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Community Acquired Pneumonia and Differentials

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🫁 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.

Severity	First-line (No Penicillin Allergy)	Penicillin Allergy	Notes
Low severity (CURB-65 0–1)	Amoxicillin 500 mg PO TDS × 5 days	Doxycycline 200 mg stat, then 100 mg OD OR Clarithromycin 500 mg BD	Outpatient; reassess if no improvement in 48h
Moderate severity (CURB-65 = 2)	Amoxicillin 500 mg–1 g PO/IV TDS + Clarithromycin 500 mg BD (PO/IV)	Levofloxacin 500 mg PO/IV OD–BD	Short hospital stay or supervised care
High severity (CURB-65 ≥3)	Co-amoxiclav 1.2 g IV TDS + Clarithromycin 500 mg IV BD	Levofloxacin 500 mg IV BD (monotherapy)	Always admit; consider ITU/critical care referral
Hospital-Acquired Pneumonia (HAP)	Mild (within 5 days of admission): Amoxicillin + Metronidazole Severe (>5 days): Piperacillin-tazobactam 4.5 g IV TDS	Levofloxacin 500 mg IV/PO BD OR Ciprofloxacin 400 mg IV TDS	Cover Gram-negative bacilli (e.g., Klebsiella, Pseudomonas)
Aspiration Pneumonia	Co-amoxiclav 1.2 g IV TDS OR Amoxicillin + Metronidazole	Clarithromycin + Metronidazole OR Levofloxacin + Metronidazole	Always cover anaerobes (Bacteroides, Fusobacterium)
Pneumocystis jirovecii (PCP, HIV/AIDS)	High-dose Co-trimoxazole IV (15–20 mg/kg/day in 3–4 divided doses)	Atovaquone or IV Pentamidine	Adjunctive steroids if PaO₂ <9.3 kPa (severe hypoxia)

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🧫 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.

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🖼️ 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.

Lobar pneumonia pathology

Bronchopneumonia histopathology

Interstitial pneumonia with ground-glass changes

Complication of pneumonia – abscess

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⚠️ 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

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📚 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.

PLEASE NOTE LEGAL ADVICE: The contents are under continuing development and improvements and may contain errors of omission or fact. Feedback vital and always welcome at drokane at gmail.com. This is not to be used for the assessment, diagnosis or management of patients. It should not be regarded as medical advice. It is only for educational purposes. Please adhere to your local protocols. If you are unwell please seek healthcare advice from your doctor. This does not replace senior or specialist advice. If you do not accept this then please do not use the website. If you need medical advice, please consult a doctor or other appropriate medical professional. If you are a medical professional and you need advice then speak to your senior or colleagues.

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