Pneumonia: Etiology, Pathogenesis, Stages, Morphology, and Complications

Introduction

Pneumonia is an inflammatory condition of the lungs primarily caused by microbial infections. It affects the alveoli and can range from mild to severe cases requiring hospitalization.

Etiology

Pneumonia can be caused by various pathogens, including:

• Bacteria: Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae
• Viruses: Influenza virus, Respiratory syncytial virus (RSV), SARS-CoV-2
• Fungi: Pneumocystis jirovecii (especially in immunocompromised patients)

Pathogenesis

Pneumonia develops when pathogens overcome the body's defense mechanisms, leading to alveolar inflammation. The infection spreads via inhalation, aspiration, or hematogenous transmission.

Mechanisms of Infection

The disease progression involves:

1. Colonization: Pathogens adhere to respiratory epithelial cells.
2. Inflammation: Cytokine release triggers immune responses.
3. Alveolar Consolidation: Fluid accumulation leads to impaired gas exchange.

Stages of Pneumonia

Pneumonia progresses through four stages:

1. Congestion: Initial inflammation with vascular engorgement.
2. Red Hepatization: Increased neutrophils and RBCs leading to lung consolidation.
3. Gray Hepatization: Macrophages replace neutrophils; lung tissue becomes fibrotic.
4. Resolution: Restoration of normal lung structure.

Morphology

Pneumonia can be classified based on its anatomical distribution:

• Lobar Pneumonia: Entire lobes affected.
• Bronchopneumonia: Patchy inflammation involving multiple lobules.
• Interstitial Pneumonia: Involves lung interstitium, often viral in origin.

Complications

If untreated, pneumonia can lead to serious complications, including:

• Pleural Effusion: Fluid accumulation in the pleural space.
• Lung Abscess: Necrotic lung tissue formation.
• Sepsis: Systemic infection leading to multi-organ failure.
• Chronic Respiratory Issues: Persistent lung damage affecting breathing.

Conclusion

Early diagnosis and appropriate treatment are critical in managing pneumonia. Preventive measures such as vaccination and good hygiene practices help reduce the risk of infection.