A 5-year-old girl presents to the emergency department with acute onset of respiratory distress and stridor. She has a 5-day history of sore throat and low-grade fever. On examination, she has a thick, adherent pseudomembrane extending into the larynx and trachea. Cervical lymph nodes are markedly enlarged. A throat swab culture on Löffler's serum medium grows black colonies. The Elek test is positive. Which of the following complications is the child at HIGHEST risk for in the next 2–3 weeks if antitoxin is not administered immediately?

Explanation

## Complication: Diphtheria Myocarditis **Key Point:** Myocarditis is the MOST COMMON and MOST LETHAL systemic complication of diphtheria, occurring in 10–25% of cases. It develops 1–3 weeks after onset of local disease and is caused by circulating diphtheria toxin, NOT by direct bacterial invasion. ### Pathophysiology of Diphtheria Toxin-Induced Myocarditis 1. **Toxin absorption:** Diphtheria toxin is absorbed from the pharyngeal pseudomembrane into the bloodstream 2. **Cardiac targeting:** The toxin circulates and reaches the myocardium 3. **Mechanism:** ADP-ribosylation of elongation factor-2 (EF-2) → inhibition of protein synthesis in cardiac myocytes 4. **Result:** Myocyte necrosis, inflammation, and conduction system damage **High-Yield:** The severity of myocarditis correlates with: - **Extent of pseudomembrane** (larger membrane = more toxin production) - **Delay in antitoxin administration** (early antitoxin neutralizes circulating toxin; late antitoxin cannot reverse tissue damage) - **Virulence of the strain** (toxigenic strains produce more toxin) ### Clinical Manifestations of Diphtheria Myocarditis | Timing | Presentation | |--------|-------------| | Days 1–5 | Tachycardia out of proportion to fever; may be asymptomatic | | Days 5–14 | Congestive heart failure (dyspnea, edema, hepatomegaly) | | Days 7–21 | **Conduction abnormalities:** heart block (1st, 2nd, 3rd degree), arrhythmias, sudden cardiac death | | Late (weeks 2–3) | Cardiogenic shock, fulminant heart failure | **Clinical Pearl:** Complete heart block (3rd degree AV block) is the most characteristic arrhythmia of diphtheria myocarditis and is a poor prognostic sign. ECG changes include prolonged PR interval, widened QRS, and ST-T wave changes. ### Why Antitoxin Timing Is Critical **Mnemonic:** **ANTITOXIN WINDOW = First 24–48 hours is optimal** - **Antitoxin mechanism:** Neutralizes circulating toxin only; does NOT reverse toxin already bound to EF-2 - **Early administration (< 24 hrs):** Prevents systemic toxicity; mortality < 5% - **Delayed administration (> 48 hrs):** Toxin already bound to tissues; myocarditis develops despite antitoxin; mortality 20–40% - **This case:** 5-day history = toxin has already circulated and damaged myocardium; antitoxin will not prevent myocarditis but may limit further progression ### Diagnosis of Myocarditis - **ECG:** Prolonged PR interval, AV block, arrhythmias, ST-T changes - **Echocardiography:** Reduced ejection fraction, wall motion abnormalities, chamber dilation - **Cardiac biomarkers:** Elevated troponin I/T, BNP - **Clinical signs:** Tachycardia, gallop rhythm, signs of heart failure ### Management of Diphtheria Myocarditis 1. **Antitoxin** (equine antitoxin) — still given to neutralize any remaining circulating toxin 2. **Antibiotics** (penicillin G or erythromycin) — eliminate bacteria 3. **Cardiac monitoring** — continuous ECG monitoring for arrhythmias and conduction blocks 4. **Pacemaker consideration** — temporary or permanent pacing for high-degree AV block 5. **Supportive care** — inotropes, diuretics, mechanical ventilation if needed **High-Yield:** Mortality from diphtheria is primarily due to myocarditis and respiratory paralysis (from cranial nerve involvement), NOT from airway obstruction by the pseudomembrane alone. [cite:Textbook of Microbiology by Ananthanarayan & Paniker, Ch 20; Harrison's Principles of Internal Medicine, 21e, Ch 139]