Primary pulmonary tb for md exams 10 marks structured answer from this pdf only
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Primary Pulmonary Tuberculosis
10-Mark Structured Answer
(Based entirely on: Sharma SK, Mohan A - Textbook of Tuberculosis and Nontuberculous Mycobacterial Diseases, 3rd Ed.)
1. Definition
Primary pulmonary tuberculosis (TB) is the disease caused by Mycobacterium tuberculosis (Mtb) in a person with no previous exposure to the organism. It occurs when Mtb is inhaled for the first time and the host has not yet developed acquired immunity. In TB-endemic areas like India, primary TB predominantly affects children, but can also occur in adults who have lost immunity due to senescence, malnutrition, or immunodeficiency.
2. Pathogenesis and Natural History
Entry and Initial Implantation
- Mtb enters via inhalation of airborne droplet nuclei (1-5 µm diameter, containing 1-5 bacilli).
- The lung is the most frequent portal of entry.
- One bout of cough produces 3,000 droplet nuclei; Mtb can survive in the dark for several hours but direct sunlight kills it quickly.
Progression after Implantation
- The organism sets up a localized infection in the periphery of the lung (usually subpleural, in the upper part of the lower lobe or lower portion of the middle lobe on the right).
- From here, bacilli disseminate via lymphatics to the draining tracheobronchial lymph nodes.
- 4-6 weeks later: tuberculin hypersensitivity develops, with mild fever and malaise.
- A bacillaemia is common at this stage, seeding bacilli haematogenously to many tissues and organs.
3. The Primary Complex (Ghon Complex)
The primary complex of Ranke comprises three elements:
- Ghon's focus - the primary parenchymal lesion (usually < 1 cm, often inapparent, subpleural, in the middle portion of the lung)
- Draining lymphatics (afferent lymphangitis)
- Enlarged ipsilateral hilar/tracheobronchial lymph nodes
Key point (Parrot's Law): "Pulmonary TB does not exist in the child without involvement of the tracheobronchial gland." The lymph nodes are invariably involved and are often much larger than the parenchymal focus - this is the opposite of post-primary TB.
- A single Ghon's complex was identified in 58% and multiple in 16% of cases (Medlar's autopsy study).
- Bilateral adenopathy is uncommon except with left-sided primary foci.
4. Histopathology
The histopathological hallmark of TB is the granuloma - specifically a necrotising epithelioid cell granuloma:
- Epithelioid histiocytes - activated macrophages with abundant pale cytoplasm, indistinct margins
- Langhans' giant cells - nuclei arranged peripherally in a rosette (pathognomonic of TB)
- Central caseous necrosis - distinguishes TB from sarcoidosis (which shows non-necrotising granulomas)
- Peripheral lymphocyte cuff
- AFB demonstrated by ZN staining in 30-40% of histological sections (up to 60-70% on cytology smears)
Reticulin: TB granulomas are reticulin-poor (lack reticulin fibres), unlike sarcoid granulomas.
5. Fate and Complications of the Primary Complex
| Outcome | Details |
|---|---|
| Healing (most common) | Resorption of caseous material → fibrosis → calcification (85% calcify; microscopic calcification within 2 months, radiologically visible after 1+ year) |
| Residual scar | Hyalinised scar / calcified Ghon focus visible on CXR = "healed primary complex" |
| Progressive primary TB | Extension of inflammatory process → TB pneumonia ("galloping consumption/pneumonia alba") - seen in HIV patients today |
| Liquefaction | Cavity formation with shaggy margins (related to delayed hypersensitivity) |
| Lymph node complications | Caseous lymph nodes may: (a) compress adjacent bronchus → epituberculosis (atelectasis) or obstructive emphysema (ball-valve effect); (b) erode through bronchial wall → bronchogenic dissemination |
| Pleural effusion | Rupture of subpleural focus into pleural cavity → TB pleurisy |
| Miliary TB / Meningitis | Haematogenous dissemination from primary complex - occurs in ~10% of children under 2 years, 2-9 months after infection |
| Simon foci | Single or multiple apical caseous nodules formed during haematogenous spread - precursor of post-primary TB |
Calcification does not imply a sterile lesion - organisms can remain dormant for decades.
6. Clinical Features
Most primary TB infections are clinically silent (inapparent). Where symptoms occur:
- Constitutional: Mild fever (low-grade, evening onset), malaise, weight loss, night sweats
- Respiratory: Cough (usually minor), breathlessness (if complications develop)
- Local compression effects: Localised wheeze (bronchial compression by enlarged nodes), obstructive emphysema
Hypersensitivity manifestations (at time of tuberculin conversion):
- Erythema nodosum
- Phlyctenular conjunctivitis
- Fever
Note: These are uncommon today.
7. Radiological Features
Chest X-ray (Primary TB - Classical):
- Parenchymal infiltrate - patchy consolidation, usually in the middle zones (not the apices, which is characteristic of post-primary TB)
- Unilateral hilar/mediastinal lymphadenopathy - the lymph node component is larger and more prominent than the parenchymal lesion; hilar enlargement persists longer than parenchymal lesions
- Calcification of lymph nodes and lung lesion (years later)
CT Findings:
- Primary TB appears as air-space consolidation with hilar or mediastinal lymphadenitis
- Lymph nodes with central low attenuation and peripheral rim enhancement on CECT strongly suggest mycobacterial infection
- CT detects subtle parenchymal changes and mediastinal involvement better than plain CXR
Complications on imaging:
- Atelectasis (epituberculosis) - from complete bronchial obstruction
- Obstructive emphysema - from ball-valve effect
- Unilateral pleural effusion
- Miliary pattern (fine mottling) if haematogenous dissemination occurs
8. Diagnosis
Definitive diagnosis involves detection and isolation of Mtb.
| Test | Details |
|---|---|
| Sputum smear (ZN stain) | Ziehl-Neelsen / Kinyoun / Auramine-Rhodamine; PPV >90% in pulmonary TB |
| Sputum culture | Gold standard for confirmation and drug susceptibility testing |
| Tuberculin Skin Test (TST) | Conversion indicates infection; not diagnostic alone |
| IGRA | Interferon-gamma release assay |
| CB-NAAT (GeneXpert) | Rapid molecular detection; also detects rifampicin resistance |
| Haematology | Raised ESR (non-specific surrogate marker); anaemia, leucocytosis may occur |
| Imaging | CXR (first-line); CT/HRCT for complex cases |
| Bronchoscopy | For smear-negative patients - yield >90% with culture; BAL, brush, biopsy |
| Gastric lavage | Preferred over BAL in children for bacteriological confirmation |
| Histopathology | Necrotising epithelioid cell granuloma = compatible with TB even if AFB not demonstrated |
9. Differences: Primary vs Post-Primary TB
| Feature | Primary TB | Post-Primary TB |
|---|---|---|
| Host immunity | Non-immune (first exposure) | Previously sensitised |
| Age | Usually children (adults if immunodeficient) | Adults |
| Parenchymal lesion | Small, often inapparent | Large, prominent; caseation/cavitation prominent |
| Lymph node involvement | Prominent, often larger than parenchymal focus | Overshadowed by parenchymal disease |
| Location | Middle zones, subpleural | Apical/posterior segments of upper lobes, superior segment of lower lobes (>95%) |
| Cavitation | Rare | Common (abundant sites for Mtb growth) |
| Miliary spread | More common | Less common |
| Chest radiograph | Consolidation + hilar adenopathy | Upper lobe infiltrates, cavities, fibrosis |
10. Treatment
Refer to standard anti-TB chemotherapy regimens (Chapter 44 in Sharma & Mohan). The general principle is DOTS-based short-course chemotherapy.
- Standard regimen: 2HRZE/4HR (2 months Isoniazid + Rifampicin + Pyrazinamide + Ethambutol, followed by 4 months Isoniazid + Rifampicin)
- Primary TB in children treated under the Revised National Tuberculosis Control Programme (RNTCP/NTP) guidelines.
- Response assessed by clinical, bacteriological, and radiological improvement.
Key Exam Points to Remember:
- Ghon's focus = primary parenchymal lesion; Ghon's complex = Ghon focus + draining lymph nodes (lung only)
- Primary complex of Ranke = general term for any organ (includes lymphatics)
- Lymph node > parenchymal lesion = primary TB; reverse = post-primary TB
- Parrot's Law: primary pulmonary TB always has tracheobronchial lymph node involvement
- Simon foci (apical haematogenous foci) = precursors of post-primary TB
- Calcification does NOT mean sterility - dormancy can last a lifetime
- Primary TB is mostly asymptomatic and self-healing; progression occurs in ~10%
- Miliary TB/meningitis occur within 2-9 months in ~10% of infants under 2 years
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