The thoracic duct is a lymphatic vessel that carries chyle from the intestines and into the chest where it empties into the bloodstream. Many of the conditions that cause chylothorax do so because they increase pressure in the blood vessels into which the thoracic duct empties. This increased pressure causes the chyle to "back up" in the duct and many new lymphatics are formed trying to bypass this obstruction. Because these lymphatics are very thin walled, they leak when there is pressure on them. Thus, the chyle seeps from the vessels and accumulates in the chest cavity.

One of the most important aspects of treating chylothorax is to determine the underlying cause so that the treatment can be tailored to the cause. Unfortunately, in many animals the underlying cause is not determined and the condition is termed idiopathic. Some of the causes of chylothorax that have been recognized in dogs include:

Mediastinal masses (lymphosarcoma, thymoma). These masses grow in the mediastinum or space between the left and right sides of the lungs. As they grow, they may partially occlude the blood vessels that the thoracic duct empties into.

Congenital heart disease (tetralogy of Fallot, tricuspid dysplasia). Because these diseases increase pressure in the heart and the blood vessels that the thoracic duct empties into, they also cause lymph to back-up in the thoracic duct. As with mediastinal masses, new lymphatics form and these new vessels simply leak chyle into the chest cavity.

Cardiomyopathy. This is an acquired heart disease where the heart does not contract normally. Similar to congenital heart disease, this leads to an increase in pressure in the blood vessels and subsequent leakage of chyle into the chest cavity.

Pericardial effusion. This is an accumulation of fluid within the sac that surrounds the heart. When fluid accumulates here, it causes an increase in pressure in the blood vessels that the thoracic duct normally empties into.

Heartworm infection. Heartworm infection may cause heart failure that increases pressures in the vessels that the thoracic duct empties into.

Fungal granulomas. These abnormal collections of cells and fungal organisms may block the vessels that they thoracic duct empties into.

Venous thrombi. Blood clots may block the vessels that the thoracic duct empties into. This can occur after catheters have been placed in the jugular vein. It has particularly been noted when liquid diets are placed into the vein.

Congenital abnormalities of the thoracic duct. Abnormalities of the thoracic duct have been described in Afghan hounds and may account for their developing this disease more commonly than most other breeds.

Trauma is an uncommon cause of chylothorax in dogs; however, it has been recognized after automobile accidents or severe vomiting. Chylothorax associated with trauma usually has a good prognosis as the thoracic duct heals fairly quickly on its own without surgery.