The thoracic duct is the largest lymphatic vessel in the human body. Around 75% of the lymph from the entire body (aside from the right upper limb, right breast, right lung and right side of the head and neck) passes through the thoracic duct. The lymphatic system is essential for the drainage of tissue fluid, and the passage of lymph around the body. This article will look at the anatomy of the thoracic duct, including the function, location and drainage. We will also discuss function and clinical relevance. We will conclude with some review questions to test the reader’s understanding of the article content.
The thoracic duct develops during the seventh and eighth week of gestational life from two vessels anterior to the aorta, which become the left and right embryonic thoracic ducts. The left one gives rise to the upper third of the adult thoracic duct while the lower two-thirds of the adult duct arise from the right embryonic thoracic duct.
The duct connects with lymph nodes over time. The thoracic duct wall and lymph nodes are not fully formed at birth. Disturbances in formation result in various structural variations. The first lymph sacs that develop are the paired jugular lymph sacs located at the border between the internal jugular and subclavian veins. A single retroperitoneal lymph sac also develops at the root of the mesentery, with the cisterna chyli also developing behind the retroperitoneal sac. The lymph vessels grow out from the lymph sacs, along the major veins of the head, neck, and arms. Longitudinal lymph vessels that ultimately form the thoracic duct gradually connect the cisterna chyli and the jugular lymph sacs.
The thoracic duct (also known as van Hoorne’s canal) is the largest lymphatic vessel of the lymphatic system of the body. It is approximately 40 cm in length in adults, and approximately 5 mm in width at its abdominal origin. The thoracic duct extends from the twelfth thoracic vertebra to the root of the neck. The thoracic duct is formed from the abdominal confluence of the left and right lumbar lymph trunks, as well as the left and right intestinal lymph trunks between T12 and L2. If the confluence of lymph trunks is saccular, it is referred to as cisterna chyli. The action of breathing helps chyle flow up the thoracic duct. The duct also contains smooth muscle within its walls, as well as interval valves (much like large veins), which prevent backflow of lymph.
The thoracic duct crosses the diaphragm at the aortic hiatus at the level of the twelfth thoracic vertebra. The aortic opening is located in the posterior mediastinum, and is formed either side by the left and right crura of the diaphragm. The duct continues to ascend, between the thoracic aorta on the left, and the azygous vein on the right, and crosses over to the left side between the fourth and sixth thoracic vertebrae. From this point, the duct ascends behind the aortic arch and the left subclavian artery. The duct will also lie anterior to the anterior scalene muscle and left phrenic nerve before its final destination. The fluid drains at the level of the venous angle (Pirogoff’s angle) between the left subclavian vein and the left internal jugular vein, where it reenters the systemic venous circulation. It is often dilated or enlarged at its terminal segment. There is a bicuspid valve located at the junction of the thoracic duct with the draining vein (e.g. left internal jugular vein or left subclavian vein depending on anatomical variations), which prevents backflow of venous blood into the lymphatic system.
Within the thoracic duct, you find a milky white fluid containing both emulsified fats, and the lymph fluid found in the rest of the lymphatic system. This fluid is called chyle. The thoracic duct drains the lymph from 75% of the body, aside from the right upper right limb, right breast, right lung and right side of the head and neck (which are drained by the right lymphatic duct).
Tributaries- The thoracic duct drains lymph from the right and left descending thoracic lymph trunks, originating from the lower 6 intercostal spaces (6 to 11). The duct also receives lymph from intercostal spaces 1 to 5 via the upper intercostal lymph trunks. Additional tributaries include the mediastinal lymph trunks, the left jugular, left bronchomediastinal and left subclavian trunks.
Variations- The thoracic duct occasionally divides into a right and left duct, with the left entering the venous system as normal, and the right draining into the right subclavian vein. The duct may also drain into the left internal jugular vein, or into the left brachiocephalic vein.
Chylothorax- Rupture of the thoracic duct causes chyle to leak into the pleural cavity. The aetiology of this condition includes traumatic and non-traumatic causes. Traumatic causes can be either iatrogenic (e.g. surgery, radiation) or non-iatrogenic (e.g. penetrating or blunt trauma to the chest wall, forceful emesis or cough). Non-traumatic causes include malignancies, other diseases such as tuberculosis, sarcoidosis, superior vena cava obstruction. Finally, it can also be congenital (e.g. Down, Noonan, Turner syndromes). A chest radiograph would show a pleural effusion while cholesterol and triglyceride measurements in the pleural fluid would point towards the right diagnosis.
Virchow’s node- Virchow’s node is a large lymph node in the left supraclavicular area that drains the lymph from the gut. When it is enlarged (positive Troisier’s sign), it can be one of the earliest signs of intra-abdominal malignancy, usually a gastric carcinoma following migration of tumour emboli through the thoracic duct. It is located in the venous angle, at the junction of the left subclavian vein and the thoracic duct.
Thoracic duct fistula- This commonly occurs when surgery takes place in the lower part of the left side of the neck. It can lead to prolonged hospital stay, due to protein, electrolyte and fluid imbalances. Presentation will be leakage of milky white fluid from the neck following the incision. If this occurs intraoperatively, it should be repaired, but surgical guidelines for postoperative management are less clear.