A 28-year-old woman from Delhi with a history of recurrent syncope and exertional dyspnoea is found to have a prolonged QT interval on ECG (QTc 520 ms). Genetic testing reveals a mutation in SDHA (succinate dehydrogenase subunit A, Complex II of the electron transport chain). She presents with acute presyncope during exercise. What is the most appropriate immediate next step?

Explanation

## Clinical Context This patient has a **Complex II (succinate dehydrogenase) deficiency** with cardiac manifestations: syncope, QT prolongation, and exertional symptoms. Complex II is unique among ETC complexes because it is the only one that also participates in the citric acid cycle (as succinate dehydrogenase). ## Pathophysiology: Complex II Deficiency and Cardiac Arrhythmia **Key Point:** SDHA mutations impair both oxidative phosphorylation and the citric acid cycle. This leads to: 1. Reduced ATP production in cardiomyocytes 2. Impaired Na⁺/K⁺-ATPase function 3. Abnormal repolarization (prolonged QT) 4. Increased risk of Torsades de Pointes and sudden cardiac death ```mermaid flowchart TD A[SDHA mutation]:::outcome --> B[Complex II deficiency]:::outcome B --> C[Impaired succinate oxidation]:::outcome C --> D[Reduced FADH₂ generation]:::outcome D --> E[Decreased ATP production]:::outcome E --> F[Impaired cardiac ion channels]:::outcome F --> G[QT prolongation + arrhythmia risk]:::outcome G --> H{Exertional trigger?}:::decision H -->|Yes| I[Syncope / sudden cardiac death]:::urgent H -->|No| J[Asymptomatic QT prolongation]:::outcome ``` ## Management of Complex II Deficiency with Cardiac Involvement | Step | Rationale | Evidence | |---|---|---| | **Acute:** IV Mg²⁺ + high-dose beta-blocker | Mg²⁺ stabilizes repolarization; beta-blocker reduces adrenergic triggers and heart rate | Standard for Long QT syndrome | | **Urgent:** Cardiology review | Assess ICD candidacy (QTc >500 ms + syncope = high risk) | Class I indication | | **Avoid:** Exercise stress testing | Provocation may trigger life-threatening arrhythmia in acute setting | Contraindicated in symptomatic Long QT | | **Avoid:** Amiodarone monotherapy | Class IA antiarrhythmic prolongs QT further (paradoxical effect) | Contraindicated in Long QT | | **Imaging:** Cardiac MRI | Rules out infiltrative/inflammatory cardiomyopathy; not acute management | Secondary role | **High-Yield:** In a symptomatic patient with QT prolongation and syncope, the immediate priority is **acute arrhythmia suppression** (Mg²⁺ + beta-blocker) followed by **ICD assessment**. Do not provoke arrhythmia or use QT-prolonging drugs. ## Why Option 0 is Correct **Clinical Pearl:** Intravenous magnesium sulphate (2 g bolus) is the first-line acute therapy for Torsades de Pointes and QT-prolongation-related syncope. High-dose beta-blockers (e.g., propranolol 80–240 mg/day or nadolol) reduce catecholamine-induced arrhythmias. Urgent cardiology review is essential to determine ICD candidacy (Class I if QTc >500 ms + syncope). **Mnemonic:** **MAGIC** for acute Long QT management: - **M**agnesium IV - **A**void triggers (exercise, stress) - **G**enetics (test family) - **I**CD (if high risk) - **C**ardiology referral ## Avoidance of Harmful Interventions **Warning:** Do NOT: - Perform exercise stress testing in symptomatic Long QT (risk of sudden death) - Use Class IA antiarrhythmics (quinidine, procainamide, disopyramide) or amiodarone monotherapy (all prolong QT) - Delay cardiology referral (risk of sudden cardiac death is high)