Tuberculosis Pathology
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Direct answer
Primary tuberculosis is the first infection of an unsensitised host: a subpleural parenchymal focus — the Ghon focus — anywhere in the lung with drained hilar lymph nodes, the two together forming the Ghon complex, which heals with calcification as the Ranke complex in most patients. Secondary (post-primary) tuberculosis arises by reactivation or reinfection in the sensitised host and prefers the apices, where high oxygen tension and poor lymphatic drainage favour the bacillus; its caseating lesions liquefy into cavities that erode vessels (haemoptysis, Rasmussen aneurysm) and seed sputum — the infectious open tuberculosis. Haematogenous dissemination gives miliary tuberculosis, and in India the same bacillus must be named first in every granuloma differential, from ileocaecal disease to Pott spine to basal meningitis.
What you must remember
- Ghon focus: 1–2 centimetre subpleural caseating lesion, usually lower or mid-zone; plus caseating hilar nodes = Ghon complex; healed and calcified = Ranke complex — the trio of eponyms every paper asks.
- Progressive primary disease: in the young, immunosuppressed or malnourished, the primary focus enlarges, cavitates, or drains into bronchi or vessels — India's under-five and HIV-positive populations are the vulnerable groups.
- Secondary tuberculosis: apical and posterior segments of upper lobes and apical lower lobes — oxygen-rich, poorly drained; limited lymph-node involvement (unlike primary); cavitary, transmissibility high.
- Cavity complications: erosions cause haemoptysis — Rasmussen aneurysm is a dilated pulmonary artery branch within a cavity wall; seeding of the lung (Simon foci); Aspergillus colonisation (aspergilloma); rarely pneumothorax and bronchiectasis.
- Miliary tuberculosis: haematogenous spread producing 1–2 millimetre yellow-white foci in lung, liver, spleen, marrow and meninges; fever of unknown origin with choroidal tubercles on fundoscopy.
- Tuberculous meningitis: gelatinous exudate at the basal cisterns causing hydrocephalus and cranial nerve palsies; cerebrospinal fluid shows lymphocytosis, low glucose, raised protein and a cobweb clot on standing; the most lethal form in children.
- Organ patterns worth quoting: ileocaecal disease with caseating granulomas (the Crohn mimic); Pott spine with paravertebral cold abscess and gibbus; adrenal destruction causing Addison disease; scrofula (cervical nodes).
- Latent infection: viable bacilli held in granulomas; roughly 5–10 per cent lifetime reactivation risk in healthy adults, rising to about 10 per cent per year with untreated HIV — the arithmetic that drives contact screening.
- Programme frame: under India's National Tuberculosis Elimination Programme, CBNAAT is the first diagnostic test (and detects rifampicin resistance), treatment is daily directly-observed therapy, and NIKSHAY notification is mandatory.
From one inhaled bacillus to every organ
A 22-year-old migrant worker inhales droplet nuclei; a single bacillus lodges in a lower-lobe subpleural alveolus and multiplies silently for two to four weeks until cell-mediated immunity matures. Then infected macrophages activate into epithelioid cells, Langhans giant cells form, and the centre caseates — the Ghon focus — while draining hilar nodes undergo the same change (the Ghon complex). In 90-odd per cent the lesion fibroses and calcifies (Ranke complex) and the person never knew: latent infection. Five years later, worn down by diabetes and overwork, reactivation begins in his right apex, where bacilli have always preferred the oxygen: the caseous centre liquefies, ruptures into a bronchus, and coughs out a cavity — open, sputum-positive tuberculosis, contagious to his household.
Follow the same bacillus one step further, into a vein or lymphatic: showers of organisms lodge as millet-seed foci through lungs, liver, spleen, marrow and meninges. Fundoscopy shows choroidal tubercles, and a bone-marrow trephine yielding caseating granulomas closes the fever of unknown origin. If the meningeal seed takes hold, basal gelatinous exudate knits around the circle of Willis, producing hydrocephalus and sixth-nerve palsy, with lymphocytic, low-glucose, high-protein fluid forming a cobweb clot on standing. One chapter of natural history, six exam questions.
How the examiner frames it
Primary versus secondary is compared as a table of the mind — site (subpleural anywhere versus apical), nodes (prominent versus scant), cavitation (rare versus typical), contagion (usually closed versus open) — and the candidate who structures it that way takes full marks. The Crohn-versus-intestinal-TB question is the Indian end of the chapter: confluent caseating granulomas favour tuberculosis, and Indian practice permits a diagnostic therapeutic trial when tissue is inconclusive. The HIV question pairs with reactivation arithmetic — 10 per cent per year, not per lifetime.
Frequently asked questions
What constitutes the Ghon complex and the Ranke complex?
The Ghon complex is the subpleural primary focus plus its caseating hilar lymph nodes; once healed and calcified it is called the Ranke complex.
Why does secondary tuberculosis favour the lung apices?
High oxygen tension favours the aerobic bacillus, and poor lymphatic drainage at the apices limits immune clearance — together making the apex the reactivation site.
What is a Rasmussen aneurysm?
A pseudoaneurysm of a pulmonary artery branch within a tuberculous cavity wall; its rupture causes massive haemoptysis.
Which form of tuberculosis involves the basal cisterns and why does it matter?
Tuberculous meningitis — gelatinous basal exudate causing hydrocephalus and cranial nerve palsies, with lymphocytic cerebrospinal fluid, low glucose and raised protein; the most lethal form, especially in children.
How does intestinal tuberculosis mimic Crohn disease and what favours TB?
Both cause ileocaecal thickening, ulcers and fistulae; confluent caseating granulomas (and acid-fast bacilli or positive CBNAAT) favour tuberculosis, the standard Indian viva answer.