Pneumonia: Pathophysiology, Etiology, and Clinical Presentation
Pneumonia is an infection of the alveolar spaces (air sacs) causing inflammation and exudation. It is a leading cause of infectious mortality worldwide.
Pathophysiology
- Pathogen reaches alveoli (aspiration, inhalation, hematogenous).
- Alveolar macrophages release cytokines (TNF-α, IL-1, IL-6) → neutrophil recruitment.
- Exudate (fibrin, neutrophils, erythrocytes, organisms) fills alveoli → consolidation.
- Impaired gas exchange (V/Q mismatch, shunt) → hypoxemia.
Major Etiologic Categories
| Category | Key Pathogens | Clinical Clues |
|---|---|---|
| Typical bacterial | S. pneumoniae, H. influenzae, M. catarrhalis | Abrupt onset, high fever, rusty sputum, lobar consolidation |
| Atypical bacterial | M. pneumoniae, C. pneumoniae, Legionella | Subacute, dry cough, headache, patchy infiltrates |
| Viral | Influenza, RSV, SARS-CoV-2, adenovirus | Seasonal, systemic symptoms, bilateral ground-glass |
| Fungal | Histoplasma, Coccidioides, Pneumocystis | Endemic exposure, immunocompromised, chronic |