A chylothorax is an abnormal accumulation of chyle, a type of lipid-rich lymph, in the pleural space surrounding the lung. The lymphatic vessels of the digestive system normally return lipids absorbed from the small bowel via the thoracic duct, which ascends behind the esophagus to drain into the left brachiocephalic vein. If normal thoracic duct drainage is disrupted, either due to obstruction or rupture, chyle can leak and accumulate within the negative-pressured pleural space. In people on a normal diet, this fluid collection can sometimes be identified by its turbid, milky white appearance, since chyle contains emulsified triglycerides.

Chylothorax is a rare but serious condition, as it signals leakage of the thoracic duct or one of its tributaries. There are many treatments, both surgical and conservative. About 2–3% of all fluid collections surrounding the lungs (pleural effusions) are chylothoraces. It is important to distinguish a chylothorax from a pseudochylothorax (a pleural effusion that happens to be high in cholesterol), which has a similar appearance visually but is caused by more chronic inflammatory processes and requires a different treatment.

The symptoms of a chylothorax depend its size and the underlying cause. A small chylothorax may not cause any symptoms and only be detected on a chest X-ray performed for another reason. A large chylothorax may lead to breathlessness or a feeling of pressure in the chest, caused by fluid restricting the expansion of the lungs, although large chylothoraces may remain asymptomatic if the chylothorax has accumulated slowly, as the lungs may have had time to become used to the pressure. Fever or chest pain are not usually associated with chylothorax, as chyle does not generate inflammation by itself.

On examination, chylothorax may lead to reduced breath sounds on the affected side, associated with a dull sound when the chest is tapped or percussed. In cases of postoperative chylothorax, the first sign may be persistent drainage from intercostal drains. Large chylothoraces may cause signs related to the loss of nutrients, including features of malnutrition or decreased ability to fight infections. Rapidly accumulating chylothoraces can cause a sudden drop in blood volume, leading to low blood pressure.

There are three main types of chylothorax: traumatic, non-traumatic, and idiopathic. Historically the most common form of chylothorax was non-traumatic, but traumatic chylothoraces now represent the majority of cases, with most arising as postoperative complications of surgery. The most common cause of non-traumatic chylothoraces is cancer. Chylothoraces can also be classified as low- or high-output based on the rate of chyle accumulation: low-output chylothoraces accumulate <500 mL of chyle per 24 hours, while high-output chylothoraces accumulate >1000 mL per 24 hours.

Malignancies are the most frequent cause of non-traumatic chylothorax. Cancers like chronic lymphocytic leukemia, lung cancer, lymphoma, Kaposi sarcoma, metastatic carcinoma or esophageal cancer are potential causes of chylothorax. Infectious causes are also observed, most often in developing countries. The most common cause of an infectious chylothorax is a complication of tuberculous lymphadenitis. Other possible causative infections include aortitis, histoplasmosis, and filariasis. Chylothorax can also be congenital, and may co-occur with other lymphatic malformations like lymphangiectasis and lymphangiomatosis. Other conditions like tuberous sclerosis, congenital heart disease, trisomy 21 (Down syndrome), Noonan syndrome, or Turner syndrome (missing X chromosome) are also possible causes of congenital chylothorax. Other, more rare causes of congenital chylothorax include Castleman's disease, yellow nail syndrome, Waldenström's macroglobulinemia, sarcoidosis, venous thrombosis, thoracic radiation, macroglobulinemia, amyloidosis, and a goiter. These diseases cause chylothorax by obstructing or destroying the thoracic duct. Also, parenteral nutrition has been a possible cause; a quick dose of total parenteral nutrition can overwhelm the thoracic duct, causing the chyle to leak into the surrounding pleural space.

Iatrogenic chylothorax after surgery is the most common variety of chylothorax. It is a common and serious complication of a pneumonectomy. It is especially common in surgeries requiring mediastinal dissection. The probability of chylothorax depends on the type of surgery. The surgery with the highest risk of chylothorax is an esophagostomy, with a 5-10% risk of chylothorax. Lung resection and mediastinal node dissection have the second highest risk, with 3-7% risk. Other operations like mediastinal tumor resection, thoracic aneurysm repair, sympathectomy, and any other surgeries that take place in the lower neck or the mediastinum can lead to chylothorax. Chylotharax after trauma but not after surgery has also been described after central line placement, pacemaker implantation, and embolization of a pulmonary arteriovenous malformation. Blunt trauma to the chest region is another cause of chylothorax, which has occurred after blast injuries and even simple injuries from coughing or sneezing.

The main mechanism of chylothorax is the leaking of chyle from the thoracic duct, usually caused by a disturbance affecting the structural integrity of the thoracic duct. For example, placement of a central venous catheter can potentially disrupt drainage of lymph into the subclavian veins, followed by the thoracic duct, resulting in chylothorax. The disturbances cause the pressure in the thoracic duct to increase. Soon, collateral channels form, which eventually drain into the thorax. Trauma affecting the thoracic duct is the most common disturbing mechanism.