3.3 Pneumonia & Lower Respiratory Tract Infections

3.3 Pneumonia & Lower Respiratory Tract Infections

Pneumonia represents acute infection and inflammation of the lung parenchyma, producing alveolar consolidation and systemic inflammatory response. In the United Kingdom and on licensing examinations, clinical competency requires distinguishing typical from atypical organisms, calculating prognostic scores (CURB-65), selecting appropriate empirical antimicrobials, and managing opportunistic and nosocomial infections.

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1. Community-Acquired Pneumonia (CAP): Pathogen Spectrum

Community-acquired pneumonia refers to acute pneumonia developing in individuals who have not been hospitalized or resided in a healthcare facility within the preceding 14 days.

Typical Pathogens

1. Streptococcus pneumoniae (Pneumococcus):

   • Accounts for $50\text{--}60\%$ of all bacterial CAP cases.
   • Classically presents with abrupt high fevers, rigors, pleuritic chest pain, tachypnoea, and rust-coloured sputum.
   • Radiograph shows dense lobar consolidation with air bronchograms. Severe disease occurs particularly in asplenic patients, older adults, and individuals with chronic organ failure.

2. Haemophilus influenzae:

   • Common cause of pneumonia and purulent bronchitis in patients with underlying COPD, bronchiectasis, and chronic smoking.

3. Moraxella catarrhalis:

   • Frequently colonizes and infects elderly patients and individuals with chronic structural lung disease.

Atypical Organisms & High-Yield Clinical Associations

Atypical pathogens lack a rigid peptidoglycan cell wall (inherently resistant to beta-lactams) or replicate intracellularly, producing systemic extrapulmonary manifestations:

• Mycoplasma pneumoniae:

  • Common in young adults, school-aged children, and university dormitory outbreaks.
  • Insidious onset with prolonged dry cough, prominent headache, sore throat, and arthralgias.
  • Extrapulmonary Hallmarks: Erythema multiforme (targetoid lesions), autoimmune haemolytic anaemia mediated by cold agglutinins (IgM antibodies), Stevens-Johnson syndrome, and neurological syndromes (Guillain-Barré, transverse myelitis).

• Legionella pneumophila:

  • Transmitted by inhaling contaminated aerosolized water droplets from hotel air conditioning towers, spa pools, or hospital plumbing systems.
  • Rapidly progressive pneumonia with high fevers and relative bradycardia (Faget's sign).
  • Extrapulmonary Hallmarks: Profuse watery diarrhoea, abdominal pain, marked encephalopathy/confusion, severe hyponatraemia (due to renal sodium wasting or SIADH), and deranged liver transaminases.
  • Diagnosis & Treatment: Urinary antigen test (detects serogroup 1); treated with clarithromycin or levofloxacin.

• Chlamydia psittaci (Psittacosis):

  • History of close contact with psittacine birds (parrots, parakeets, cockatoos) or poultry.
  • Features splenomegaly, high fever, epistaxis, and bilateral interstitial shadowing. Treated with doxycycline.

• Coxiella burnetii (Q Fever):

  • Contact with livestock (cattle, sheep, goats) during calving or lambing seasons.
  • Manifests as acute pneumonia, culture-negative endocarditis, or granulomatous hepatitis.

• Staphylococcus aureus (Post-Viral Secondary Bacterial Pneumonia):

  • Typically supervenes during or following recovery from acute influenza virus infection.
  • Causes rapid, necrotizing cavitating pneumonia with multiple abscesses, pneumatoceles, and high mortality. Requires treatment with IV flucloxacillin or co-amoxiclav (vancomycin/linezolid if MRSA is suspected).

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2. Risk Stratification: The CURB-65 Score

The British Thoracic Society (BTS) and NICE recommend the CURB-65 score to stratify 30-day mortality risk and guide admission decisions in adults with CAP.

Scoring Tiers and Clinical Disposition

• Score 0 to 1 (Low Risk - Mortality $< 3\%$):

  • Suitable for outpatient home management.
  • Provide oral antibiotic therapy, rest, hydration, and clear safety netting.

• Score 2 (Moderate Risk - Mortality $\sim 9\%$):

  • Inpatient hospital admission indicated.
  • Short inpatient stay or closely monitored outpatient ambulatory care with dual oral/IV antimicrobial therapy.

• Score 3 to 5 (High Risk - Mortality $15\text{--}40\%$):

  • Urgent hospital admission required.
  • Immediately evaluate for critical care unit (HDU or ICU) admission, particularly if multi-lobar consolidation or refractory hypoxaemia is present.

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3. Empirical Antimicrobial Regimens for CAP (NICE Guidelines)

Antibiotic therapy should be initiated within 4 hours of arrival in hospital (or within 1 hour if sepsis or septic shock is present):

Essential Diagnostic Investigations in Inpatient CAP

• Moderate to High Severity: Sputum microscopy and culture, blood cultures (before antibiotics), pneumococcal and Legionella urinary antigen tests, full blood count, renal and liver function panels, CRP, and arterial blood gas.
• Repeat Radiography: NICE CG191 advises not routinely offering a follow-up chest X-ray to people whose symptoms have resolved. Consider one (usually about 6 weeks after treatment) for persistent symptoms or a higher risk of underlying lung cancer, such as smokers and people over 50.

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4. Hospital-Acquired Pneumonia (HAP)

Hospital-Acquired Pneumonia is defined as pneumonia developing $> 48\text{ hours}$ after hospital admission that was neither present nor incubating at admission.

Microbiology & Resistance

Unlike CAP, HAP is dominated by nosocomial Gram-negative bacilli and hospital-adapted pathogens:

• Pseudomonas aeruginosa
• Klebsiella pneumoniae and other Enterobacteriaceae ($E.\text{ coli}$, Proteus)
• Staphylococcus aureus (including Methicillin-Resistant $S.\text{ aureus}$ [MRSA])
• Acinetobacter species in ventilated intensive care patients (VAP)

Empirical Management

• Mild / Non-Severe HAP (Early Onset): Oral co-amoxiclav $625\text{ mg}$ TDS (or doxycycline/ceftriaxone) for 5 days.
• Severe HAP / High Risk of Resistance (Late Onset $> 5\text{ days}$, ICU, Prior Antibiotics): Broad-spectrum intravenous antipseudomonal therapy with Piperacillin-tazobactam (Tazocin) $4.5\text{ g}$ TDS IV, ceftazidime $2\text{ g}$ TDS IV, or meropenem $1\text{ g}$ TDS IV. Add vancomycin or linezolid if MRSA is suspected.

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5. Aspiration Pneumonia

Aspiration pneumonia results from the inhalation of oropharyngeal secretions or gastric contents into the lower respiratory tract, triggering a chemical pneumonitis and secondary polymicrobial bacterial infection.

Risk Factors

• Depressed conscious level (acute stroke, head trauma, general anaesthesia, alcohol or drug intoxication, post-ictal state).
• Bulbar or pseudobulbar palsy (motor neurone disease, Parkinson's disease, myasthenia gravis).
• Gastro-oesophageal reflux, persistent vomiting, or presence of a nasogastric feeding tube.

Anatomical Localization

Due to gravitational drainage and bronchial anatomy (the right main bronchus is wider, shorter, and more vertical):

• Recumbent / Supine Patient: Apical (superior) segment of the right lower lobe or posterior segment of the right upper lobe.
• Erect / Sitting Patient: Basilar segments of the right lower lobe.

Microbial Flora and Treatment

• Dominated by oral anaerobes (Peptostreptococcus, Fusobacterium nucleatum, Prevotella, Bacteroides) alongside oral streptococci and Gram-negative bacilli.
• First-Line Antibiotics: Intravenous Co-amoxiclav $1.2\text{ g}$ TDS (or cephalosporin/ciprofloxacin plus intravenous Metronidazole $500\text{ mg}$ TDS for anaerobic coverage).

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6. Pneumocystis jirovecii Pneumonia (PCP / PJP)

Pneumocystis jirovecii is an opportunistic fungal pathogen that causes severe, life-threatening pneumonia in immunocompromised individuals—classically patients with HIV infection with CD4 count $< 200\text{ cells/}\mu\text{L}$, solid organ transplant recipients, haematological malignancies, or those on prolonged high-dose corticosteroid therapy.

Clinical Triad & Hallmarks

1. Insidious Progression: Subacute dry, non-productive cough, low-grade fevers, and progressive breathlessness evolving over weeks.
2. Disproportionate Exertional Desaturation: Chest auscultation is frequently unremarkable, but oxygen saturation plunges dramatically upon minimal exertion (e.g., walking a short distance).
3. Radiography: Bilateral, symmetrical perihilar ground-glass interstitial infiltrates ("bat-wing" distribution). Normal CXR occurs in up to 10% of cases; high-resolution CT shows diffuse ground-glass attenuation.

Diagnosis & Management

• Microbiological Confirmation: Direct visualization of cysts on induced sputum or bronchoalveolar lavage (BAL) using silver stain (Grocott's methenamine silver) or immunofluorescence/PCR.
• First-Line Antimicrobial Therapy: High-dose Co-trimoxazole (Trimethoprim-Sulfamethoxazole) given intravenously ($120\text{ mg/kg/day}$ in 3–4 divided doses) for 21 days.
• Indications for Adjunctive Systemic Corticosteroids:
  • Corticosteroids (oral prednisolone or IV methylprednisolone) must be administered before or simultaneously with co-trimoxazole if moderate-to-severe hypoxaemia is present:
    • Room air $\mathbf{\text{PaO}_2 < 9.3\text{ kPa}}$ ($< 70\text{ mmHg}$), OR
    • Alveolar-arterial oxygen gradient $\ge 35\text{ mmHg}$.
  • Mechanism: Lysis of organisms by antimicrobials causes intense inflammatory alveolar exudation; steroids prevent acute respiratory deterioration and fatal ARDS.