Pneumonia - A Comprehensive Overview

Definition

• Goldman-Cecil Medicine, p. 992

Epidemiology

• One of the most common serious health conditions and the leading infectious cause of hospitalization and death in the United States
• Accounts for ~7 health care visits/1000 young adults but 96 visits/1000 adults aged 85+
• Hospitalization rates increase exponentially with age: 1-2/1000 in young adults up to ~40/1000 in those 85+ years old
• ~35% of CAP hospitalizations occur in patients with recent health care exposure; ~25% in immunocompromised patients
• Pneumonia peaks in November through March, coinciding with influenza and other respiratory virus seasons
• During the COVID-19 pandemic, SARS-CoV-2 became the leading cause of pneumonia-related death
• Goldman-Cecil Medicine, p. 990-992

Risk Factors

• Goldman-Cecil Medicine, p. 992

Pathobiology

1. Immigration - oropharyngeal colonization, aspiration events, supine position, GERD
2. Elimination - ciliary function, cough, mucosal immunity
3. Relative reproduction rates - altered by local pH, oxygen tension, surfactant, and host immunity

• Goldman-Cecil Medicine; Murray & Nadel's Respiratory Medicine

Causative Organisms

CAP - Pathogens by Setting (IDSA/ATS Classification)

• Fishman's Pulmonary Diseases, p. 2202

• Typical bacteria (S. pneumoniae, H. influenzae, S. aureus, GNRs) cause up to 30% of cases
• Atypical organisms (Mycoplasma, Chlamydia, Legionella) cause <5% of cases
• Respiratory viruses historically cause 20-30% of cases (dramatically increased during COVID-19)
• No pathogen is confirmed in >50% of cases

Clinical Presentation

• Fever, chills
• Cough (productive or dry)
• Dyspnea
• Pleuritic chest pain
• Sputum production

• Crackles / bronchial breath sounds over consolidation
• Dullness to percussion
• Increased tactile fremitus
• Tachypnea, tachycardia

Severity Assessment

CURB-65 Score

• Score 0-1: Outpatient management
• Score 2: Short hospitalization or supervised outpatient
• Score 3+: Hospitalization; consider ICU if score ≥4

ATS/IDSA Criteria for Severe CAP (ICU admission)

• Invasive mechanical ventilation
• Hemodynamic compromise requiring vasopressor support

• RR ≥30 breaths/min
• PaO₂/FiO₂ ratio ≤250
• Multilobar infiltrates
• Confusion/disorientation
• BUN ≥20 mg/dL
• WBC <4000 cells/mm³
• Platelet count <100,000 cells/mm³
• Core temperature <36°C
• Hypotension requiring aggressive fluid resuscitation
• Fishman's Pulmonary Diseases, p. 2204

Diagnosis

1. Clinical assessment - history and physical examination
2. Chest imaging - CXR (lobar consolidation, interstitial infiltrates, pleural effusion); CT chest if CXR equivocal
3. Microbiologic studies:
   • Blood cultures (before antibiotics in hospitalized patients)
   • Sputum Gram stain and culture
   • Urine antigen tests (Legionella, S. pneumoniae)
   • Multiplex PCR / respiratory panel (including influenza, SARS-CoV-2)
   • Procalcitonin: professional societies recommend against using procalcitonin alone to decide whether to give antibiotics
4. Laboratory: CBC, BMP (BUN/creatinine), LFTs, ABG (in severe cases)

Treatment

Outpatient (Low Risk, No Comorbidities)

• Amoxicillin
• Doxycycline
• Azithromycin

Outpatient (With Comorbidities / Risk for Resistant Organisms)

• Respiratory fluoroquinolone monotherapy (levofloxacin or moxifloxacin), OR
• Beta-lactam (amoxicillin-clavulanate, cefpodoxime, or cefuroxime) + macrolide or doxycycline

Inpatient, Non-Severe CAP (No MRSA/Pseudomonas Risk)

• Respiratory fluoroquinolone monotherapy, OR
• Beta-lactam + macrolide or doxycycline

Severe CAP (ICU)

• Beta-lactam + macrolide, OR
• Beta-lactam + respiratory fluoroquinolone
• Monotherapy should not be used in ICU patients
• Add MRSA coverage (vancomycin or linezolid) if: prior MRSA isolation, recent hospitalization with IV antibiotics in past 90 days, or locally validated risk factors
• Add Pseudomonas coverage if: prior isolation, recent hospitalization with IV antibiotics in past 90 days, or locally validated risk factors

CA-MRSA Necrotizing Pneumonia

• Linezolid alone, OR
• Vancomycin + clindamycin (to inhibit exotoxin production)

Viral Pneumonia

• Influenza: antivirals (oseltamivir) + empirical antibacterials for possible bacterial coinfection
• SARS-CoV-2: antivirals/monoclonal antibodies; antibacterials only if bacterial coinfection suspected
• Goldman-Cecil Medicine; Fishman's Pulmonary Diseases, p. 2202-2204

Complications

Prognosis

• Mortality for CAP requiring hospitalization: ~6% during hospitalization, ~15% by 1 month
• Recovery can be slow, particularly in older adults
• Cardiac complications contribute significantly to early mortality

Prevention

Recent Evidence (2024-2026)

• A 2025 Lancet Respiratory Medicine meta-analysis (PMID 39892408) examined adjuvant corticosteroids in CAP, suggesting that only certain subgroups derive benefit - routine steroid use remains investigational
• A 2025 European Respiratory Review systematic review (PMID 40107661) identified risk factors for drug-resistant pathogens in CAP, supporting selective (rather than empiric broad-spectrum) coverage
• A 2024 Pediatric Pulmonology meta-analysis (PMID 39239917) confirmed lung ultrasound has comparable accuracy to chest radiography for diagnosing CAP in children and may be preferred to avoid radiation

Lobular Pneumonia (Bronchopneumonia) and Stages of Pneumonia

Terminology: Lobular vs. Lobar

• Robbins & Kumar Basic Pathology, p. 470; Robbins Cotran & Kumar Pathologic Basis of Disease, p. 662

Lobular Bronchopneumonia - Detailed Morphology

Gross Appearance

• Foci of consolidation (acute suppurative inflammation) - often multilobar, bilateral, and basal due to gravitational pooling of secretions
• Lesions are slightly elevated, dry, granular, gray-red to yellow, with poorly defined margins

Microscopic Appearance

• Neutrophil-rich exudate fills the bronchi, bronchioles, and adjacent alveolar spaces
• Centered on bronchioles, spreading outward into surrounding alveoli
• Patchy areas separated by relatively normal lung parenchyma

Key Point

The 4 Classical Stages of Lobar Pneumonia

Stage 1 - Congestion (Days 1-2)

Stage 2 - Red Hepatization (Days 2-4)

Stage 3 - Gray Hepatization (Days 4-8)

Stage 4 - Resolution (Days 8 onward)

Visual Summary of the 4 Stages

Stage 1: Congestion          Stage 2: Red Hepatization
 ┌─────────────────────┐      ┌─────────────────────────┐
 │ Heavy, boggy, RED   │  →   │ Firm, airless, RED      │
 │ Vascular engorgement│      │ Neutrophils + RBCs +    │
 │ Few neutrophils     │      │ Fibrin in alveoli       │
 │ Many bacteria       │      │ Liver-like consistency  │
 └─────────────────────┘      └─────────────────────────┘

Stage 3: Gray Hepatization   Stage 4: Resolution
 ┌─────────────────────┐      ┌─────────────────────────┐
 │ Grayish-brown, firm │  →   │ Lung re-aerates         │
 │ RBCs disintegrate   │      │ Macrophages dominant    │
 │ Neutrophils persist │      │ Exudate resorbed or     │
 │ Bacteria disappear  │      │ expelled                │
 │ Infection contained │      │ ± Pleural adhesions     │
 └─────────────────────┘      └─────────────────────────┘

Why the 4 Stages Matter Clinically

• Stage 1 (congestion): Radiograph may be normal or show early haziness; patient appears sick but CXR "lags"
• Stage 2 (red hepatization): Classic dense lobar consolidation on CXR; maximum clinical severity
• Stage 3 (gray hepatization): Patient may begin improving clinically; consolidation persists on imaging
• Stage 4 (resolution): Radiographic clearance lags behind clinical recovery by days to weeks - especially in elderly

Important caveat from Harrison's (22e): This 4-stage pattern applies best to lobar pneumococcal pneumonia and may not apply to all etiologies. In ventilator-associated pneumonia (VAP), respiratory bronchiolitis often precedes visible infiltrates. The bronchopneumonia pattern predominates in nosocomial infections, while lobar pattern is more common in bacterial CAP.

Complications (Both Types)

1. Lung abscess - tissue destruction and necrosis
2. Empyema - spread to pleural cavity → fibrinopurulent intrapleural reaction
3. Bacteremic dissemination - seeding of heart valves (endocarditis), brain (meningitis/abscess), joints (septic arthritis), kidneys, pericardium

Aetiopathogenesis of Lobular (Bronchopneumonia) - Step by Step

1. Predisposing Factors (Why defences fail)

2. Routes of Infection

• Inhalation of contaminated droplets → upper airway → bronchioles
• Aspiration of oropharyngeal / gastric contents (most important in bronchopneumonia)
• Haematogenous spread from a distant septic focus

3. Breakdown of Host Defences

• Mucociliary escalator impaired → bacteria not cleared upward
• Alveolar macrophages overwhelmed by bacterial load
• Surfactant disrupted → bacterial adhesion increases
• Secretory IgA reduced → less neutralisation

4. Bacterial Colonisation of Bronchioles

• Organisms adhere to bronchiolar epithelium
• Toxins and proteases released → epithelial damage
• Complement activated → IL-1, TNF-α, IL-6, IL-8 cytokine storm

5. Acute Inflammatory Response (the Pathological Core)

• Massive neutrophil recruitment via chemokines
• Vascular congestion and oedema
• Exudate (neutrophils + fibrin + RBCs) floods bronchioles and adjacent alveoli
• This produces the characteristic patchy, centribronchiolar (lobular) consolidation - multilobar, bilateral, basal

This is why bronchopneumonia does NOT go through the 4 hepatization stages - inflammation is centred on bronchioles, patchy, and does not flood an entire lobe in one continuous wave.

6. Outcomes

• Macrophages phagocytose debris
• Enzymatic digestion of exudate
• Re-aeration → complete recovery

• Lung abscess - necrosis by proteases and toxins (especially S. aureus, Klebsiella)
• Empyema - spread through visceral pleura
• Bacteraemia / Sepsis - haematogenous dissemination
• ARDS - bilateral alveolar flooding, respiratory failure
• Carnification - organisation of exudate by fibroblasts → permanent fibrosis (failed resolution)

Aetiopathogenesis of Lobar Pneumonia - Detailed Walkthrough

Step 1 - Aetiology

Step 2 - Route of Entry & Why Defences Fail

• Organism-laden droplets inhaled → reach alveoli
• S. pneumoniae polysaccharide capsule is the master virulence factor - it resists opsonisation and phagocytosis by alveolar macrophages
• Pneumolysin and neuraminidase disrupt alveolar epithelium
• Once one alveolus is seeded, the organism spreads rapidly to the entire lobe through the pores of Kohn (inter-alveolar communications) - this is what makes lobar pneumonia LOBAR, unlike the centribronchiolar spread of bronchopneumonia

Step 3 - Innate Immune Activation

• Complement activated → C3a, C5a → vascular permeability increases
• IL-1, TNF-α, IL-6, IL-8 released → systemic fever, acute phase response
• Massive neutrophil chemotaxis begins

Step 4 - The 4 Morphological Stages

The Critical Difference from Bronchopneumonia

Pores of Kohn allow the exudate to spread continuously from alveolus to alveolus across an entire lobe, producing uniform lobar consolidation. In bronchopneumonia, spread is centribronchiolar and patchy - the pores are not the primary route.

Step 5 - Outcomes

• Empyema - pleural spread (~3-5%)
• Lung abscess - necrosis (more common with Klebsiella, type 3 pneumococcus)
• Carnification - failed resolution → fibroblasts organise the exudate → permanent fibrosis
• Bacteraemia (10-15%) → endocarditis, meningitis, septic arthritis
• Acute cardiac events (AF, MI, worsening heart failure) in ~20-25% of hospitalised patients
• ARDS in severe/bilateral disease