Long QT Syndrome
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Long QT syndrome (LQTS) is characterized by prolongation of the QT interval on the surface electrocardiogram (EKG) that can result in symptomatic ventricular arrhythmias. The symptoms can include palpitations, syncope, and sudden cardiac arrest. The prolonged QT interval reflects abnormal ventricular repolarization associated with cardiac ion channel dysfunction that results in electrical instability.
- Prevalence of LQTS is estimated to be approximately 1 in 2,500.
- Cardiac events occur in ~50% of congenital LQTS patients; most occur in preteens, adolescents, and young adults.
- Autosomal dominant (Romano-Ward syndrome)
- Autosomal recessive, sometimes associated with congenital bilateral sensorineural hearing loss (Jervell and Lange-Nielsen syndrome)
- Genetic studies have demonstrated that nearly 500 genetic mutations among 15 cardiac ion channel genes account for nearly 80% of congenital LQTS.
- Genotype-phenotype–based research studies have identified gene-specific electrocardiographic profiles, gene-specific arrhythmia triggers, gene-directed treatment strategies, and gene-specific risk stratification.
- Preventive measures focus on targeted screening for the electrocardiographic abnormality, especially in individuals who appear to be at risk of having the diagnosis based on symptoms and family history.
- Patients with LQTS should avoid exposure to stimulants; medications that are known to prolong the QT interval or provoke ventricular arrhythmias; and situations such as exercise, emotional stress, or auditory stimuli that may aggravate abnormalities in ventricular repolarizations resulting in ventricular arrhythmias, in particular torsades de pointes.
Two hypotheses have been proposed to explain the pathogenesis of congenital LQTS syndrome:
- An abnormality or imbalance in sympathetic innervation to the heart, which helps explain the findings of sinus bradycardia, abnormal repolarization, adrenergic dependence of arrhythmias, and response to adrenergic antagonist medications (i.e., β-blockers) associated with the syndrome
- Intrinsic cardiac ion (potassium, sodium, or calcium) channel defects are also responsible for cardiac repolarization abnormalities and abnormal cardiac myocyte excitability. These genetic mutations alter cardiac ion channel proteins resulting in gain or loss of function mutations that lead to altered cardiac ion flows into and out of cardiac cells.