Genetic Reassessment Reveals Catecholaminergic Polymorphic Ventricular Tachycardia in Sisters Initially Diagnosed With Long QT Syndrome

BACKGROUND: The QT interval is often prolonged in the immediate postcardiac arrest period, but not all patients with a prolonged QT interval have long QT syndrome (LQTS). CASE SUMMARY: A 29-year-old female was diagnosed with LQTS in childhood based on borderline QTc interval and a family history of a sister with presumed LQTS after cardiac arrest. Updated genetic testing in the proband revealed a likely pathogenic RYR2 variant and a benign KCNE1 variant. Familial testing confirmed that the patient carried the RYR2 variant, establishing a diagnosis of catecholaminergic polymorphic ventricular tachycardia. DISCUSSION: This case underscores the value of confirming the underlying molecular diagnosis in determining arrhythmic risk and guiding family evaluation. Broader or updated genetic testing can clarify disease mechanism, prevent misdirected management, and support targeted risk assessment in relatives. TAKE-HOME MESSAGES: The differential diagnosis of cardiac arrest is broad, including arrhythmia syndromes, arrhythmogenic cardiomyopathy, and nongenetic causes. Comprehensive genetic testing and counseling are essential for informing management of sudden cardiac arrest survivors and their relatives.