Bit · Micro
Tuberculosis — Primary, Latent, Secondary, Miliary
Four stages of TB infection. The pivot is whether the organism is contained or breaking out, and what the imaging shows.
Mechanism
Mycobacterium tuberculosis is an acid-fast bacillus (Ziehl-Neelsen stain), aerobic, slow-growing, intracellular pathogen of macrophages. It causes four distinct clinical pictures depending on host immunity:
- Primary TB — first infection. Most cases are asymptomatic. Bacilli reach alveolar macrophages → form a small subpleural lesion (Ghon focus). Hilar lymph nodes drain it → form the Ghon complex (Ghon focus + ipsilateral hilar node). Most contain the infection. T-cell hypersensitivity develops (PPD becomes positive). If host immunity fails: progressive primary TB.
- Latent TB — contained but not eradicated. No symptoms, no transmission. PPD or IGRA positive. Treat to prevent reactivation: isoniazid + B6 for 9 months, or rifampin for 4 months, or INH/rifampin for 3–4 months.
- Secondary (reactivation) TB — bacilli reactivate, often when host immunity wanes. Classically involves apex of lung (highest pO₂). Cavitary lesions in upper lobes. Symptoms: chronic cough, weight loss, night sweats, hemoptysis, fever. Acid-fast smear of sputum positive.
- Miliary TB — massive hematogenous dissemination. CXR shows millet seed pattern. Multi-organ involvement (lung, liver, bones, kidneys, meninges, adrenals).
- Extra-pulmonary TB — Pott disease (vertebral), TB meningitis (basilar), scrofula (cervical lymph nodes), TB peritonitis, Addison disease (adrenals), urogenital.
Differentiator Table
| Stage | Imaging | Symptoms | PPD | Sputum AFB |
|---|---|---|---|---|
| Primary | Ghon complex (focus + hilar node); often resolves | Often asymptomatic | Converts to positive | Often negative |
| Latent | Normal or stable Ghon complex | NONE | Positive | Negative |
| Secondary (reactivation) | Upper-lobe cavitary lesions | Chronic cough, weight loss, night sweats, hemoptysis | Positive | Positive (smear) or PCR |
| Miliary | Diffuse 'millet seed' nodules; multi-organ | Severe systemic + organ-specific | Often anergic in severe cases | Variable; PCR |
| Extra-pulmonary | Site-dependent (Pott: vertebra; TB meningitis: basilar) | Site-dependent | Often positive | Variable |
The Pivot
Three questions:
- Is the patient symptomatic? No, just positive PPD/IGRA → latent. Yes, with cavitary upper-lobe disease → secondary.
- Imaging — apex vs widespread? Apex cavitary → secondary. Diffuse miliary pattern → disseminated.
- Treatment regimen? Latent: INH 9 mo (with B6). Active: RIPE for 2 months (Rifampin, Isoniazid, Pyrazinamide, Ethambutol), then RI for 4 more months.
NBME-Style Stem
A 36-year-old immigrant from Eritrea presents with 3 months of productive cough, drenching night sweats, and a 7 kg weight loss. Chest x-ray shows cavitary lesions in the right upper lobe. Sputum acid-fast smear is positive. Which of the following is the most appropriate initial regimen?
Concept Anchor
TB has four faces: a contained first encounter (primary), a quiet detente (latent), a reactivated cavitary apex disease (secondary), and a hematogenous storm (miliary). Host immunity decides which face shows.