A 6-month-old female infant from rural Maharashtra presents with a 3-week history of paroxysmal cough preceded by a 1-week prodrome of coryza and low-grade fever. The cough is characterized by rapid, staccato bursts followed by a high-pitched inspiratory 'whoop' sound. The mother reports the infant has turned cyanotic during coughing episodes and has vomited post-tussively. On examination, the child appears malnourished with mild subcutaneous emphysema over the chest. There is no rash. Chest X-ray shows bilateral perihilar infiltrates with a 'shaggy heart' appearance. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Pertussis (Whooping Cough) ### Key Clinical Features **Key Point:** Pertussis is caused by *Bordetella pertussis* and presents in three classic stages: catarrhal (1–2 weeks), paroxysmal (2–8 weeks), and convalescent (weeks to months). This infant is in the **paroxysmal stage**, which is pathognomonic: - Rapid, staccato bursts of cough ("machine-gun" cough) - High-pitched inspiratory whoop (due to laryngeal spasm during recovery of breath) - Post-tussive vomiting (from forceful expulsion) - Cyanosis during paroxysms (hypoxemia from prolonged expiration) - Subcutaneous emphysema (from rupture of alveoli under pressure) ### Radiological Findings **High-Yield:** The "shaggy heart" or "shaggy heart border" appearance on CXR is characteristic of pertussis — caused by peribronchial infiltration and atelectasis, not cardiac pathology. ### Prodromal Phase Recognition The initial 1-week history of coryza, fever, and mild cough (catarrhal stage) is often mistaken for a viral upper respiratory infection, delaying diagnosis. By the time the paroxysmal stage begins, the diagnosis becomes obvious. ### Why This Is Pertussis, Not Diphtheria | Feature | Pertussis | Diphtheria | |---------|-----------|----------| | **Onset** | Gradual (catarrhal → paroxysmal) | Acute (sore throat, low-grade fever) | | **Cough** | Paroxysmal with whoop | Absent or mild, non-paroxysmal | | **Membrane** | None | Pseudomembrane (pharyngeal, laryngeal, or nasal) | | **Toxin** | Pertussis toxin (systemic effects) | Diphtheria toxin (myocarditis, neuropathy) | | **CXR** | Shaggy heart, perihilar infiltrates | Normal or mild infiltrates | **Clinical Pearl:** Diphtheria presents with a **pseudomembrane** (gray-white, adherent, bleeds if removed) and systemic toxin effects (myocarditis, cranial nerve palsies); pertussis does not produce a membrane. ### Diagnostic Confirmation 1. **Culture:** Nasopharyngeal swab on Bordet-Gengou or Regan-Lowe medium (best in catarrhal/early paroxysmal stage) 2. **PCR:** Highly sensitive, especially in paroxysmal stage 3. **Serology:** IgM/IgG antibodies (useful in late paroxysmal or convalescent stage) 4. **Lymphocytosis:** Absolute lymphocytosis (15,000–50,000/µL) is typical ### Management Implications **Key Point:** Azithromycin (10–12 mg/kg/day for 5 days) is the antibiotic of choice; it shortens infectivity if given in the catarrhal or early paroxysmal stage but does not shorten cough duration once established. Close contacts (unvaccinated or partially vaccinated) require post-exposure prophylaxis with azithromycin.