## Diphtheria: Immediate Antitoxin Administration ### Why Antitoxin First? **Key Point:** Diphtheria antitoxin (DAT) must be administered **immediately on clinical suspicion**, without waiting for culture confirmation. Delaying antitoxin administration increases mortality and the risk of systemic toxin complications (myocarditis, neuropathy). **High-Yield:** The diphtheria toxin is produced by *Corynebacterium diphtheriae* lysogenized with β-phage carrying the *tox* gene. The toxin inhibits protein synthesis by inactivating elongation factor 2 (EF-2), causing tissue necrosis and systemic effects. Once the toxin binds to tissue receptors, antitoxin cannot neutralize it — only circulating toxin is neutralized. ### Clinical Diagnosis of Diphtheria | Feature | Diphtheria | |---------|----------| | **Pseudomembrane** | Gray-white, adherent, bleeds when scraped, extends beyond tonsils | | **Onset** | Insidious (sore throat, low-grade fever) | | **Systemic signs** | Bull neck (cervical edema), lethargy, myocarditis (arrhythmias), cranial nerve palsies | | **Complications** | Airway obstruction, myocarditis (heart block, cardiogenic shock), neuropathy (palatal, pharyngeal, laryngeal, ocular) | | **Culture** | Loeffler's medium or tellurite agar; gray-black colonies | **Clinical Pearl:** The "bull neck" appearance (cervical edema and lymphadenopathy) is a sign of severe disease and increased risk of myocarditis. ### Management Algorithm ```mermaid flowchart TD A[Clinical suspicion of diphtheria]:::outcome --> B{Pseudomembrane present?}:::decision B -->|Yes| C[Administer DAT immediately]:::action B -->|No| D[Consider other diagnoses] C --> E[Start antibiotics: Penicillin G IV or Erythromycin]:::action E --> F[Supportive care: airway management, cardiac monitoring]:::action F --> G[Respiratory isolation until 2 negative cultures]:::action G --> H[Send throat swab for culture confirmation]:::action H --> I[Notify public health authorities]:::action ``` ### Antitoxin Administration **Key Point:** Diphtheria antitoxin is a **horse serum-derived preparation** and carries a risk of serum sickness (5–10% of recipients). However, the risk of not giving antitoxin far outweighs the risk of serum sickness. **Dosing:** - Mild/moderate diphtheria: 20,000–40,000 units IV - Severe diphtheria (bull neck, respiratory involvement): 40,000–60,000 units IV - Laryngeal diphtheria: 40,000 units IV **Administration:** 1. Test for horse serum sensitivity (intradermal test with 0.1 mL of 1:100 dilution) 2. If negative, give full dose IV slowly (over 30–60 minutes) 3. If positive, desensitize before giving antitoxin ### Antibiotic Therapy **After antitoxin**, start antibiotics: - **Penicillin G:** 100,000 units/kg/day IV in 4 divided doses (for 7–10 days) - **Alternative:** Erythromycin 40–50 mg/kg/day IV in 4 divided doses (for 7 days) **Oral penicillin V is inadequate** for systemic diphtheria and does not achieve sufficient tissue levels. ### Supportive Care 1. **Airway management:** Prepare for emergency intubation or tracheostomy if respiratory involvement develops 2. **Cardiac monitoring:** Continuous ECG to detect myocarditis (heart block, arrhythmias) 3. **Respiratory isolation:** Until 2 consecutive negative cultures (48 hours apart) after completing antibiotics 4. **Public health notification:** Mandatory reporting; close contacts require prophylaxis ### Why Waiting for Culture Is Wrong **Warning:** Culture confirmation takes 24–48 hours. During this delay, circulating toxin continues to damage myocardium and nerves. Mortality increases significantly if antitoxin is delayed beyond 48 hours of symptom onset.
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