Its cause is usually leakage from the thoracic duct or one of the main lymphatic vessels that drain to it. The most common causes are lymphoma and trauma caused by thoracic surgery. If the patient is on a normal diet, the effusion can be identified by its white and milky appearance, as it contains high levels of triglycerides. This chyle composition of triglycerides is mostly in the form of chylomicrons.
The condition is rare but serious, and appears in all mammals. In animals, chylothorax usually results from diseases that cause obstruction to the thoracic duct, preventing lymph from draining normally into the venous system. Examples include tumors, heartworm disease, right-sided cardiac failure, or idiopathic lymphangiectasia.
Since the mechanism behind chylothorax is not well understood, treatment options are limited. Drainage of the fluid out of the pleural space is essential to obviate damage to organs, especially the inhibition of lung function by the counter pressure of the chyle. Another treatment option is pneumoperitoneal shunting (creating a communication channel between pleural space and peritoneal cavity). By this surgical technique loss of essential triglycerides that escape the thoracic duct can be prevented. Omitting fat (in particular FFA) from the diet is essential. Either surgical or chemical pleurodesis are options: the leaking of lymphatic fluids is stopped by irritating the lungs and chest wall, resulting in swelling and closure of the pleural space. The medication octreotide has been shown to be beneficial and in some cases will stop the chylothorax after a few weeks.
In animals, the most effective form of treatment until recently has been surgical ligation of the thoracic duct combined with partial pericardectomy. There is at least one case report (in a cat) of clinical response to treatment with rutin.