A 52-year-old man with a history of recurrent syncope presents to the cardiology clinic. ECG shows a prolonged QT interval (QTc 520 ms). To confirm the diagnosis of Long QT syndrome and identify the underlying ion channel defect, which investigation is the most appropriate next step?

## Investigation of Choice for Long QT Syndrome Confirmation **Key Point:** Genetic testing is the gold standard for confirming Long QT syndrome and identifying the specific ion channel mutation responsible for the phenotype. ### Why Genetic Testing is Definitive Long QT syndrome results from mutations in genes encoding cardiac ion channels that regulate the action potential duration. The three most common genes are: | Gene | Ion Channel | Inheritance | Frequency | | --- | --- | --- | --- | | KCNQ1 | K~v~7.1 (slow delayed rectifier K^+^) | Autosomal dominant (LQT1) | ~40% | | KCNH2 | K~v~11.1 (rapid delayed rectifier K^+^) | Autosomal dominant (LQT2) | ~35% | | SCN5A | Na~v~1.5 (L-type Na^+^ channel) | Autosomal dominant (LQT3) | ~10% | **High-Yield:** Identifying the specific mutation allows for: - Risk stratification (LQT3 has higher arrhythmia risk) - Personalized β-blocker therapy (LQT1 responds best) - Family screening and genetic counseling - Avoidance of QT-prolonging drugs ### Mechanism: How Mutations Prolong the Action Potential Mutations in repolarizing K^+^ channels (KCNQ1, KCNH2) reduce outward K^+^ current → delayed repolarization → prolonged QT interval → increased risk of early afterdepolarizations (EADs) → torsades de pointes. Mutations in Na^+^ channels (SCN5A) increase inward Na^+^ current during late phase 2 → prolonged plateau phase → prolonged action potential duration. **Clinical Pearl:** The genotype-phenotype correlation is clinically important: - **LQT1 (KCNQ1):** Syncope triggered by exercise or emotional stress; β-blockers highly effective - **LQT2 (KCNH2):** Syncope triggered by auditory stimuli or emotional stress; β-blockers moderately effective - **LQT3 (SCN5A):** Syncope at rest or during sleep; sodium channel blockers (flecainide) preferred **Mnemonic:** **KCNQ1-Exercise, KCNH2-Emotions, SCN5A-Sleep** (triggers for syncope) ## Why Other Investigations Are Insufficient **Holter monitoring** documents arrhythmias but does not identify the underlying genetic defect or confirm the diagnosis. **Stress testing** may provoke arrhythmias in LQT1 but does not provide genetic confirmation or guide mutation-specific therapy. **Cardiac MRI** is useful for structural assessment but has no role in diagnosing inherited channelopathies like Long QT syndrome.