6. Pulmonary infections

Question 1

Types of pulmonary infections

Answer 1

1. Pneumonia
2. Lung abscess
3. Pulmonary TB

Question 2

Definition of pneumonia

Answer 2

Infection of the lung; can be caused by a variety of microbes which presents in 3 main pathological forms of pneumonia

Question 3

Predisposing factors for pneumonia

Answer 3

1. Loss or suppression of the cough reflex
   - Coma, anaesthesia, neuromuscular disorders, drugs
2. Injury to the mucociliary escalator
   - Cigarette smoking, inhalation of hot or corrosive gases, viral infections, genetic defects (e.g. Kartagener syndrome)
3. Accumulation of secretions
   - Bronchial obstruction, cystic fibrosis
4. Phagocytic cell dysfunction
   - Alcohol, cigarette smoking
5. Pulmonary congestion & edema

Question 4

Clinical classification of microbial causes of pneumonia

Answer 4

1. Community-acquired typical pneumonia
2. Community-acquired atypical pneumonia
3. Hospital-acquired pneumonia
4. Immunosuppression-related pneumonia
5. Aspiration pneumonia

Question 5

Microbes that result in community-acquired typical pneumonia

Answer 5

1. Streptococcus pneumoniae
2. Haemophilus influenzae
3. Moraxella catarrhalis
4. Staphylococcus aureus
5. Legionella pneumophila
6. Klebsiella pneumoniae
7. Pseudomonas spp.

Question 6

Microbes that result in community-acquired atypical pneumonia

Answer 6

1. Mycoplasma pneumoniae
2. Chlamydia spp.
3. Coxiella burnetii
4. Viruses (RSV, parainfluenza, influenza, adenovirus, SARS virus)

Question 7

Microbes that result in hospital-acquired pneumonia (nosocomial pneumonia)

Answer 7

1. Gram-negative rods (Pseudomonas spp. &
   enterobacteriaceae like Escherichia coli, Klebsiella spp.)
2. Staphylococcus aureus (usually penicillin resistant)

Question 8

Microbes that result in immunosuppression-related pneumonia

Answer 8

1. Cytomegalovirus
2. Pneumocystis jiroveci
3. Aspergillosis
4. Candidiasis

Question 9

Microbes that result in aspiration pneumonia

Answer 9

1. Anaerobic oral flora (Bacteroides, Fusobacterium, Peptostreptococcus)
2. Admixed aerobic (Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, Pseudomonas aeruginosa)

Question 10

Pathological classification of pneumonia

Answer 10

1. Lobar pneumonia
2. Bronchopneumonia
3. Atypical pneumonia (interstitial pneumonitis)

Question 11

Lobar pneumonia

Answer 11

1. Consolidation of a whole lobe or a large part of it
2. Usually seen in adults with poor social & medical
   care (alcoholics, vagrants)
3. Usual pathogens: Streptococcus pneumoniae, Klebsiella
4. 4 stages of inflammatory response: congestion → red
   hepatization → grey hepatization → resolution
5. Usually resolves with treatment

Question 12

Bronchopneumonia

Answer 12

1. Patchy consolidation (involves bronchioles & adjacent alveoli) commonly in lower lobes
2. Usually seen in infancy, old age & immunocompromised individuals
3. Can be caused by a variety of infective organisms depending on the circumstance
4. Usually results in focal organization & fibrosis

Question 13

Atypical pneumonia

Answer 13

1. Inflammation confined to the alveolar septa & lung interstitium without alveolar exudation (hence no physical findings of consolidation)
2. Usually seen in community-acquired atypical pneumonias (Mycoplasma, Chlamydia, Coxiella, viral)
3. Superimposed bacterial infection may produce the features of typical pneumonia (either lobar or bronchopneumonia)

Question 14

Morphology of lobar pneumonia

Answer 14

1. [Gross] Consolidation of whole or part of a lobe, imparting a liver-like consistency to it, hence ‘hepatization’ (firm, airless)
2. [Histology] Alveoli of affected lobe diffusely infiltrated by acute inflammatory exudate (neutrophils, fibrin)
3. Red hepatization: massive confluent exudation with neutrophils, red cells & fibrin filling the alveolar spaces
4. Grey hepatization: follows red hepatization, with progressive disintegration of red cells & persistence of the original fibrinosuppurative exudate

Question 15

Morphology of bronchopneumonia

Answer 15

1. [Gross] Patchy areas of consolidation

2. [Histology] Acute inflammatory infiltration of bronchioles & adjacent alveoli

Question 16

Morphology of atypical pneumonia

Answer 16

1. Widened edematous alveolar septa with mononuclear infiltrates
2. Neutrophils may be present in certain cases
3. No alveolar exudation

Question 17

Pathological effects & complications of pneumonia

Answer 17

1. Pleurisy
2. Lung abscess
3. Septicaemia
4. Lung fibrosis
   - Bronchopneumonia: focal fibrosis
   - Atypical pneumonia: interstitial fibrosis, leading to restrictive lung disease

Question 18

Definition of lung abscess

Answer 18

Localized pus-filled cavity in the lung due to local inflammatory destruction of lung tissue

Question 19

Causes of lung abscess

Answer 19

1. Aspiration of infective material (most common cause)
   - Particularly common in acute alcoholism, coma, anaesthesia, sinusitis, gingivodental sepsis, debilitation in which the cough reflex is suppressed
2. Antecedent lung infection (pneumonia, bronchiectasis)
   - Usually associated with Staphylococcus aureus,
   Klebsiella pneumoniae, Streptococcus pneumoniae
3. Septic embolism
   - Trapping of infected emboli from systemic venous
   circulation or infected vegetations from right-sided
   infective endocarditis in the lung
4. Neoplasms
   - Due to secondary infection of a bronchopulmonary segment obstructed by a malignancy
5. Spread of infection from other sites
   - Contiguous spread of a suppurative process from
   adjacent structures (esophagus, spine, subphrenic
   space, pleural cavity)
   - Hematogenous seeding by circulating organisms

Question 20

Pathological effects & complications of lung abscess

Answer 20

1. Rupture in pleural space, producing empyema

2. Bacteraemia

Question 21

Clincial features of lung abscess

Answer 21

1. Cough, foul-smelling purulent sputum
2. Fever, chest pain, weight loss
3. Clubbing (may appear within a few weeks from onset)

Question 22

Definition of pulmonary tuberculosis

Answer 22

Granulomatous inflammation of the lung due to infection by Mycobacterium tuberculosis

Question 23

Pathogenesis of primary TB

Answer 23

1. Occurs in individuals who have not been exposed to mycobacteria before (or in immunosuppressed)
2. Infection usually begins at periphery of the lung (subpleural) with the formation of a Ghon focus
3. Ghon focus may heal or spread to involved regional lymph nodes (Ghon focus + affected lymph nodes = Ghon complex)
4. Outcomes of primary tuberculosis:
   - Complete resolution
   - Latent tuberculosis
   - Progressive primary tuberculosis, in which
   tuberculous infection spreads to the pleura (tuberculous pleurisy), through the bronchi (tuberculous bronchopneumonia) & to distant sites through the blood (miliary tuberculosis)

Question 24

Pathogenesis of secondary TB

Answer 24

1. Occurs during exogenous infection or reactivation of
   latent tuberculosis in individuals who have been
   exposed or sensitised to mycobacteria previously
2. Infection usually at apical region of the upper lobe,
   with the formation of an Assmann focus
3. Granulomatous inflammation & destruction of lung tissue leads to the formation of cavities surrounded by fibrous tissue (accompanied by coughing out of bacilli)
4. Can lead on to progressive secondary tuberculosis with similar outcomes as progressive primary TB

Question 25

Morphology of caseating granuloma seen in TB

Answer 25

Central caseous necrosis surrounding by a peripheral ring of epithelioid cells (histiocytes), lymphocytes & Langhans giant cells (multinucleated, nuclei arranged along periphery in horseshoe formation)