A 4-year-old partially vaccinated child develops diphtheria with pseudomembrane formation. Which is the most common life-threatening complication in the acute phase of this illness?

Explanation

## Most Common Life-Threatening Complication of Diphtheria **Key Point:** Myocarditis is the most common life-threatening complication of diphtheria overall, occurring in 10–25% of cases and accounting for the majority of diphtheria-related deaths. It is caused by diphtheria exotoxin-mediated myocardial necrosis and typically manifests in the acute phase (days 10–14 onward). ### Complications Timeline ``` Diphtheria Onset │ ├── Days 1–7: Airway obstruction (laryngeal extension) — serious but less common │ ├── Days 10–14: Myocarditis — MOST COMMON life-threatening complication │ ├── Conduction abnormalities (AV block, bundle branch block) │ ├── Arrhythmias │ └── Heart failure → death │ └── Weeks 3–6: Neuropathy (palatal palsy, cranial nerve palsies, peripheral neuropathy) ``` ### Myocarditis: Mechanism and Features 1. **Diphtheria exotoxin** inhibits protein synthesis (EF-2 inactivation via ADP-ribosylation) 2. **Myocardial necrosis** and fatty degeneration of cardiac muscle 3. **Conduction system involvement** → AV block, bundle branch block, ventricular arrhythmias 4. **Incidence:** 10–25% of diphtheria cases; responsible for ~50–60% of diphtheria deaths 5. **ECG changes:** ST-T wave changes, prolonged PR interval, complete heart block **High-Yield (Nelson's Textbook of Pediatrics / Harrison's):** Myocarditis is the most common cause of death in diphtheria. Cardiac involvement is seen in up to 25% of patients and is the leading cause of mortality, even though airway obstruction can cause early deaths in laryngeal diphtheria. ### Why Airway Obstruction Is NOT the Most Common Life-Threatening Complication - Laryngeal diphtheria occurs in only **5–10%** of cases - Airway obstruction is a serious but **less frequent** complication compared to myocarditis - With prompt airway management (intubation/tracheostomy), mortality from airway obstruction is reduced - Myocarditis, by contrast, affects a far larger proportion of patients and is harder to prevent once toxin is absorbed ### Management - **Diphtheria Antitoxin (DAT):** Must be given immediately on clinical suspicion — neutralizes circulating toxin before it binds myocardium - **Antibiotics:** Penicillin G or erythromycin to eliminate the organism - **Cardiac monitoring:** Continuous ECG monitoring for all diphtheria patients - **Pacemaker:** May be required for complete heart block **Clinical Pearl:** The stem specifies "most common life-threatening complication" — this is myocarditis (B), not airway obstruction (C). Airway obstruction is the most dramatic early presentation but myocarditis is the most common cause of death overall in diphtheria (Nelson's Pediatrics, 21st ed.; Harrison's Principles of Internal Medicine, 21st ed.).