A 35-year-old woman from Delhi presents with a 6-month history of persistent dry cough and constitutional symptoms. She is HIV-negative and has no known TB contact. Chest X-ray reveals multiple small nodular opacities (1–2 mm) distributed bilaterally and diffusely throughout both lung fields, with a 'millet seed' appearance. There is no cavitation. Sputum smear microscopy is negative. What is the most likely radiological diagnosis?

Explanation

## Miliary Tuberculosis: Radiological Recognition **Key Point:** Miliary TB is characterized by bilateral, diffuse, small (1–2 mm) nodular opacities resembling millet seeds, resulting from hematogenous dissemination of *Mycobacterium tuberculosis*. This is a medical emergency with high mortality if untreated. ### Pathophysiology of Miliary TB 1. **Mechanism of dissemination** - Rupture of a caseous focus into a pulmonary vein (erosion) - Hematogenous spread throughout the lungs - Simultaneous seeding of multiple organs (extrapulmonary TB) 2. **Timeline** - Can occur during primary TB (weeks to months after infection) - More commonly occurs as reactivation TB (years after initial infection) - In this case, the 6-month history suggests progressive hematogenous dissemination ### Radiological Features of Miliary TB **Classic "Millet Seed" Pattern:** - Small, uniform nodules (1–2 mm, sometimes up to 3 mm) - Bilateral and diffuse distribution - No cavitation (unlike active secondary TB) - Acinar pattern (air bronchograms may be present) - Hilar lymphadenopathy may or may not be present **High-Yield:** The **absence of cavitation** combined with **bilateral, diffuse, small nodular opacities** is the key distinguishing feature from active cavitary TB. ### Why Sputum Smear is Negative In miliary TB, the nodules are small and distributed throughout the lung parenchyma without cavitation. There is no endobronchial involvement, so acid-fast bacilli are rarely found in sputum. Diagnosis requires: - **Bronchoscopy with bronchoalveolar lavage (BAL)** — higher yield - **Transbronchial lung biopsy** — can show caseating granulomas - **CT chest** — better sensitivity for detecting small nodules - **Blood cultures** (in some cases) - **Clinical correlation** with constitutional symptoms and risk factors ### Comparison of TB Radiological Patterns | Feature | Active Cavitary TB | Miliary TB | Primary TB | |---------|-------------------|------------|------------| | **Nodule size** | Large (cavities >1 cm) | Small (1–2 mm) | Variable | | **Distribution** | Upper lobe, unilateral | Bilateral, diffuse | Any lobe | | **Cavitation** | Present | Absent | Absent | | **Sputum smear** | Often positive | Usually negative | Negative | | **Lymphadenopathy** | Hilar | May be present | Prominent hilar/mediastinal | | **Urgency** | High | **Very high** (medical emergency) | **Clinical Pearl:** Miliary TB is a medical emergency with mortality rates of 20–40% even with treatment if diagnosis is delayed. Any patient with constitutional symptoms and bilateral diffuse nodular opacities should raise suspicion for miliary TB, and empiric anti-TB therapy should be considered while awaiting confirmatory tests. **Mnemonic: MILIARY TB — Millet-seed appearance, Immediate dissemination via hematogenous route, Lethal if untreated, Involves multiple organs, Absence of cavitation, Requires urgent diagnosis and treatment, Yields negative sputum smear** [cite:Harrison 21e Ch 205, Robbins 10e Ch 8]