MBBS Block 2: Pneumonia Classification and Pathogenesis

Study Notes

Pneumonia is defined as a collection of inflammatory exudate in lung parenchyma distal to terminal bronchioles, resulting in consolidation (solidification) of lung parts.

Anatomical pattern:
- Bronchopneumonia
- Lobar pneumonia
Clinical circumstances:
- Primary (in an otherwise healthy person)
- Secondary (with local or systemic defects in defense)
Aetiological agents:
- Bacterial (Streptococcus pneumoniae, Staphylococcus aureus, Mycobacterium tuberculosis, etc.)
- Viral (Influenza, measles, etc.)
- Fungal (Cryptococcus, Candida, Aspergillus, etc.)
- Other (Pneumocystis jiroveci, Mycoplasma, Aspiration, lipid, eosinophilic)
Host reaction:
- Fibrinous
- Suppurative

Pneumonia occurs when defense mechanisms of the respiratory system are impaired or immunity of the host is low.
Factors that compromise pulmonary defense mechanisms:
- Loss or suppression of the cough reflex
- Dysfunction of the mucociliary apparatus
- Interference with phagocytic and bactericidal activities of alveolar macrophages
- Pulmonary congestion and edema

Affects anatomically delineated segment(s) or the entirety of a lobe or lung
Relatively uncommon in infancy and old age
Affects males more than females
90% due to Streptococcus pneumoniae (pneumococcus)
Pneumococcal pneumonia typically affects otherwise healthy adults between 20 and 50 years of age

Congestion:
- Outpouring of a protein-rich exudate into alveolar spaces, with venous congestion
- The lung is heavy, oedematous, and red
Red hepatization:
- Massive accumulation of polymorphs, together with some lymphocytes and macrophages
- Many red cells are also extravasated from the distended capillaries
- The overlying pleura bears a fibrinous exudate
- The lung is red, solid, and airless, with a consistency resembling fresh liver
Grey hepatization:
- Further accumulation of fibrin, with destruction of white cells and red cells
- The lung is now grey-brown and solid
Resolution:
- Resorption of exudate and enzymatic digestion of inflammatory debris, with preservation of the underlying alveolar wall architecture

Has a characteristic patchy distribution, centred on inflamed bronchioles and bronchi with subsequent spread to surrounding alveoli
Often several lobes or bilateral
It occurs most commonly in old age, in infancy, and in patients with debilitating diseases
Typical organisms include staphylococci, streptococci, and Haemophilus influenzae

Abscess formation:
- Results from tissue destruction (more in case of Klebsiella or type III Pneumococcal infections)
Empyema:
- Virulent bacterial strains induce suppuration in the pleural cavity, causing the intra-pleural fibrosuppurative reaction
Fibrosis:
- Organization of intra-alveolar exudate may convert affected lung into solid fibrous tissue
Bacterial dissemination:
- Dissemination of bacteria may lead to endocarditis, pericarditis, meningitis, suppurative arthritis, and formation of metastatic abscesses in various organs

Pneumonias caused by organisms other than traditional bacteria are often referred to as ‘atypical pneumonias’
Causative organisms:
- Mycoplasma pneumoniae
- Influenza virus type A and B
- Respiratory syncytial viruses, adenovirus, rhinovirus, rubeola, and varicella virus
- Chlamydia
- Coxiella burnetii

Cryptogenic organising pneumonia
Aspiration pneumonia:
- When fluid or food is aspirated into the lung
Lipid pneumonia:
- Endogenous: associated with airway obstruction causing distal collections of foamy macrophages and giant cells
- Exogenous: due to aspiration of material containing a high concentration of lipid

Definition:
- Bronchopneumonia: Patchy consolidation of multiple lobes (bilateral)
- Lobar pneumonia: Involvement of a large part of a lobe or entire lobe, diffuse consolidation
Predisposing illness:
- Bronchopneumonia: Bronchitis/bronchiolitis, chronic debility
- Lobar pneumonia: Affects healthy individuals
Immune status:
- Bronchopneumonia: Usually affects immunosuppressed individuals
- Lobar pneumonia: Affects previously healthy individuals