. This acquired form resolves when the cause is treated or the offending drug is stopped. Diagnosis is based on the QTc (corrected QT interval) on a resting ECG; a QTc above 480 ms in women or 470 ms in men raises suspicion, and genetic testing can confirm the diagnosis and identify the subtype.
Beta-blockers are the mainstay of treatment for congenital LQTS, particularly LQT1 and LQT2. Avoidance of QT-prolonging drugs is important for all subtypes. An ICD is recommended for those who have survived cardiac arrest or have breakthrough arrhythmias on beta-blockers. Left cardiac sympathetic denervation (LCSD) is a surgical option for selected patients.
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