Thoracic duct anatomy must be understood in the context of its embryology.
The anlage of the thoracic duct appears in the sixth to seventh week of fetal life as lymphatic clefts surrounded with mesenchyme near large veins. Connecting with each other, the clefts form the jugular and retroperitoneal lymph sacs and a well branching network of canals. Further development of the thoracic duct is connected with the lymph nodes formation; their germs appear on the 9th-10th week along the course of the left trunk, as well as along the ductal branches and anastomoses. The formation of the lymph nodes results in reduction of some trunks and plexuses of the thoracic duct. Disturbances in the formation processes of the lymph nodes can result in various structural variants of the thoracic duct in children and adults. The thoracic duct wall and the lymph nodes formation are not completed by birth. [1, 2, 3]
The first lymph sacs to develop are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. The jugular lymph sacs communicate inferiorly with the single retroperitoneal lymph sac at the root of the mesentery of the intestine and with the cisterna chyli, a lymph sac below the developing diaphragm on the posterior abdominal wall. The retroperitoneal lymph sac develops from mesonephric (primitive kidney) veins and the primitive vena cava. Capillary plexuses and lymphatics expand from the retroperitoneal lymph sac to the abdominal viscera and diaphragm. Channels that join the jugular lymph sacs to the cisterna chyli become the thoracic duct (or left lymphatic duct) and the right lymphatic duct. The retroperitoneal sac establishes connections with the cisterna chyli but loses its connections with the nearby veins.
The cisterna chyli loses its connections with the surrounding veins but produces the inferior portion of the thoracic duct. The primitive lymph sacs become united by longitudinal connections, and each sac associates with lymphatic vessels that drain specific areas. New vessels sprout from the primitive lymphatic system and extend to most tissues of the body, mostly along the course of the main veins. Lymphatics never occur in the central nervous system, meninges, eyeball and cornea, inner ear, cartilage, epidermis, and spleen. Eventually, lymphatic vessels replace the lymph sacs.
The thoracic duct is a tubular structure that is 2-3 mm in diameter, varies in length from 38-45 cm, and extends from the second lumbar vertebra to the root of the neck (see the following image). It begins in the abdomen by a triangular dilatation, the cisterna chyli, which is situated on the front of the body of the second lumbar vertebra, to the right side of and behind the aorta, by the side of the right crus of the diaphragm. It enters the thorax through the aortic opening of the diaphragm between the aorta and the azygos vein. In the posterior mediastinum, the thoracic duct lies anterior to the vertebral column, the right intercostal arteries, and the hemiazygos veins as they cross to open into the azygos vein. Anterior to it are the diaphragm, esophagus, and pericardium. The pericardium is separated from it by a recess of the right pleural cavity. [2, 3, 4, 5, 6]
At the level of the fifth thoracic vertebra, the thoracic duct inclines toward the left side to enter the superior mediastinum and ascends behind the aortic arch and the thoracic part of the left subclavian artery, between the left side of the esophagus and the left pleura, to the thoracic inlet. In the neck, it forms an arch which rises about 3-4 cm (up to 6 cm) above the clavicle and crosses anterior to the subclavian and vertebral arteries and veins, as well as the thyrocervical trunk or its branches.
It is also anterior to the phrenic nerve and the medial border of the scalenus anterior muscle but is separated from these structures by the prevertebral fascia and the carotid tubercle (anterior tubercle of transverse process of the sixth cervical vertebra). In front of it are the left common carotid artery, vagus nerve, and internal jugular vein (IJV). It ends by opening into the angle of junction of the left subclavian vein with the IJV.
The thoracic duct, at its commencement, is about equal in diameter to a goose quill, but it diminishes considerably in caliber in the middle of the thorax and is again dilated just before its termination. It is generally flexuous and constricted at intervals, presenting a varicose appearance. Not infrequently, it divides in the middle of its course into 2 vessels of unequal size that soon reunite or divides into several branches, which form a plexuslike structure.
The thoracic duct occasionally divides at its upper part into 2 branches, right and left: The left ends in the usual manner, whereas the right opens into the right subclavian vein, in connection with the right lymphatic duct (see the following image). At its termination, it is provided with a pair of valves that prevent the passage of venous blood into the duct.
Tributaries of the thoracic duct include the following:
A descending trunk from the posterior intercostal lymph nodes of the lower 6 or 7 intercostal spaces opens into the beginning of the duct on either side
In the thorax, the duct is joined, on either side, by a trunk that drains the upper lumbar lymph glands and pierces the crus of the diaphragm
It also receives the efferents from the posterior mediastinal lymph nodes and from the posterior intercostal lymph glands of the upper 6 left spaces
The left jugular and subclavian trunks (and sometimes by the left bronchomediastinal trunk) join it in the trunk; the left bronchomediastinal trunk, however, usually opens independently into the junction of the left subclavian and internal jugular veins
The right lymphatic duct (ductus lymphaticus dexter) is about 1.25 cm in length, courses along the medial border of the scalenus anterior muscle at the root of the neck, and ends in the right subclavian vein, at its angle of junction with the right IJV; its orifice is guarded by 2 semilunar valves that prevent the passage of venous blood into the duct; the right lymphatic duct receives the lymph from the right side of the head and neck through the right jugular trunk, from the right upper extremity through the right subclavian trunk, and from the right side of the thorax, right lung, right side of the heart, and part of the convex surface of the liver through the right bronchomediastinal trunk; these 3 collecting trunks frequently open separately in the angle of union of the 2 veins
The cisterna chyli receives the 2 lumbar lymphatic trunks, right and left, and the intestinal lymphatic trunk. The lumbar trunks are formed by the union of the efferent vessels from the lateral aortic lymph nodes. They receive the lymph from the lower limbs, from the walls and viscera of the pelvis, from the kidneys and adrenal glands, and from the deep lymphatics of the greater part of the abdominal wall. The intestinal trunk receives the lymph from the stomach, intestine, pancreas, spleen, and from the lower and front part of the liver. The abdominal and thoracic parts of the thoracic duct are widely believed to exhibit active peristalsis, whereas the cervical portion functions like a vein.
The structure of the thoracic duct is considered to be similar to that of a vein. However, the duct is more muscular, and its adventitia and media are less demarcated. The thoracic duct also has also an internal elastic lamina that is more prominent in the thoracic portion. Longitudinal smooth muscle fibers are also described in the subendothelial layer of the midthoracic section. The cervical part is less muscular.
Thoracic duct valves
At the lymphovenous junction, 3 types of valves are described, as follows (see the following image):
Venous valves at the internal jugular vein (IJV) (with 2 big semilunar valves) and the subclavian vein (unicuspid): The cusps of these valves close when the refluxing blood fills them
The ostial valves are bicuspid and extend obliquely across the junction; they prevent blood from entering the thoracic duct
The third type of valves are the ordinary type found in the rest of the thoracic duct; their cusps are closed by the refluxing lymph; however, the last 5 mm of the duct is devoid of any valves [5, 7]Transmission electron microscopy (TEM) micrographs showing the different appearances of the downstream (O) and upstream (I) surfaces of the thoracic duct valves.
Layers of the thoracic duct
Lee et al described the 3 layers of the thoracic duct in the monkey using scanning and transmission electron microscopy (TEM)  .
The luminal and valvular surfaces of the duct and cisterna chili are covered by endothelial cells, which measure 1-2 μm in thickness at the perinuclear region, whereas attenuated areas of the cytoplasm are as thin as 0.1 μm or less. Bicuspid valves are formed by folds of 2 endothelial layers with a thin intervening layer of connective tissue. Many small fingerlike surface projections and numerous plasmalemmal vesicles are associated with the endothelial cells on the downstream surface of the valves (see the following image). However, endothelial cells on the upstream surface appear relatively smooth with few vesicles.
The tunica media is the thickest coat of the thoracic duct and consists of 4-9 layers of smooth muscle cells organized into inner longitudinal, middle longitudinal and oblique, and outer circular layers. The thickest coat exists in the abdominal thoracic duct. The thoracic portion has a moderately thick wall and the cervical portion has the thinnest wall. The muscle bundles are separated by abundant connective tissue. The smooth muscle cells are characterized by large bundles of myofilaments, and their long axes are parallel with the long axes of the cells.
The tunica adventitia is the outermost layer of the thoracic duct. It consists of a connective tissue complex which includes fibroblasts, collagen fibrils, nerves, and vasa vasorum. Near the tunica media, there is a layer of collagen fibrils disposed mainly longitudinally. This layer merges with the surrounding connective tissue. The cisterna chyli and the thoracic duct contain vasa vasorum and unmyelinated nerve fibers near the outer border of the tunica media. 
Single thoracic duct that passes cephalad in the thorax on the right side of the aorta; it crosses to the left side at the level of the fifth cervical vertebra and opens in the venous system as described in Gross Anatomy; this is the most common course as described in most anatomy texts, with an incidence of 60-65%.
The thoracic duct may be partially doubled and opens in the left venous system; this occurs in 15-20% of cases
The thoracic duct is described as double throughout its entire course, one duct on each side of the aorta; the ducts open in the venous system of the corresponding side; this anatomic variation occurs in 12-15% of cases.
The thoracic duct may lie on the right side of the aorta in its entire length and open in the right venous system in 4% of cases
The thoracic duct may lie on the left side of the aorta in its entire length and opens in the left venous system in 4% of cases
The anatomy of the duct may vary in children with complex congenital heart disease. In patients with dextrocardia, left periaortic mass ligation should be considered in patients with chylothoraces that persist after the right-sided thoracotomy. 
The termination of the thoracic duct in the venous system also has many variations. It could end as a single duct in the left internal jugular vein (IJV), left subclavian, left angulus venosus, left innominate, or end as a double, triple, or even quadruple mode in any of the aforementioned venous channels.
Management of Thoracic Duct Fistula
Chylous leak in the neck could occur as a complication of oncologic cervical surgery or during thoracic surgical procedures of the posterior mediastinum. The best treatment for chylous leak is to prevent its occurrence. The reported incidence of chylous fistula after neck dissection is 1-2.5%. [4, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
Keep in mind that thoracic duct injury and or chylothorax could occur due to other causes such as penetrating neck trauma, after surgery for congenital heart disease in newborns and infants, and rarely after left internal jugular venous cannulation. Chylothorax could lead to metabolic and immunologic disorders that can be life threatening, with a mortality rate reaching 50%.
The management of chyle leaks is dependent on the etiology and daily output. The right lymphatic duct enters the internal jugular vein (IJV) below the level of the clavicle; therefore, it is less likely to be traumatized during cervical surgery. The thoracic duct, on the other hand, opens in the left IJV above the clavicle and can extend up to 6 cm in the neck. [19, 20, 21]
Great care should be exercised when the lower end of the IJV is dissected and ligated. Before closing the neck, inspecting the area for any lymphatic fluid that may have accumulated during surgery is important. If a leak is identified, it should be isolated and ligated or clipped. It is difficult to use hemoclips due to the fragility of the lymphatic vessels and the surrounding fatty tissue. It would be advisable to use fine silk (size 5.0) sutures tied over a hemostatic sponge to stop the leak.
In the immediate postoperative period, the serum and drainage levels of cholesterol and triglyceride levels are good predictors for early occurrence of chylous leak. The management of the leak depends on the amount and time of occurrence as well as the accumulation of fluid under the flaps. If the accumulation is immediate and exceeds 800 mL, conservative treatment is less likely to be sufficient to stop the leakage. In these cases, early exploration is advisable before the tissues interact with the chyle and the inflammation process jeopardizes important structures in the area such as the vagus and phrenic nerves.
Sarkaria et al. (2015) published on the complications associated with robotic video-assisted thoracic surgery (RVATS) for lung resections. They described chylothax and recurrent nerve injury (RLNI) associated with mediastinal lymph node dissection (MLND).
Following initiation of tube feeding
Delayed chyle fistula can also occur when tube feedings are started. A milky fluid is noticed in the drainage system. The fat-laden chyle stimulates an inflammatory dermal reaction with violaceous woody induration and discoloration. Dietary measures are instituted to eliminate fat that passes through the lacteals to the thoracic duct. Medium-chain triglyceride directs fat into the portal system and away from the lymphatic channels. Pressure dressings may then be applied without suction drainage.
An alternative method is to apply suction drainage. It usually takes 2 weeks for a delayed leak to stop. Recalcitrant cases need wound exploration with the use of an operating microscope. If conservative measures fail, exploration becomes necessary. The use a clavicular periosteal flap or fibrin blue may be helpful to stop the leak. Thoracoscopic thoracic duct ligation is another technique that provides a safe and efficient means of treating chyle leaks refractory to repeated surgical and medical intervention.
The use of octreotide to stop chylous leak has also been reported. Octreotide reduces pancreatic and gastrointestinal secretions, thus decreasing intrahepatic venous pressure and splanchnic blood flow. These effects decrease the flow within the thoracic duct and lower the concentration of triglycerides.
The occurrence of unilateral chylothorax can complicate neck dissection. Bilateral chylothorax is extremely rare, but it is a very serious and potentially fatal complication. Chylothorax after esophagectomy should be suspected when an unexplained high-volume chest tube output turns milky white after enteral tube feedings are started. The diagnosis is confirmed by a triglyceride level of 110 mg/dL or greater, the presence of chylomicrons in the chest tube drainage, a positive Sudan stain, or lipoprotein electrophoresis.
The results of early closure of the thoracic duct for a high output iatrogenic chylothorax after esophagectomy are excellent. When reoperation is not delayed and simple duct closure of any type is performed, patients have little added morbidity, and the reported success rates are around 90%.
A few points should be kept in mind when the wound is explored, as follows:
The use of methylene blue is not advised; it can stain the surrounding tissue and make finding the leak difficult; sewing the duct has to be done using pledgets; the thoracic duct is paper thin and chyle contains no fibrin
Nonpledgeted sutures will tear it further
Sewing has to be done above and below the leak, and the stitches should not be through the duct but into the surrounding tissue around the duct and should allow the pledgets to close the duct; use of clips may be an adjunct, but pledgeted sutures are preferred
Nadolski and Itkin (2013) published on the recent advancements in the minimally invasive treatment of chylothorax.  Intranodal lymphangiography has gradually replaced traditional pedal lymphangiography for thoracic duct embolization (TDE). The accumulation of experience in treating chylous effusions has significantly broadened the adoption of TDE to treat chylothorax. This technique has low morbidity and is more effective than the surgical and conservative traditional therapies. Of course, in cases of nontraumatic chylothorax, an MRI should precede lymphangiography to exclude causes of chylothorax that cannot be managed by TDE (eg, lymphatic malformations, chylous ascites). Future advancements in imaging guidance during TDE will continue to refine the management of chylous effusions.
Hoeppner and Hopt (2013) described transabdominal mass ligation of the thoracic duct following en bloc esophagectomy as a simple and safe method for the prevention of postoperative chylothorax.