Pneumonia

Overview

Acute inflammation of lung parenchyma distal to terminal bronchioles (resp. bronchiole+alveolar ducts+sacs+alveoli). “Consolidation” = gross/radiologic appearance (solidification).

Pathogenesis

Entry routes: inhalation, aspiration (naso/oropharynx), haematogenous spread, direct spread. Normal lung sterile via: nasopharyngeal filtering, mucociliary clearance, alveolar macrophages, immunoglobulins. Predisposing defence failures:

1. Altered consciousness — aspiration (coma, trauma, seizures, CVA, drug OD, alcoholism)
2. ↓Cough/glottic reflex — gastric aspiration (old age, post-surgical pain, neuromuscular disease, malnutrition, kyphoscoliosis, severe COPD, intubation)
3. ↓Mucociliary transport — smoking, viral infection, immotile cilia syndrome, hot/corrosive gas, old age
4. ↓Alveolar macrophage function — smoke, hypoxia, starvation, anaemia, pulmonary oedema, viral infection
5. Endobronchial obstruction — tumour, foreign body, CF, chronic bronchitis
6. Immunocompromise — AIDS, debility, senility, immunosuppression, granulocyte defects

Classification

By anatomy: lobar / bronchopneumonia / interstitial. By setting: community-acquired / healthcare-associated / ventilator-associated. By etiology: bacterial / viral / other (fungal, non-infective).

Bacterial = commonest cause, produces the 2 classic patterns below.

Lobar pneumonia

Acute bacterial infection of part/whole lobe(s), one or both lungs.

Etiology: >90% Streptococcus pneumoniae (lancet diplococcus, type 3 most virulent), usually community-acquired. Others: Staph aureus (haematogenous/post-viral), β-haemolytic strep (children post-measles/flu, debilitated elderly, diabetics), gram-negatives (H. influenzae — children <3 post-viral; Klebsiella/Friedländer’s bacillus; Pseudomonas, Proteus, E. coli).

4 sequential stages (Laennec) — direct histologic timeline of immune response

1. Congestion (1-2 days): Gross — enlarged, heavy, dark red, congested, blood-stained frothy exudate. Micro — dilated congested capillaries, pale eosinophilic oedema fluid, few RBCs+neutrophils, numerous bacteria (Gram stain).

2. Red hepatisation (2-4 days): “hepatisation” = liver-like consistency. Gross — red, firm, consolidated, airless/dry/granular, liver-like; serofibrinous pleurisy. Micro — fibrin strands replace oedema fluid, marked neutrophilic exudate+RBC extravasation, neutrophils show ingested bacteria, septa less prominent.

3. Grey hepatisation (4-8 days): Gross — firm, heavy, dry/granular/GREY, liver-like; colour change hilum→periphery; fibrinous pleurisy prominent. Micro — denser fibrin, REDUCED neutrophils (pyknotic, disintegrating), fewer RBCs, MACROPHAGES appearing, exudate separated from septa by clear space, fewer/degenerated organisms.

4. Resolution (day 8-9 untreated, 1-3wk; antibiotics→~day 3): progressive, central→peripheral. Gross — fibrinous material liquefies enzymatically, aeration restored; grey-red/dirty-brown, frothy, creamy fluid expressible; pleural reaction resolves or organises. Micro — MACROPHAGES predominant (engulfing neutrophils/debris), neutrophils ↓, fibrin granular/fragmented (enzymatic digestion), engorged capillaries, fluid+exudate cleared mainly via LYMPHATICS.

Complications (uncommon post-antibiotics; neglected/immunocompromised cases)

1. Organisation/CARNIFICATION (~3%) — exudate fails to resolve, fibroblasts grow in from septa → fibrosed, tough, airless, leathery lung
2. Pleural effusion (~5%) — usually resolves, may organise/adhere
3. Empyema (<1%) — encysted pleural pus
4. Lung abscess — rare, secondary infection
5. Metastatic infection — pericarditis, endocarditis, myocarditis; rarely otitis media, mastoiditis, meningitis, brain abscess, arthritis

Clinical features

Sudden onset: shaking chills, fever, malaise, pleuritic pain, dyspnoea, cough (mucoid/purulent/bloody). Fever, tachycardia, tachypnoea, ±cyanosis. Neutrophilic leucocytosis; blood culture +ve ~30%. CXR: consolidation. Antibiotic response rapid (48-72hr improvement).

Bronchopneumonia (lobular pneumonia)

Terminal bronchiole infection extending into surrounding alveoli → PATCHY consolidation (mechanistically distinct from lobar’s confluent pattern). Extremes of life (infancy/elderly), terminal event in debility, secondary to viral infection (flu, measles).

Etiology: staph, strep, pneumococci, Klebsiella, H. influenzae, gram-negative bacilli.

Morphology: Gross — patchy red/grey consolidation, 1+ lobes, often bilateral, LOWER zones (gravitating secretions). Lesions 3-4cm, dry/granular/firm, centred on a BRONCHIOLE. Micro — acute bronchiolitis, suppurative (neutrophilic) peribronchiolar exudate, thickened septa (congested capillaries+leucocytes), less-involved alveoli have oedema fluid only.

Complications: lobar list applies but COMPLETE RESOLUTION UNCOMMON — bronchiolar destruction → fibrosis foci → can → bronchiectasis.

Clinical: infants/elderly, preceding bed-ridden illness/debility/aspiration/URI. First 2-3 days = acute bronchitis picture, then lobar-like signs. Neutrophilic leucocytosis. CXR: mottled focal opacities, lower zones.

Comparison

Clinical relevance

4-stage sequence = direct histologic timeline of immune response (vascular→cellular/fibrinous→neutrophil disintegration/macrophage recruitment→enzymatic clearance) — one continuous process, each stage predictable from the last, not 4 disconnected snapshots. Carnification = pathologic endpoint when resolution FAILS — fibroblasts substitute for macrophages in clearing exudate, permanent fibrous airless lung is the direct structural consequence. Lobar vs bronchopneumonia distinction matters beyond morphology — same organism can produce either pattern depending on host factors/spread route, not organism identity alone; lobar = staged/self-limited in healthy host, broncho = incomplete resolution/bronchiectasis risk. 6 predisposing-factor categories = covers every level of lung defence in sequence (consciousness→mucociliary→macrophage→mechanical→systemic immunity) — triage new scenarios by which level is disrupted, not as unconnected list.

Overview

Pneumonia is acute inflammation of the lung parenchyma distal to the terminal bronchioles (respiratory bronchiole, alveolar ducts, alveolar sacs, alveoli). “Pneumonia” and “pneumonitis” are used interchangeably; “consolidation” describes the gross/radiologic appearance (solidification) that results.

Pathogenesis

Organisms reach the lung by one of four routes: inhalation of airborne microbes, aspiration from the naso-/oropharynx, haematogenous spread from a distant focus, or direct spread from an adjoining infected site.

The normal lung is sterile because of layered defences — nasopharyngeal filtering, mucociliary clearance, alveolar macrophage phagocytosis, and immunoglobulins. Pneumonia results when these defences fail, which happens through several distinct predisposing mechanisms:

1. Altered consciousness — aspiration of oropharyngeal contents in coma, cranial trauma, seizures, cerebrovascular accident, drug overdose, alcoholism.
2. Depressed cough/glottic reflexes — allows aspiration of gastric contents; old age, post-traumatic/thoracoabdominal surgical pain, neuromuscular disease, malnutrition-related weakness, kyphoscoliosis, severe obstructive lung disease, endotracheal intubation/tracheostomy.
3. Impaired mucociliary transport — smoking, viral respiratory infection, immotile cilia syndrome, hot/corrosive gas inhalation, old age.
4. Impaired alveolar macrophage function — cigarette smoke, hypoxia, starvation, anaemia, pulmonary oedema, viral infection.
5. Endobronchial obstruction — tumour, foreign body, cystic fibrosis, chronic bronchitis.
6. Immunocompromise — congenital/acquired immunodeficiency (AIDS), debility, senility, immunosuppressive therapy, granulocyte abnormalities.

Classification

• By anatomic pattern: lobar pneumonia, bronchopneumonia (lobular pneumonia), interstitial pneumonia.
• By clinical setting: community-acquired, health-care-associated (including hospital-acquired), ventilator-associated.
• By etiology: bacterial, viral, and other (fungal, non-infective — aspiration, hypostatic, lipid pneumonia).

Bacterial infection is the most common cause of pneumonia/consolidation, and produces the two classic, etiologically and morphologically distinct patterns detailed below.

Lobar Pneumonia

Acute bacterial infection of part of a lobe, an entire lobe, or two lobes, of one or both lungs.

Etiology: >90% caused by Streptococcus pneumoniae (a lancet-shaped diplococcus; type 3 is particularly virulent), usually community-acquired. Other causes: Staphylococcus aureus (haematogenous spread or post-viral), β-haemolytic streptococci (children post-measles/influenza, debilitated elderly, diabetics), and gram-negative aerobes (H. influenzae — classically children under 3 post-viral-infection; Klebsiella pneumoniae/Friedländer’s bacillus; Pseudomonas, Proteus, E. coli).

Pathophysiology — The Four Sequential Stages (Laennec’s Classic Description)

Untreated lobar pneumonia evolves through four sequential stages — the anatomic and histologic correlate of the immune response’s own time-course against the organism. Modern antibiotic therapy interrupts this sequence early, so the full classic progression is now seen less often, but the stages remain the conceptual backbone of the disease. Lower lobes affected most commonly, sometimes bilaterally.

1. Stage of congestion (initial phase, 1–2 days) — the earliest acute inflammatory response.

• Gross: affected lobe enlarged, heavy, dark red, congested; cut surface exudes blood-stained frothy fluid.
• Micro: dilated, congested alveolar wall capillaries; pale eosinophilic oedema fluid in air spaces; scattered red cells and neutrophils in the fluid; numerous bacteria demonstrable on Gram stain.

2. Red hepatisation (early consolidation, 2–4 days) — “hepatisation” refers to the liver-like consistency the lobe acquires.

• Gross: red, firm, consolidated; cut surface airless, red-pink, dry, granular, liver-like; accompanied by serofibrinous pleurisy.
• Micro: oedema fluid replaced by fibrin strands; marked neutrophilic cellular exudate with red cell extravasation; many neutrophils show ingested bacteria; alveolar septa less prominent (obscured by the cellular exudate).

3. Grey hepatisation (late consolidation, 4–8 days).

• Gross: firm, heavy; cut surface dry, granular, grey, liver-like — colour change from red to grey begins at the hilum and spreads peripherally; fibrinous pleurisy prominent.
• Micro: fibrin strands denser and more numerous; neutrophil numbers reduced (disintegrating, pyknotic nuclei) and red cells fewer; macrophages begin appearing; cellular exudate separated from septal walls by a thin clear space; organisms fewer, degenerated.

4. Resolution (begins day 8–9 untreated, complete in 1–3 weeks; antibiotics accelerate resolution to ~day 3) — proceeds progressively, centrally to peripherally.

• Gross: solid fibrinous material liquefies enzymatically, restoring aeration; cut surface grey-red/dirty-brown, frothy; yellow creamy fluid expressible on pressure; pleural reaction may resolve or organise into fibrous pleural obliteration.
• Micro: macrophages become predominant, neutrophils diminish, many macrophages contain engulfed neutrophils/debris; fibrin strands granular and fragmented from progressive enzymatic digestion; alveolar capillaries engorged; fluid and cellular exudate progressively cleared — partly by expectoration, mainly via lymphatics — restoring normal aerated parenchyma.

Complications (Uncommon Since Antibiotics; Seen in Neglected/Immunocompromised Cases)

1. Organisation (carnification) — in ~3% of cases, the exudate fails to resolve and instead organises: fibroblasts grow in from the alveolar septa, producing fibrosed, tough, airless, leathery lung tissue — this post-pneumonic fibrosis is termed carnification.
2. Pleural effusion — ~5% of treated cases; usually resolves, occasionally organises into fibrous pleural adhesions.
3. Empyema — <1% of treated cases; encysted pus in the pleural cavity.
4. Lung abscess — rare; typically from secondary infection by other organisms.
5. Metastatic infection — spread to pericardium/heart (purulent pericarditis, bacterial endocarditis, myocarditis), or rarely otitis media, mastoiditis, meningitis, brain abscess, purulent arthritis.

Clinical Features

Classically sudden onset: shaking chills, fever, malaise, pleuritic chest pain, dyspnoea, cough (mucoid/purulent/bloody sputum). Fever, tachycardia, tachypnoea, cyanosis if severely hypoxaemic. Marked neutrophilic leucocytosis; blood cultures positive in ~30%. CXR shows consolidation. Sputum culture/sensitivity guides antibiotic choice. Response to antibiotics typically rapid — clinical improvement within 48–72 hours.

Bronchopneumonia (Lobular Pneumonia)

Infection of the terminal bronchioles extending into surrounding alveoli, producing patchy consolidation — mechanistically and morphologically distinct from lobar pneumonia’s confluent lobar pattern. Particularly frequent at the extremes of life (infancy, old age), as a terminal event in chronic debilitating disease, and as secondary infection following viral respiratory infections (influenza, measles).

Etiology: staphylococci, streptococci, pneumococci, Klebsiella pneumoniae, H. influenzae, gram-negative bacilli (Pseudomonas, coliforms).

Morphology:

• Gross: patchy red/grey consolidation, one or more lobes, frequently bilateral, more often lower zones (secretions gravitate down). Patchy lesions 3–4 cm, dry, granular, firm, slightly elevated, centred around a bronchiole — best appreciated by palpating the cut surface.
• Micro: acute bronchiolitis; suppurative (mainly neutrophilic) exudate in peribronchiolar alveoli; alveolar septa thickened by congested capillaries and leucocytic infiltration; less-involved alveoli contain oedema fluid only.

Complications: the lobar pneumonia complication list applies here too, but complete resolution is uncommon — some degree of bronchiolar destruction typically occurs, leaving foci of bronchiolar fibrosis that can eventually produce bronchiectasis (see Bronchiectasis).

Clinical features: typically infants or elderly, often with a preceding bed-ridden illness, chronic debility, aspiration, or upper respiratory infection. First 2–3 days resemble acute bronchitis, then lobar-pneumonia-like signs/symptoms appear. Neutrophilic leucocytosis. CXR: mottled, focal opacities, mainly lower zones.

Comparing Lobar Pneumonia and Bronchopneumonia

Clinical Relevance

• The four-stage sequence of lobar pneumonia is not an arbitrary description but a direct histologic timeline of the immune response — congestion (vascular response) → red hepatisation (cellular/fibrinous exudation) → grey hepatisation (neutrophil disintegration, macrophage recruitment) → resolution (enzymatic clearance) — understanding it as one continuous process rather than four disconnected snapshots is what makes each stage’s gross and microscopic findings predictable from the one before it.
• Carnification is the pathologic endpoint that results specifically when the resolution stage fails — fibroblasts, rather than macrophages, take over the exudate-clearing role, and the outcome (permanent fibrous, airless lung) is the direct structural consequence of that substitution.
• The lobar-versus-bronchopneumonia distinction matters clinically beyond morphology: lobar pneumonia’s staged, self-limited course in an otherwise healthy host contrasts with bronchopneumonia’s tendency toward incomplete resolution and bronchiectasis risk, which is why the same organism (e.g. pneumococcus) can produce either pattern depending on host factors and route of spread, not organism identity alone.
• The six-category list of pneumonia-predisposing defence failures is best understood as covering every level of the lung’s normal defence in sequence (consciousness/reflexes → mucociliary clearance → macrophage function → mechanical patency → systemic immunity) — a new clinical scenario can be triaged by asking which specific level of defence it disrupts, rather than treated as an unconnected list of risk factors.

• Four stages of lobar pneumonia: congestion → red hepatisation → grey hepatisation → resolution, gross and microscopic findings matched at each stage
• Lobar pneumonia vs bronchopneumonia: pattern, organism, patient, and resolution compared
• Six levels of lung defence and the predisposing factor that fails at each

Short Essays (1)

Long Essays (1)

Multiple Choice Questions (1)