Community Acquired Pneumonia and Differentials

Related Subjects: Asthma |Acute Severe Asthma |Exacerbation of COPD |Pulmonary Embolism |Cardiogenic Pulmonary Oedema |Pneumothorax |Tension Pneumothorax |Respiratory (Chest) infections Pneumonia |Fat embolism |Hyperventilation Syndrome |ARDS |Respiratory Failure |Diabetic Ketoacidosis

🫁 Introduction

• Community-acquired pneumonia (CAP) is an acute infection of the lungs causing inflammation of the alveoli and interstitium.
• 🌍 Most cases occur outside hospital; affects all ages but highest morbidity in elderly, infants, and immunocompromised patients.

📖 About CAP

• 🦠 Acute infection with alveolar exudation ± interstitial inflammation.
• 🏥 Can present as lobar pneumonia, bronchopneumonia, or interstitial (atypical) pneumonia.
• ⚠️ Causes include bacteria (most common), viruses, fungi, aspiration, and rare immunological/allergic mechanisms.

🧬 Pathogenesis

• 🌐 Colonisation: Oropharyngeal flora aspirated → reach alveoli.
• 🛡️ Immune evasion: Pathogen overcomes mucociliary clearance, IgA, macrophages.
• 🔥 Inflammation: Neutrophils + cytokines (IL-1, TNFα) → alveolar filling with exudate.
• 💨 Gas exchange failure: Exudate blocks O₂ diffusion → hypoxaemia, V/Q mismatch.
• ⚡ Bacteraemia risk: Pathogen gains access to blood via damaged alveolar-capillary barrier.

🔎 Types of Pneumonia (Pathology)

• 🌑 Lobar Pneumonia: Entire lobe affected, usually S. pneumoniae. Pathological progression through 4 stages (see below).
• 🟡 Bronchopneumonia: Patchy, often basal consolidation spreading from bronchioles. Elderly, bedridden, aspiration-prone.
• 🌫️ Interstitial (Atypical): Alveolar septal inflammation; usually viral (influenza, RSV) or atypical bacteria (Mycoplasma).
• 🟢 Necrotising Pneumonia: Severe destruction ± abscess (e.g. S. aureus, Klebsiella).
• 🦠 Opportunistic: Pneumocystis jirovecii in AIDS, fungal pneumonias (Aspergillus, Cryptococcus).

📆 Classical Stages of Lobar Pneumonia

• 1️⃣ Congestion (Day 1–2): Heavy, red lung. Vascular engorgement, intra-alveolar fluid, few neutrophils, numerous bacteria.
• 2️⃣ Red Hepatization (Day 3–4): Firm, liver-like lung. Alveoli packed with neutrophils, RBCs, fibrin → red, airless tissue.
• 3️⃣ Gray Hepatization (Day 5–7): RBC breakdown → gray-brown tissue. Dense fibrinopurulent exudate, neutrophil dominance.
• 4️⃣ Resolution (Day 8+): Enzymatic digestion, macrophage clearance → lung architecture restored (or scarred if incomplete).

🧫 Histopathology

• Lobar Pneumonia: Intra-alveolar neutrophilic infiltrate, fibrin, RBCs. Thickened alveolar septa. Exudate fills airspaces.
• Bronchopneumonia: Suppurative inflammation centred on bronchi/bronchioles, patchy consolidation.
• Interstitial Pneumonia: Mononuclear infiltrates (lymphocytes, macrophages) in alveolar walls; airspaces often clear.
• Viral Pneumonia: Cytopathic effects (giant cells in RSV, inclusion bodies in CMV).
• Pneumocystis jirovecii: Foamy intra-alveolar exudate, cup-shaped organisms on silver stain.

🧍 Host Factors Influencing Pathology

• 👶 Infants: Immature immunity.
• 👵 Elderly: Decline in mucociliary clearance, comorbidities.
• 🛡️ Immunosuppression: HIV/AIDS, chemotherapy, steroids.
• 🫁 Chronic lung disease: COPD, bronchiectasis, CF.
• 🚬 Smoking: Impairs mucociliary clearance, macrophage function.
• 🍷 Alcohol: Aspiration risk, poor neutrophil function.

⚠️ Complications

• 🕳️ Lung Abscess: Liquefactive necrosis (esp. S. aureus, Klebsiella, anaerobes).
• 💧 Pleural Effusion & Empyema: Infected exudate in pleural space.
• 🩸 Bacteraemia & Sepsis: May seed endocardium, brain, kidneys.
• 🧱 Fibrosis & Organisation: Chronic scarring if resolution incomplete.
• 🌬️ Bronchiectasis: Permanent bronchial dilation from necrosis.
• 🫀 ARDS: Severe inflammatory response → diffuse alveolar damage.

🧪 Investigations (Pathology-Oriented)

• 🩸 Bloods: FBC (↑ WCC/CRP), U&E, LFTs, blood cultures.
• 🧫 Microbiology: Sputum Gram stain, culture, PCR for atypical/viral pathogens.
• 💧 Urine antigen: Pneumococcal & legionella.
• 🖼️ Imaging: CXR (lobar vs patchy vs interstitial). CT for complications.
• 🔬 Histopathology: BAL, lung biopsy in atypical or non-resolving cases.

💊 Management (Pathology Integration)

• 🎯 Antimicrobials: Guided by severity (CURB-65), local resistance, culture results.
• 🫁 Supportive care: Oxygen, fluids, antipyretics, analgesia, physiotherapy.
• 💉 Severe CAP: IV broad-spectrum antibiotics ± dual therapy (e.g. β-lactam + macrolide).
• ⚠️ Complication-directed: Drain empyema, resect abscess if not responding.
• 🌱 Immunocompromised: Add antifungal (e.g. voriconazole for Aspergillus) or co-trimoxazole for PCP.

🫁 Community-Acquired Pneumonia (CAP) – Antibiotic Therapy

Empirical treatment should be given as soon as diagnosis is made (ideally within 1 hour). Tailor based on CURB-65 severity score and local resistance patterns.

🧫 Additional Pathogen-Specific Notes

• Mycoplasma pneumoniae: Atypical; use macrolide (Clarithromycin/Azithromycin) or doxycycline. No cell wall → resistant to β-lactams.
• Legionella: Severe, systemic illness; treat with Azithromycin IV or Levofloxacin. Add Rifampicin in severe cases.
• Staphylococcus aureus: Post-influenza, cavitating lesions; Flucloxacillin (MRSA → add Vancomycin/Linezolid).
• Klebsiella: Alcoholics, diabetics; “red currant jelly” sputum. Treat with Cefuroxime + Gentamicin.
• Pseudomonas: Immunocompromised, CF, ventilated patients; Ciprofloxacin or Piperacillin-tazobactam.

🖼️ Imaging & Pathology Correlation

• Lobar Pneumonia: Homogenous consolidation of a lobe ± air bronchograms (classical pneumococcus).
• Bronchopneumonia: Patchy, bilateral peribronchial infiltrates (elderly, post-viral, aspiration).
• Interstitial Pneumonia: Ground-glass opacities (viral, Mycoplasma, PCP).
• Abscess Formation: Cavity with air-fluid level (S. aureus, Klebsiella, aspiration).
• Pleural Effusion/Empyema: Meniscus sign on CXR, confirmed with USS/CT.

⚠️ ICU Referral Criteria

• PaO₂ <8 kPa despite 60% O₂
• PaCO₂ >6.4 kPa
• Septic shock (hypotension despite fluids)
• Multilobar disease on imaging
• Altered consciousness, exhaustion, or worsening acidosis

📚 References

• British Thoracic Society (BTS) Guidelines for CAP (2019 update)
• NICE Guideline [CG191] – Pneumonia (community- and hospital-acquired)
• Harrison’s Principles of Internal Medicine
• Robbins & Cotran Pathologic Basis of Disease

🛡️ Prevention

• 💉 Vaccination: Pneumococcal + annual influenza.
• 🚭 Smoking cessation, 🍷 alcohol reduction.
• 🧼 Infection control: Hand hygiene, masks in outbreak settings.

📚 References

• NICE & BTS Guidelines on Pneumonia.
• Robbins & Cotran Pathologic Basis of Disease.
• Harrison’s Principles of Internal Medicine.