The pleura is the membranous lining of the chest cavity. A pleural effucsion is an excessive production or accumulation of free fluid in the thoracic space outside of the lungs. Fluids can be made up of 4 main types causing disease:
1. Pyothorax: Accumulation of pus in the thorax
2. Chylothorax: Accumulation of lymph fluid in the thorax.
3. Hemothorax: Accumulation of blood in the thorax.
4. Non-Transudate Pleural Effusion: Accumulation of a non-infectious serum in pleural spaces.
Causes of pleural effusions can be due to: trauma to the chest, heartworm disease in dogs, low serum protein levels, rupture of the thoracic duct, bacterial, liver disease, twisting of a lung lobe amongst others.
The most common type of pleural effusion in cats is chylothorax. It is seen commonly in practice. The second common type seen is pyothorax.