Pneumonia is an acute or chronic infection of the lung parenchyma, characterized by inflammation of the alveoli and/or interstitial tissue, leading to impaired gas exchange. It may involve alveolar consolidation, bronchioles, or the interstitium.
Etiology and Causative Agents
Pneumonia may be caused by:
- Infectious agents :
- Bacteria : Streptococcus pneumoniae , Haemophilus influenzae , Mycoplasma pneumoniae , Legionella pneumophila , Chlamydophila pneumoniae
- Viruses : Influenza virus, RSV, Parainfluenza virus, Adenovirus, SARS-CoV-2
- Fungi : Pneumocystis jirovecii , Histoplasma capsulatum , Aspergillus spp.
- Non-infectious agents : Gastric acid aspiration, toxic inhalants (smoke, chemical fumes)
🧠High-Yield Note :
- S. pneumoniae is the leading cause of community-acquired pneumonia (CAP) .
- Aspiration pneumonia is common in stroke, elderly, or sedated patients.
- Ventilator-associated pneumonia (VAP) is linked to Pseudomonas , Acinetobacter , MRSA .
Classification of Pneumonia
A. By Anatomic Pattern of Involvement
- Lobar Pneumonia
- Consolidation of an entire lobe
- Typically caused by S. pneumoniae
- Bronchopneumonia
- Patchy consolidation centered around bronchi
- Common in elderly or debilitated individuals
- Interstitial (Atypical) Pneumonia
- Involves alveolar walls and interstitium
- Often viral or caused by Mycoplasma
- Miliary Pneumonia
- Hematogenous spread leading to diffuse micronodular infiltrates
- Associated with TB, fungal, or disseminated infections
B. By Clinical Setting
| Classification | Description | Common Pathogens |
|---|---|---|
| Community-Acquired (CAP) | Acquired outside hospitals | S. pneumoniae , H. influenzae , M. pneumoniae , respiratory viruses |
| Hospital-Acquired (HAP) | Onset ≥48h after admission | P. aeruginosa , Klebsiella , MRSA , Enterobacter |
| Ventilator-Associated (VAP) | Occurs ≥48h after intubation | Similar to HAP but more resistant organisms |
| Healthcare-Associated (HCAP) | Exposure to healthcare settings (dialysis, nursing homes) | Similar to HAP organisms |
📌 High-Yield Tip :
Hospital-acquired organisms often show multidrug resistance (MDR) . Empiric therapy should consider local antibiogram data.
C. By Causative Agent
- Typical Pneumonia
- Extracellular bacteria (e.g. S. pneumoniae )
- Presents with productive cough, high fever, lobar consolidation
- Atypical Pneumonia
- Caused by Mycoplasma , Chlamydia , viruses
- Dry cough, milder symptoms, diffuse interstitial infiltrates
- Aspiration Pneumonia
- Inhalation of gastric contents
- Common in impaired consciousness, poor gag reflex
- Affects right lower lobe most commonly
- Opportunistic Pneumonia
- Occurs in immunocompromised patients (e.g. AIDS, transplant)
- Caused by Pneumocystis jirovecii , Aspergillus , CMV
- Cryptogenic Organizing Pneumonia (COP)
- Non-infectious, inflammatory disorder resembling pneumonia
- Patchy subpleural consolidation, responds to steroids
- Legionnaire's Disease
- Caused by Legionella pneumophila
- Found in water systems, presents with GI symptoms, hyponatremia, confusion
Risk Factors
- Extremes of age (<5 years, >65 years)
- Smoking, COPD, asthma
- Immunosuppression (HIV/AIDS, chemotherapy)
- Neurological disease (stroke, Parkinson’s)
- Recent hospitalization or surgery
- Mechanical ventilation (VAP)
- Dysphagia, GERD, sedation
Pathophysiology
- Invasion of lung parenchyma by pathogen
- Inflammatory response triggers cytokine release (e.g., IL-1, TNF-α)
- Alveolar exudation causes consolidation and impaired oxygenation
- Resolution or progression to complications (e.g., abscess, ARDS)
Clinical Manifestations
| Typical Pneumonia | Atypical Pneumonia |
|---|---|
| Sudden onset | Insidious onset |
| High-grade fever | Low-grade fever |
| Productive cough with purulent sputum | Dry cough |
| Pleuritic chest pain | Mild/no chest pain |
| Dyspnea | Mild or absent dyspnea |
| Lobar consolidation on CXR | Diffuse, patchy infiltrates |
Diagnostic Evaluation
- History and physical exam
- Chest X-ray (CXR) : Lobar vs interstitial infiltrates
- Sputum culture & Gram stain
- Blood cultures
- Pulse oximetry/ABG : Assess oxygenation
- CBC : Leukocytosis in bacterial infections
- CRP, Procalcitonin : Help differentiate bacterial vs viral causes
🧠USMLE Tip :
- Legionella requires urine antigen test
- P. jirovecii needs special stains (e.g., silver stain) or PCR
- Look for hypoxia out of proportion in interstitial pneumonia
Management
1. Empiric Antibiotic Therapy (Adults)
| Setting | First-Line |
|---|---|
| Outpatient (no comorbidity) | Amoxicillin, Doxycycline, Macrolide (if resistance <25%) |
| Outpatient (comorbid) | Amoxicillin-clavulanate + Macrolide or Doxycycline |
| Inpatient (non-ICU) | IV Beta-lactam + Macrolide or Fluoroquinolone |
| ICU | Beta-lactam + Azithromycin or Fluoroquinolone ± MRSA/Pseudomonas coverage |
✅ High-Yield :
- Macrolides are preferred in atypical pneumonia.
- Vancomycin or Linezolid added if MRSA is suspected.
- Piperacillin-tazobactam for HAP/VAP with Pseudomonas risk.
2. Supportive Care
- Oxygen therapy
- IV fluids
- Antipyretics
- Chest physiotherapy (selected patients)
Complications
- Lung abscess
- Pleural effusion/empyema
- ARDS (acute respiratory distress syndrome)
- Sepsis
- Respiratory failure
Prevention
- Vaccination :
- Pneumococcal vaccine (PCV13/PPSV23)
- Influenza vaccine annually
- COVID-19 vaccine
- Hand hygiene
- Smoking cessation
- Head elevation in tube-fed or sedated patients
High-Yield Summary for NCLEX & USMLE
| Feature | Typical Pneumonia | Atypical Pneumonia |
|---|---|---|
| Causative agent | S. pneumoniae , Klebsiella | Mycoplasma , Chlamydia , viruses |
| Onset | Sudden | Gradual |
| Sputum | Purulent | Scant or absent |
| WBC | Elevated | Normal/slightly elevated |
| CXR | Lobar consolidation | Diffuse interstitial |
| Response to β-lactams | Good | Poor (needs macrolide/tetracycline) |
