Pathology of the pleura and pericardium

Pleura — Anatomy

• Visceral pleura: wraps around the lungs
• Parietal pleura: lines the inside of the chest wall
• Pleural cavity/space: thin space between the layers; filled with lubricating pleural fluid
• Pathologic involvement of the pleura is usually a secondary complication of underlying pulmonary disease
• Primary pleural disorders: primary intrapleural bacterial infections + malignant mesothelioma

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A) Pleural Effusion and Pleuritis

• Excessive fluid accumulates in the pleural space → limits lung expansion → impairs breathing

Transudate (Hydrothorax)

• Low protein content (ultrafiltrate of blood)
• Most commonly caused by CHF

Exudate

• High protein content (>2.9 g/dL); suggests pleuritis when inflammatory cells are present
• Causes:
  1. Suppurative pleuritis / empyema: microbial invasion from pulmonary infection or blood dissemination
  2. Cancer: suspect in any patient >40 years with exudative effusion, no fever, pain, or positive tuberculin test
  3. Pulmonary infarction
  4. Viral pleuritis

Outcomes

• Transudates and serous exudates: usually resorbed without residual effects
• Fibrinous, hemorrhagic, or suppurative exudates → fibrous organization → adhesions or fibrous pleural thickening + sometimes calcification
• Treatment: drainage of pleural effusion

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B) Pneumothorax

• Air (or other gases) in the pleural space

Types

• Spontaneous: occurs in young healthy males without known pulmonary disease
• Secondary: rupture of a pulmonary lesion near the pleural surface (emphysema, abscess, TB, carcinoma)

Complications

1. Tension pneumothorax: progressive air build-up that cannot escape (“one-way-valve effect”) → mediastinal shift to opposite hemithorax → compromised pulmonary circulation → fatal; if not re-expanded → scarring → hydropneumothorax
2. Infection: prolonged collapse → susceptibility to infection
3. Empyema: pus in pleural cavity → pyopneumothorax

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C) Hemothorax

• Hemorrhage within the pleural cavity
• Cause: ruptured intrathoracic aortic aneurysm → almost always fatal

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D) Chylothorax

• Pleural collection of milky lymphatic fluid containing lipid microglobules
• Caused by obstruction of major lymphatic ducts (usually by intrathoracic cancer)

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E) Malignant Mesothelioma

• Rare cancer of mesothelial cells; arising from visceral/parietal pleura (less commonly peritoneum/pericardium)
• Strongly related to asbestos exposure → tumor develops 25–40 years after initial exposure
• Mechanism: asbestos fibers accumulate near mesothelial cells → generate ROS → DNA damage → oncogenic mutations

Morphology

• Often preceded by extensive pleural fibrosis and plaque formation
• Spreads from localized area to encase the lung
• At autopsy: lung ensheathed by yellow-white, firm, sometimes gelatinous tumor that obliterates the pleural space

💡 High-yield: Transudate = low protein, CHF. Exudate = high protein; empyema, cancer (>40 yrs, no fever), infarction. Pneumothorax: spontaneous = young males; tension = fatal mediastinal shift. Malignant mesothelioma = asbestos exposure; 25–40 yr latency; yellow-white tumor obliterating pleural space.