Physiopathology of the effusions

The pleural, pericardial and peritoneal cavities are lined by a single layer of flat or cuboidal mesothelial cells called the serosa. In normal conditions these cavities contain only a small amount of fluid, enough to lubrificate the adjacent visceral and parietal surfaces as they move over each other. In disease states, a greater amount of fluid, known as an effusion, accumulates.

Effusions are classified as transudative or exudative. Transudates result from an imbalance of hydrostatic and oncotic pressure, as occurs in congestive hearts failure, cirrhosis and the nephrotic syndrome. These fluids have a low specific gravity and a low protein concentration.

Exudates result from injury to the mesothelium, as with infections, lupus or rheumatoid pleuritis, pancreatitis, radiation, or malignant tumours. These fluids have a high specific gravity (>1.015) and a high protein concentration (>3 g/dl) and contain inflammatory cells. Malignant tumours, either primary (mesothelioma) or metastatic, are a common cause of exudative effusions; the serosal surfaces are a site of metastasis for many tumours.