long QT syndrome
Summary
Long QT syndrome (LQTS) is a cardiac channelopathy characterized by prolonged ventricular repolarization (QTc >450ms men, >470ms women), predisposing to torsades de pointes (TdP), a polymorphic ventricular tachycardia that can cause syncope, seizures, or sudden cardiac death. It can be congenital (genetic ion channel mutations) or acquired (drugs, electrolyte abnormalities).
Detail
Pathophysiology: LQTS results from prolonged myocardial action potential duration due to delayed repolarization, most commonly from loss-of-function mutations in potassium channels (KCNQ1 - LQT1, KCNH2/HERG - LQT2) or gain-of-function mutations in sodium channels (SCN5A - LQT3), leading to increased risk of early afterdepolarizations (EADs) that trigger TdP.
Congenital forms: Romano-Ward syndrome (autosomal dominant, most common, no deafness) and Jervell-Lange-Nielsen syndrome (autosomal recessive, associated with sensorineural deafness). LQT1 triggered by exercise/swimming, LQT2 by auditory stimuli/emotional stress, LQT3 during sleep/rest.
Acquired causes: Drugs (Class IA/III antiarrhythmics, antipsychotics like haloperidol, antibiotics like macrolides/fluoroquinolones, antidepressants, antiemetics like ondansetron - remember mnemonic 'ABCDE': Antibiotics/Antiarrhythmics, Bradycardia, Cardiac disease, Drugs (antidepressants, antipsychotics), Electrolyte disturbances). Electrolyte abnormalities: hypokalemia, hypomagnesemia, hypocalcemia. Other causes: bradycardia, hypothyroidism, CNS lesions.
Clinical presentation: Syncope, palpitations, seizure-like activity, or sudden cardiac death, often triggered by exercise, emotional stress, or auditory stimuli depending on subtype. May be asymptomatic and found incidentally on ECG.
Diagnosis: ECG showing prolonged QTc interval; genetic testing for congenital forms; Schwartz score can help risk-stratify.
Management: Beta-blockers (first-line for congenital LQTS, especially LQT1/LQT2), implantable cardioverter-defibrillator (ICD) for high-risk patients or those with prior cardiac arrest, avoidance of QT-prolonging drugs, electrolyte correction. Acute TdP treated with IV magnesium sulfate regardless of magnesium level, plus correcting underlying cause; unstable patients require defibrillation.
High-yield boards association: Always screen for drug interactions and electrolyte status when a patient presents with new-onset arrhythmia or syncope, and check QTc before starting/continuing QT-prolonging medications.
Sources
- First Aid for the USMLE Step 1
- UpToDate: Congenital long QT syndrome
- Braunwald's Heart Disease
- Goldberger's Clinical Electrocardiography
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