Which feature best distinguishes catecholaminergic polymorphic ventricular tachycardia (CPVT) from Long QT syndrome in a 28-year-old woman with recurrent syncope triggered by emotional stress and exercise?

Explanation

## CPVT vs Long QT Syndrome: Key Discriminator ### The Critical Distinction: Arrhythmia Morphology and Trigger **Key Point:** The morphology of the arrhythmia and its relationship to catecholamine surge is the best discriminator. CPVT presents with bidirectional or polymorphic VT (beat-to-beat alternation in QRS axis) triggered by exercise or emotional stress, whereas Long QT presents with torsades de pointes (twisting QRS axis) often triggered by bradycardia, pause, or QT-prolonging drugs. ### CPVT: Bidirectional/Polymorphic VT Pattern **Mechanism:** 1. Mutations in RYR2 (ryanodine receptor) or CASQ2 (calsequestrin) cause abnormal calcium handling in the sarcoplasmic reticulum. 2. During catecholamine surge (exercise, stress), calcium overload triggers delayed afterdepolarizations (DADs). 3. Ectopic beats from different foci in the ventricles produce beat-to-beat alternation in axis (bidirectional or polymorphic pattern). **Clinical Features:** - **Baseline ECG**: Normal QT interval (this is crucial) - **Arrhythmia**: Bidirectional VT (alternating axis, 180° apart) or polymorphic VT - **Trigger**: Exercise, emotional stress, catecholamine infusion - **Inheritance**: Autosomal dominant (RYR2) or autosomal recessive (CASQ2) **Clinical Pearl:** In CPVT, the baseline ECG is often completely normal—no QT prolongation. The arrhythmia only appears with catecholamine surge. ### Long QT Syndrome: Torsades de Pointes **Mechanism:** 1. Mutations in ion channels (KCNQ1, KCNH2, SCN5A) prolong repolarization. 2. Prolonged QT creates a substrate for re-entry and early afterdepolarizations (EADs). 3. Torsades de pointes (polymorphic VT with twisting QRS axis around isoelectric line) results. **Clinical Features:** - **Baseline ECG**: Prolonged QT interval (diagnostic finding) - **Arrhythmia**: Torsades de pointes (characteristic twisting morphology) - **Trigger**: Bradycardia, pause, QT-prolonging drugs, hypokalemia, hypomagnesemia - **Inheritance**: Autosomal dominant (Romano-Ward) or autosomal recessive (Jervell and Lange-Nielsen) ### Comparison Table: CPVT vs Long QT | Feature | CPVT | Long QT Syndrome | |---------|------|------------------| | **Baseline QT** | Normal | Prolonged (QTc > 460 ms in females) | | **Arrhythmia Morphology** | Bidirectional or polymorphic VT | Torsades de pointes | | **Primary Trigger** | Exercise, emotional stress, catecholamines | Bradycardia, pause, QT-prolonging drugs | | **Genetic Basis** | RYR2, CASQ2 (calcium handling) | KCNQ1, KCNH2, SCN5A (ion channels) | | **Inheritance** | AD (RYR2) or AR (CASQ2) | AD (Romano-Ward) or AR (Jervell-Lange-Nielsen) | | **Mechanism** | Delayed afterdepolarizations (DADs) | Early afterdepolarizations (EADs) | | **Management** | Beta-blockers, flecainide, ICD | Beta-blockers, QT-shortening drugs, ICD | **High-Yield:** The bidirectional/polymorphic VT morphology triggered by catecholamine surge in the setting of a NORMAL baseline QT interval is pathognomonic for CPVT. ### Why Other Options Fail - **Prolonged QT interval**: This is a feature of Long QT, NOT CPVT. In CPVT, the baseline QT is normal, which is why CPVT is often missed initially. - **Autosomal recessive inheritance**: While CASQ2-related CPVT is autosomal recessive, most CPVT (RYR2) is autosomal dominant. Long QT can also be autosomal recessive (Jervell and Lange-Nielsen). Inheritance pattern alone is not a reliable discriminator. - **Torsades de pointes**: This is the characteristic arrhythmia of Long QT, not CPVT. CPVT produces bidirectional or polymorphic VT, which is morphologically different from torsades. **Mnemonic:** **CPVT = Catecholamine-triggered, Polymorphic/Bidirectional, normal QT**. **LQTS = Long QT, Torsades (twisting), triggered by pause/drugs**. **Tip:** When you see a young patient with syncope on exertion and a normal baseline ECG, think CPVT. When you see syncope with a prolonged QT on baseline ECG, think Long QT.