The anatomy of the thoracic duct can be variable; however, it usually originates at the confluence of the cisterna chili on the right side of the aorta at the L1-L2 level. It courses cephalad through the aortic hiatus and crosses the midline behind the aortic arch at the level of T4-T5. The duct then travels along the left side of the esophagus into the neck to drain into the venous system at the confluence of the left subclavian and jugular veins. The thoracic duct transports up to 4 L/day of chyle, depending on diet, drug intake, and intestinal function. Chyle is primarily lymphatic fluid from the gastrointestinal tract, with a smaller contribution from lymphatic drainage of the chest. Since 60% to 75% of absorbed dietary fat passes through this system mainly as chylomicrons, chyle is primarily composed of triglycerides, cholesterol, and fat-soluble vitamins. These components give chyle its characteristic milky white appearance. The thoracic duct is also the main conduit for the return of extravasated proteins to the circulation. The cellular component is predominantly lymphocytes.

Chyle leaks are most commonly secondary to trauma or malignancy (e.g., lymphoma, chronic lymphocytic leukemia, metastatic disease); however, they can also be congenital or secondary to sarcoidosis or infectious etiologies (e.g., histoplasmosis, tuberculosis). Traumatic chyle leaks are often iatrogenic, usually as a complication of thoracic procedures. Esophagectomy and surgical correction of congenital heart disease are surgical procedures with particularly high risk. Abdominal procedures such as open aortic aneurysm repair, retroperitoneal lymph node dissection, Nissen fundoplication, liver transplantation, and peritoneal dialysis catheter placement have also been implicated in chylous ascites. Approximately 20% of traumatic chyle leaks are secondary to penetrating or blunt trauma.