6. Pulmonary infections Flashcards
Types of pulmonary infections
- Pneumonia
- Lung abscess
- Pulmonary TB
Definition of pneumonia
Infection of the lung; can be caused by a variety of microbes which presents in 3 main pathological forms of pneumonia
Predisposing factors for pneumonia
- Loss or suppression of the cough reflex
- Coma, anaesthesia, neuromuscular disorders, drugs - Injury to the mucociliary escalator
- Cigarette smoking, inhalation of hot or corrosive gases, viral infections, genetic defects (e.g. Kartagener syndrome) - Accumulation of secretions
- Bronchial obstruction, cystic fibrosis - Phagocytic cell dysfunction
- Alcohol, cigarette smoking - Pulmonary congestion & edema
Clinical classification of microbial causes of pneumonia
- Community-acquired typical pneumonia
- Community-acquired atypical pneumonia
- Hospital-acquired pneumonia
- Immunosuppression-related pneumonia
- Aspiration pneumonia
Microbes that result in community-acquired typical pneumonia
- Streptococcus pneumoniae
- Haemophilus influenzae
- Moraxella catarrhalis
- Staphylococcus aureus
- Legionella pneumophila
- Klebsiella pneumoniae
- Pseudomonas spp.
Microbes that result in community-acquired atypical pneumonia
- Mycoplasma pneumoniae
- Chlamydia spp.
- Coxiella burnetii
- Viruses (RSV, parainfluenza, influenza, adenovirus, SARS virus)
Microbes that result in hospital-acquired pneumonia (nosocomial pneumonia)
- Gram-negative rods (Pseudomonas spp. &
enterobacteriaceae like Escherichia coli, Klebsiella spp.) - Staphylococcus aureus (usually penicillin resistant)
Microbes that result in immunosuppression-related pneumonia
- Cytomegalovirus
- Pneumocystis jiroveci
- Aspergillosis
- Candidiasis
Microbes that result in aspiration pneumonia
- Anaerobic oral flora (Bacteroides, Fusobacterium, Peptostreptococcus)
- Admixed aerobic (Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, Pseudomonas aeruginosa)
Pathological classification of pneumonia
- Lobar pneumonia
- Bronchopneumonia
- Atypical pneumonia (interstitial pneumonitis)
Lobar pneumonia
- Consolidation of a whole lobe or a large part of it
- Usually seen in adults with poor social & medical
care (alcoholics, vagrants) - Usual pathogens: Streptococcus pneumoniae, Klebsiella
- 4 stages of inflammatory response: congestion → red
hepatization → grey hepatization → resolution - Usually resolves with treatment
Bronchopneumonia
- Patchy consolidation (involves bronchioles & adjacent alveoli) commonly in lower lobes
- Usually seen in infancy, old age & immunocompromised individuals
- Can be caused by a variety of infective organisms depending on the circumstance
- Usually results in focal organization & fibrosis
Atypical pneumonia
- Inflammation confined to the alveolar septa & lung interstitium without alveolar exudation (hence no physical findings of consolidation)
- Usually seen in community-acquired atypical pneumonias (Mycoplasma, Chlamydia, Coxiella, viral)
- Superimposed bacterial infection may produce the features of typical pneumonia (either lobar or bronchopneumonia)
Morphology of lobar pneumonia
- [Gross] Consolidation of whole or part of a lobe, imparting a liver-like consistency to it, hence ‘hepatization’ (firm, airless)
- [Histology] Alveoli of affected lobe diffusely infiltrated by acute inflammatory exudate (neutrophils, fibrin)
- Red hepatization: massive confluent exudation with neutrophils, red cells & fibrin filling the alveolar spaces
- Grey hepatization: follows red hepatization, with progressive disintegration of red cells & persistence of the original fibrinosuppurative exudate
Morphology of bronchopneumonia
- [Gross] Patchy areas of consolidation
2. [Histology] Acute inflammatory infiltration of bronchioles & adjacent alveoli
Morphology of atypical pneumonia
- Widened edematous alveolar septa with mononuclear infiltrates
- Neutrophils may be present in certain cases
- No alveolar exudation
Pathological effects & complications of pneumonia
- Pleurisy
- Lung abscess
- Septicaemia
- Lung fibrosis
- Bronchopneumonia: focal fibrosis
- Atypical pneumonia: interstitial fibrosis, leading to restrictive lung disease
Definition of lung abscess
Localized pus-filled cavity in the lung due to local inflammatory destruction of lung tissue
Causes of lung abscess
- 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 - Antecedent lung infection (pneumonia, bronchiectasis)
- Usually associated with Staphylococcus aureus,
Klebsiella pneumoniae, Streptococcus pneumoniae - Septic embolism
- Trapping of infected emboli from systemic venous
circulation or infected vegetations from right-sided
infective endocarditis in the lung - Neoplasms
- Due to secondary infection of a bronchopulmonary segment obstructed by a malignancy - 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
Pathological effects & complications of lung abscess
- Rupture in pleural space, producing empyema
2. Bacteraemia
Clincial features of lung abscess
- Cough, foul-smelling purulent sputum
- Fever, chest pain, weight loss
- Clubbing (may appear within a few weeks from onset)
Definition of pulmonary tuberculosis
Granulomatous inflammation of the lung due to infection by Mycobacterium tuberculosis
Pathogenesis of primary TB
- Occurs in individuals who have not been exposed to mycobacteria before (or in immunosuppressed)
- Infection usually begins at periphery of the lung (subpleural) with the formation of a Ghon focus
- Ghon focus may heal or spread to involved regional lymph nodes (Ghon focus + affected lymph nodes = Ghon complex)
- 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)
Pathogenesis of secondary TB
- Occurs during exogenous infection or reactivation of
latent tuberculosis in individuals who have been
exposed or sensitised to mycobacteria previously - Infection usually at apical region of the upper lobe,
with the formation of an Assmann focus - Granulomatous inflammation & destruction of lung tissue leads to the formation of cavities surrounded by fibrous tissue (accompanied by coughing out of bacilli)
- Can lead on to progressive secondary tuberculosis with similar outcomes as progressive primary TB
Morphology of caseating granuloma seen in TB
Central caseous necrosis surrounding by a peripheral ring of epithelioid cells (histiocytes), lymphocytes & Langhans giant cells (multinucleated, nuclei arranged along periphery in horseshoe formation)
