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Classify pneumonia. Describe the stages of lobar pneumonia and differentiate it from bronchopneumonia.

Pathology · 2nd Professional — Para-Clinical

Definition and Classification

Pneumonia is inflammation of the lung parenchyma with consolidation of the alveolar spaces by an inflammatory exudate.

Basis	Types
Anatomical	Lobar pneumonia, bronchopneumonia (lobular), interstitial (atypical) pneumonia
Setting	Community-acquired, hospital-acquired (nosocomial), ventilator-associated, aspiration, pneumonia in the immunocompromised
Aetiology	Bacterial, viral, fungal, parasitic, chemical

Lobar Pneumonia

Consolidation of a large part OR the whole OF A lobe, uniformly and at the same stage throughout
Streptococcus pneumoniae causes about 90%; also Klebsiella (in alcoholics and diabetics, giving thick "red-currant jelly" sputum and a bulging fissure)
Typically affects previously healthy adults aged 20 to 50

Stage	Timing	Naked eye	Microscopy
1. Congestion	Day 1–2	Heavy, boggy, red lung; pits on pressure	Vascular engorgement, intra-alveolar fluid with few neutrophils; numerous bacteria
2. Red hepatisation	Day 2–4	Red, firm, airless, liver-like consistency	Alveoli packed with neutrophils, red cells and fibrin
3. Grey hepatisation	Day 4–8	Grey-brown, dry, firm surface	Red cells disintegrate; fibrinosuppurative exudate persists
4. Resolution	Day 8 onward	Exudate liquefies; lung returns to normal	Enzymatic digestion of exudate; macrophages clear the debris; it is coughed up or absorbed

CLINICAL PEARL

Lobar pneumonia resolves completely, and the reason is the alveolar framework. The exudate fills the airspaces but the alveolar walls are not destroyed, so once the exudate is digested the architecture is intact and the lung is restored to normal. Compare this with abscess formation, where tissue is destroyed and healing must be by scar.

Bronchopneumonia

Feature	Lobar pneumonia	Bronchopneumonia
Distribution	Uniform consolidation of a whole lobe	Patchy consolidation centred on bronchi and bronchioles; often bilateral and basal
Stage of lesions	All at the same stage	Lesions of different ages side by side
Age	Healthy adults 20–50	Extremes of age — infants and the elderly; and the debilitated
Organisms	Streptococcus pneumoniae (90%)	Staphylococcus aureus, Haemophilus influenzae, Klebsiella, Pseudomonas, coliforms; often mixed
Underlying disease	Usually none	Almost always present — a terminal event in the chronically ill
Pleura	Usually involved — pleuritis	Often spared
Resolution	Complete	Often incomplete, with organisation and scarring

Atypical (interstitial) Pneumonia

Organisms — mycoplasma pneumoniae (the commonest), Chlamydia, Coxiella, respiratory viruses (influenza, respiratory syncytial virus, adenovirus, SARS-CoV-2), Legionella
"Atypical" because the exudate is interstitial rather than alveolar — the alveolar walls are inflamed and the airspaces relatively clear
Clinical mismatch is the hallmark — marked constitutional upset and extensive radiographic changes with few chest signs and little sputum
Microscopy — mononuclear interstitial infiltrate; in severe cases hyaline membranes resembling diffuse alveolar damage
Mycoplasma may cause cold agglutinin haemolysis, erythema multiforme and neurological complications

Complications

Lung abscess — tissue destruction and suppuration, particularly with Staphylococcus and Klebsiella
Empyema — pus in the pleural cavity; requires drainage
Organisation ("carnification") — the exudate is converted to fibrous tissue and the lung becomes solid and airless
Bacteraemia and metastatic infection — meningitis, endocarditis, arthritis, brain abscess
Respiratory failure and septic shock
Pleural effusion, parapneumonic and usually sterile

Pneumonia in Special Groups

Group	Typical organisms	Notes
Nosocomial (hospital-acquired)	Gram-negative rods — Klebsiella, Pseudomonas, E. Coli, Enterobacter; Staphylococcus aureus including MRSA	Onset 48 hours or more after admission; often resistant; carries a high mortality
Aspiration	Mixed anaerobes from the oropharynx	In unconsciousness, alcoholism, stroke, poor dentition; tends to abscess
Immunocompromised	Pneumocystis jirovecii, cytomegalovirus, Aspergillus, Candida, Nocardia, mycobacteria	Pneumocystis gives a foamy intra-alveolar exudate; cysts shown by silver (GMS) staining
Neonates and infants	Group B streptococcus, E. Coli, respiratory syncytial virus	A leading cause of under-five mortality in India
Chronic pneumonia	Mycobacteria, Histoplasma, Blastomyces, Cryptococcus	Granulomatous; often mistaken for malignancy

Applied Aspects

Pneumonia is a leading cause of childhood death in India, and vaccination against pneumococcus, Haemophilus influenzae b, measles and influenza is the most effective prevention
CURB-65 — Confusion, Urea, Respiratory rate, Blood pressure, age 65 — a simple score guiding whether to admit; a score of 0 to 1 may be treated at home
Sputum Gram stain and culture before antibiotics, with blood cultures in severe cases; treatment must not be delayed for them
Aspiration pneumonia affects dependent segments — the posterior segment of the right upper lobe and apical segment of the right lower lobe when supine, because the right main bronchus is wider and more vertical
Failure to resolve should raise suspicion of an obstructing bronchial carcinoma, tuberculosis, an abscess, or a resistant organism, and warrants bronchoscopy
Always consider tuberculosis in India in any pneumonia that is slow to resolve or that recurs in the same site
Pneumococcal pneumonia may show a "bulging fissure" with Klebsiella, from the voluminous inflammatory exudate — a classical radiographic sign
Antibiotic resistance is rising, and restricting empirical broad-spectrum use to those who need it is the only way to slow it
Legionella should be considered with hyponatraemia, diarrhoea and confusion, and requires a urinary antigen test since it does not grow on routine media
A pleural effusion complicating pneumonia should be sampled if it is more than minimal, since a simple parapneumonic effusion and an empyema are managed quite differently

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