Pneumonia

Introduction
Pneumonia is an acute infection of the pulmonary parenchyma, characterized by inflammation of the alveoli and interstitial tissue, leading to consolidation and impaired gas exchange. It is caused by a variety of microorganisms, including bacteria, viruses, fungi, and parasites. Pneumonia remains a major cause of morbidity and mortality worldwide, especially in children under 5 years, elderly individuals, and immunocompromised patients.

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Classification
Pneumonia can be classified by the setting in which it develops, the causative organism, or the clinical/radiologic pattern:

1. By Setting

   • Community-acquired pneumonia (CAP): Occurs in individuals not recently hospitalized or residing in long-term care facilities.

   • Hospital-acquired pneumonia (HAP): Occurs ≥48 hours after hospital admission, not incubating at the time of admission.

   • Ventilator-associated pneumonia (VAP): Develops ≥48–72 hours after endotracheal intubation.

   • Healthcare-associated pneumonia (HCAP): In patients with significant healthcare contact (definition now largely replaced by risk-based assessments).

2. By Causative Agent

   • Bacterial: Streptococcus pneumoniae (most common in CAP), Haemophilus influenzae, Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila, Staphylococcus aureus.

   • Viral: Influenza, respiratory syncytial virus (RSV), adenovirus, coronaviruses (including SARS-CoV-2).

   • Fungal: Histoplasma capsulatum, Coccidioides immitis, Cryptococcus neoformans, Aspergillus spp. (more common in immunocompromised).

3. By Radiologic Pattern

   • Lobar pneumonia: Consolidation of an entire lobe (e.g., pneumococcal pneumonia).

   • Bronchopneumonia: Patchy consolidation around bronchioles.

   • Interstitial pneumonia: Diffuse interstitial infiltrates, often viral or atypical bacterial pathogens.

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Risk Factors

• Extremes of age (<5 years, >65 years).

• Chronic lung diseases (COPD, bronchiectasis, asthma).

• Cardiovascular disease.

• Diabetes mellitus.

• Chronic kidney or liver disease.

• Immunosuppression (HIV/AIDS, chemotherapy, transplant).

• Smoking, alcohol abuse.

• Aspiration risk (neurological disorders, impaired swallowing).

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Pathophysiology
Pathogens gain access to the lower respiratory tract via inhalation of droplets, aspiration of oropharyngeal contents, or hematogenous spread. Inflammatory cells infiltrate alveoli, leading to consolidation, impaired gas exchange, and the characteristic clinical and radiologic findings. The immune response, while protective, contributes to the symptoms and signs of pneumonia.

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Clinical Features

Typical bacterial pneumonia (e.g., S. pneumoniae):

• Sudden onset fever, chills.

• Productive cough with purulent sputum.

• Pleuritic chest pain.

• Dyspnea.

• Tachypnea, tachycardia.

Atypical pneumonia (e.g., Mycoplasma pneumoniae, viruses):

• Gradual onset.

• Dry cough.

• Low-grade fever.

• Headache, myalgia, malaise.

Elderly patients: May present with confusion, falls, or worsening chronic illness rather than respiratory symptoms.

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Physical Examination

• Fever, tachypnea, tachycardia.

• Reduced chest expansion on affected side.

• Dullness to percussion.

• Bronchial breath sounds, crackles.

• Increased vocal fremitus and resonance.

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Diagnosis

Laboratory Tests

• Complete blood count: Leukocytosis (bacterial) or normal/low count (viral/severe infection).

• CRP, ESR, procalcitonin (may help assess severity and etiology).

• Sputum Gram stain and culture.

• Blood cultures (especially in severe cases).

• Antigen tests: Urinary antigen for S. pneumoniae and L. pneumophila.

• PCR for respiratory viruses and atypical bacteria.

Imaging

• Chest X-ray: Consolidation, interstitial pattern, or multilobar disease.

• CT chest: For unclear cases or suspected complications.

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Severity Assessment

• CURB-65 (Confusion, Urea >7 mmol/L, Respiratory rate ≥30/min, BP <90/60 mmHg, Age ≥65) for CAP.

• Pneumonia Severity Index (PSI): Stratifies mortality risk and guides hospitalization.

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Management

1. General Principles

• Early diagnosis and prompt initiation of appropriate antimicrobial therapy.

• Supportive care: Oxygen for hypoxia, fluids for dehydration, analgesia for pleuritic pain.

2. Empirical Antimicrobial Therapy (adults, initial choice guided by severity, comorbidities, and local resistance patterns)

a. Community-Acquired Pneumonia (CAP)

• Outpatient, healthy, no recent antibiotic use:

  • Amoxicillin: 1 g orally three times daily for 5–7 days.

  • Alternative: Doxycycline 100 mg orally twice daily or Clarithromycin 500 mg orally twice daily (if atypical pathogens suspected).

• Outpatient with comorbidities/recent antibiotics:

  • Amoxicillin-clavulanate 875/125 mg orally twice daily plus macrolide (e.g., Azithromycin 500 mg day 1, then 250 mg daily for 4 days).

  • Or respiratory fluoroquinolone (Levofloxacin 750 mg once daily or Moxifloxacin 400 mg once daily).

• Inpatient (non-ICU):

  • Ceftriaxone 1–2 g IV once daily plus Azithromycin 500 mg IV/PO daily.

  • Or Levofloxacin 750 mg IV/PO once daily.

• Inpatient (ICU):

  • Ceftriaxone or Cefotaxime IV plus Azithromycin or Levofloxacin.

b. Hospital-Acquired Pneumonia (HAP) and Ventilator-Associated Pneumonia (VAP)

• Broader coverage due to risk of resistant organisms:

  • Piperacillin-tazobactam 4.5 g IV every 6–8 hours

  • Or Cefepime 2 g IV every 8–12 hours

  • Consider MRSA coverage if risk factors: Vancomycin 15 mg/kg IV every 8–12 hours or Linezolid 600 mg IV/PO twice daily.

3. Duration of Therapy

• Typically 5–7 days for uncomplicated CAP; longer for severe, complicated, or immunocompromised cases.

4. Supportive Measures

• Antipyretics (e.g., paracetamol 500–1000 mg every 4–6 h).

• Adequate hydration.

• Oxygen therapy to maintain SpO₂ ≥ 92% (≥88% in COPD).

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Complications

• Parapneumonic effusion/empyema.

• Lung abscess.

• Sepsis and septic shock.

• Acute respiratory distress syndrome (ARDS).

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Prevention

• Vaccination:

  • Pneumococcal vaccines (PCV13, PPSV23) for at-risk populations.

  • Annual influenza vaccination.

• Smoking cessation.

• Infection control in healthcare settings.