A 7-year-old child with diphtheria is brought to the hospital. Which of the following complications is most likely to occur if antitoxin is not administered within the first 48 hours of symptom onset?

Explanation

## Complication: Myocarditis in Diphtheria **Key Point:** Diphtheria toxin causes myocarditis and conduction abnormalities as the most serious and life-threatening complication. This occurs due to direct toxin-mediated damage to cardiac myocytes and the conduction system, leading to arrhythmias and heart block. **Pathophysiology of Diphtheria Toxin:** - Inhibits EF-2, blocking protein synthesis in myocardial cells - Causes myocyte necrosis and inflammation - Affects the conduction system (AV node, bundle branches) - Results in: - Myocarditis (10–25% of cases) - Conduction blocks (1st, 2nd, 3rd degree) - Arrhythmias - Cardiogenic shock **Clinical Pearl:** - Myocarditis typically develops in the 2nd–3rd week of illness - ECG changes: prolonged PR interval, ST-T changes, AV block - Early antitoxin administration (within 48 hours) significantly reduces toxin-mediated complications - Mortality in untreated diphtheria is 5–10%, primarily due to myocarditis **High-Yield:** Antitoxin is effective only against circulating toxin, not toxin already bound to tissue. Hence, early administration is critical. Antibiotics (penicillin or erythromycin) eliminate the organism but do not neutralize toxin.