Effusions are increased amounts of fluid in the peritoneal, pleural or pericardial cavities. They are not diseases in themselves but they indicate the presence of a pathological process which has altered the local balance of fluid production and removal.
The classification of effusions is based on the total cell count and total protein content of the fluid:
- Transudates and modified transudates have the lowest protein contents and cell counts. They usually result from imbalances in the Starling forces.
- Exudates have the highest cell counts and protein contents and usually occur with an inflammatory or neoplastic disease process.
Effusions may also occur when other fluids leak into the abdominal or thoracic cavities. Fluids that may be involved include:
- Urine, producing a uroabdomen (a modified transudate or exudate).
- Bile, producing bile peritonitis (a modified transudate or exudate)).
- Pancreatic juices leaking in pancreatitis (producing a modified transudate or exudate).
- Blood, producing a haemoabdomen (a haemorrhagic effusion).
- Chyle (or lymph), producing a chylous effusion in the thorax or abdomen.
- Pseudochyle, a mixture of cellular debris and lipid that may resemble a chylous effusion.