A 5-year-old child with diphtheria develops acute myocarditis with congestive heart failure on day 10 of illness. Which is the most common cause of mortality in diphtheria?

Explanation

## Most Common Cause of Mortality in Diphtheria **Key Point:** Diphtheria toxin-induced myocarditis is the most common cause of death in diphtheria, accounting for approximately 50–70% of all diphtheria-related mortality. ### Pathophysiology of Diphtheria Toxin **High-Yield:** Diphtheria toxin is an exotoxin produced only by *Corynebacterium diphtheriae* strains lysogenized with a β-phage carrying the *tox* gene. The toxin: 1. Catalyzes ADP-ribosylation of elongation factor 2 (EF-2) 2. Inhibits protein synthesis in host cells 3. Causes myocardial necrosis, conduction abnormalities, and heart failure ### Timeline and Complications of Diphtheria | Complication | Onset | Frequency | Mortality Risk | |--------------|-------|-----------|----------------| | **Myocarditis** | Day 5–10 | 50–70% of deaths | **Highest** | | Airway obstruction | Day 1–3 | 5–10% of deaths | Moderate (if untreated) | | Cranial nerve palsies | Week 2–4 | Non-fatal | Low | | Respiratory paralysis | Week 3–4 | Rare | Moderate (if ventilation unavailable) | | Aspiration pneumonia | Variable | ~5% of deaths | Low | **Clinical Pearl:** Myocarditis in diphtheria presents insidiously: - Tachycardia disproportionate to fever - Cardiomegaly on chest X-ray - Conduction abnormalities (prolonged PR interval, heart block) - Arrhythmias and sudden cardiac death - Fulminant heart failure in severe cases Early antitoxin administration (within 48 hours) reduces myocarditis risk significantly. **Mnemonic:** **TOXIN KILLS THE HEART** — The diphtheria toxin, not the pseudomembrane obstruction, is the primary killer. Antitoxin must be given early to neutralize circulating toxin before it damages the myocardium. [cite:Harrison 21e Ch 131]