A 35-year-old woman from Delhi with known HIV infection (CD4 count 120 cells/μL) presents with acute onset dyspnea, fever, and dry cough for 2 weeks. She denies hemoptysis. Chest X-ray shows bilateral diffuse micronodular opacities with a 'snowstorm' appearance involving all lung fields, with relative sparing of the apices. Sputum smear is negative for acid-fast bacilli. What is the most likely diagnosis?

Explanation

## Miliary Tuberculosis: Clinical and Radiological Diagnosis ### Pathophysiology of Miliary TB **Key Point:** Miliary TB results from hematogenous dissemination of Mycobacterium tuberculosis, typically occurring during primary infection or reactivation with erosion into a blood vessel. It is a medical emergency with high mortality if untreated. ### Why This Patient Has Miliary TB 1. **Severe immunosuppression** (CD4 <200) — permits rapid dissemination 2. **Diffuse bilateral micronodular pattern** — pathognomonic for miliary disease 3. **Negative sputum smear** — miliary TB often has lower bacillary load in sputum despite widespread lung involvement 4. **Apical sparing** — classic feature distinguishing miliary from other diffuse patterns ### Radiological Features of Miliary TB | Feature | Characteristic | |---------|----------------| | **Pattern** | Diffuse, bilateral, 1–2 mm micronodules ("millet seed" appearance) | | **Distribution** | Uniform throughout both lungs; **apices often spared** | | **Cavitation** | Absent (unlike reactivation TB) | | **Hilar adenopathy** | Variable; may be minimal | | **Pleural involvement** | Uncommon unless complicated | | **Timeline** | Acute onset (days to weeks) | ### Clinical Clues for Miliary TB - **Immunocompromised state** (HIV, malignancy, immunosuppressive therapy) - **Acute presentation** with constitutional symptoms - **Negative sputum smear** despite radiological severity - **High mortality** if diagnosis delayed (>50% if untreated) **High-Yield:** The combination of **bilateral diffuse micronodules + apical sparing + immunocompromised host + negative sputum** = miliary TB until proven otherwise. **Mnemonic: MILIARY TB** — **M**icronodules, **I**mmune suppression, **L**ung fields (all), **I**nfectious (hematogenous), **A**pices spared, **R**apid onset, **Y** — Yes, urgent treatment needed. **Warning:** Do not confuse miliary TB with other diffuse micronodular patterns (sarcoidosis, silicosis, hypersensitivity pneumonitis). The clinical context (immunosuppression, fever, constitutional symptoms) and apical sparing favor TB. **Clinical Pearl:** Miliary TB is a medical emergency. Even with negative sputum, if clinical and radiological suspicion is high, initiate anti-TB therapy immediately — waiting for culture confirmation can be fatal. [cite:Harrison 21e Ch 205]