Lung Infections - Tuberculosis

Question 1

Features of TB

Answer 1

• chronic pneumonia
• communicable, granulomatous
• caused by Mycobacterium tuberculosis
• initial infection usually inhaled, but does not only affect lungs
• within the lung, spread via bronchi, lymphatic, into pleura, blood (miliary)

Question 2

Features of Mycobacteria

Answer 2

• slender rod-shaped, waxy cell wall
• resistant to destruction by neutrophils
• only macrophages can phagocytose & contain mycobacteria, but it can still proliferate
• own cells killed while killing mycobacteria - cavities
• bind Ziehl-Neelsen stain, resist discolouration (AFB)

Question 3

Pattern of disease in TB

Answer 3

1. Primary TB - no previous exposure or poor immune system
2. Secondary TB - previous exposure, sensitised
3. Miliary TB

Question 4

Pathogenesis in primary TB

Answer 4

• inhalation - infection & necrosis at lung periphery, just beneath the pleura (Ghon focus)
• heal or spread to involved local lymph nodes at hilum (Ghon focus + affected lymph nodes = Ghon complex)
• enlarge through granulomatous inflammation & caseation (necrosis)

Question 5

Outcomes of primary TB (3)

Answer 5

1. Complete resolution (95%), only residual evidence of infection remaines
2. Latent TB - viable organisms lie dormant in foci, infected but inactive - cannot transmit
3. Progressive primary TB - severe pneumonia & dissemination, continuining enlargement of caseating granulomas in lymph nodes, spread by enlarging nodes eroding through pleura, wall of bronchus/vessel

Question 6

Pathogenesis of secondary TB

Answer 6

• exogenous infection (acq new organisms) or reactivation of primary complex
• usually in the apex of the lung - Assmann focus - central area of caseous necrosis surrounded by granulomas
• immune system recognises infection, rapid mobilisation of defence reaction - tries to contain it, less lymph node involvement
• destruction of lung tissue + granulomatous inflammation - cavitation

Question 7

Outcomes of secondary TB (2)

Answer 7

1. Healing - leaves central area of caseous necrotic material surrounded by a thick dense collagenous wall - calcifies - fibrocaseous TB (can reactivate)
2. Progression - enlargement of apical lesion, continued tissue destruction, risk of erosion into vessels/airways - spread