Anatomy and Physiology of the Thoracic Duct

Chyle is a milky fluid primarily composed of emulsified fat (high triglyceride content in the form of chylomicrons, along with fat-soluble vitamins), T lymphocytes, electrolytes, proteins, and immunoglobulins.

The thoracic duct is the largest lymphatic vessel, responsible for transporting chyle from the intestines to the bloodstream. It drains approximately 75% of the body's lymphatic fluid, carrying 1–2 liters of chyle per day. Its flow increases with dietary fat intake, particularly long-chain triglycerides.

Histologically, the thoracic duct consists of three layers (intima, media, and adventitia). Except in small-sized ducts, it contains numerous valves, which, along with smooth muscle fibers in the media layer, facilitate lymphatic fluid movement.

The thoracic duct typically extends from the cisterna chyli to the left jugulosubclavian angle (Figure 1). It originates from the cisterna chyli, which forms from the confluence of intra-abdominal lymphatic vessels with retroperitoneal and lower limb lymphatics. The cisterna chyli is located at the D12-L2 level, to the right of the aorta and behind the right diaphragmatic crus.

Cranially, the duct ascends along the right anterior surface of the dorsolumbar vertebral bodies, between the aorta and the azygos vein, entering the thorax through the aortic hiatus. The intrathoracic portion follows an oblique ascending course, crossing to the left behind the aorta and to the left of the esophagus at the D5-D6 vertebral level. As it travels through the thorax, it receives tributary vessels from the mediastinum, pulmonary parenchyma, and parietal pleura.

The thoracic duct exhibits numerous anatomical variations, attributed to its embryological development, which involves selective atrophy of various lymphatic channels (Figure 2). Only 50% of cases present a "normal" anatomy, while the remaining cases show variations in the origin, course, or termination of the duct (Figure 3).

Etiology and Pathophysiology of Chylothorax

Disruption of chyle flow through the thoracic duct is the primary mechanism leading to chylothorax. This can occur due to various etiologies, which are broadly classified into traumatic and non-traumatic causes (Table 1).

• Malignant neoplasms are the most common non-traumatic cause.
• Thoracic surgeries are the most frequent traumatic cause of chylothorax.

The pathophysiological mechanism varies depending on the etiology, with key mechanisms including:

• Lymphatic abnormalities (e.g., Gorham-Stout syndrome, generalized lymphatic anomaly, lymphangiomatosis).
• Lymphatic flow dysfunction (retrograde chyle flow into the lung).
• Lymphatic obstruction (due to malignancy).
• Direct involvement (malignant or infectious lymphadenitis, penetrating or blunt trauma, surgery).
• Excessive chyle production (mainly from vascular malformations and lymphatic masses), leading to rupture or leakage of lymphatic vessels.
• Chyle passage through the diaphragm from abdominal or retroperitoneal accumulations.

Clinical manifestations

The most common symptom in patients with chylothorax is dyspnea, particularly in those with high daily chyle output. Other findings include fatigue and weight loss, which result from nutrient loss due to continuous leakage. Fever and chest pain are uncommon, partly because accumulated lymphatic fluid in the pleural space does not trigger an inflammatory response, and the risk of infection is limited by the bacteriostatic effect of immunoglobulins within the fluid. However, due to the chronic immunosuppression seen in these patients, the risk of infections is increased.

Non-traumatic chylothorax tends to present with a more insidious onset, whereas traumatic chylothorax can develop immediately when volumes exceed 500 mL, or within a few days if the output is lower.

In post-surgical patients with slow chyle leakage, fluid accumulation may begin after the resumption of oral nutrition. It is important to recognize that a post-pneumonectomy cavity that rapidly fills with fluid, causing contralateral mediastinal shift, should raise suspicion for chylothorax, as normal post-surgical filling usually occurs over four weeks.

Chylothorax may also be accompanied by other signs and symptoms related to the underlying pathological process.

Diagnosis

The diagnosis of chylothorax requires an analysis of pleural fluid obtained via thoracentesis or from an existing drainage tube. Measurement of triglycerides and cholesterol levels is essential, along with the usual parameters assessed according to Light’s criteria. In unclear cases, lipoprotein electrophoresis should be performed to identify chylomicrons. A diagnostic algorithm is provided for reference (Figure 4).

Management

Currently, there is no standardized treatment for chylothorax, as most clinical evidence is based on non-randomized studies, mainly in postoperative patients, and due to the variety of factors influencing disease progression.

As a general rule, management depends on the daily chyle output, categorizing patients as:

• Low-flow chylothorax (<1,000 mL/day)
• High-flow chylothorax (>1,000 mL/day) (Fig. 5)

Taking this into account, we will describe the different therapeutic options available.

Dietary modifications can reduce chyle production and promote healing, sometimes eliminating the need for treatment. An MCT(medium-chain triglycerides)-rich diet is helpful because these fats are absorbed through the portal vein to the liver without passing through the thoracic duct.

Medications like somatostatin and its analogs (octreotide) inhibit secretions and cause constriction of lymphatic vessels, reducing lymph production and circulation. Alpha-adrenergic agents such as etilefrine and midodrine have also been reported to have similar effects.

However, conservative treatment is not sufficient in cases of recurrent or massive chylothorax, and in traumatic chylothorax, definitive treatment is required to close the injury.

The primary aim of pleural drainage is the alleviation of respiratory symptoms through the placement of a chest tube or the use of thoracentesis. In the patient who is postoperative, imaging is important in evaluating the condition. Continuous drainage versus intermittent thoracentesis is chosen according to the rate of fluid accumulation, the level of symptoms, and the progression of the process of healing.

In a desperate move, in the patient in whom conservative treatment is not succeeding and who is not a candidate and is not responsive to definitive treatment, chemical pleurodesis with a chest tube is a consideration.

Surgical management involves ligation of the thoracic duct or ligation of the tributaries and pleural space control. It is accompanied with a low-fat diet and, in a subset, with the addition of somatostatin analogues. The least invasive techniques are preferably utilized in the surgery, e.g., video-assisted thoracoscopic surgery (VATS) or robot-assisted surgery, but in a few situations, thoracotomy is necessary.

Thoracic duct ligation is also a treatment with a success rate of more than 90% in postoperative chylothorax but is not very useful in non-surgical states.

For refractory situations, pleuroperitoneal shunts or pleurovenous shunts could be considered. These treatments are not without risks, however, such as infection or shunt dysfunction.

Percutaneous lymphatic interventions include visualization of the dilated retroperitoneal lymphatic channels or the cisterna chyli using lymphangiography, cannulation of the thoracic duct, and embolization at the point of leakage.

Traditional pedal lymphangiography is largely substituted with the use of the intranodal approach, with entry through the inguinal nodes. In the absence of visualization of the cisterna chyli, the thoracic duct can be approached in the neck under ultrasonographic as well as under the control of the fluoroscope or retrograde transvenous entry through the brachial or the basilic vein.

The clinical effectiveness of these procedures is approximately 60%, which is lower compared with surgery but, with their low morbidity and being minimally invasive, they are a safe and effective modality in the treatment of chylothorax.

Management of traumatic chylothorax

This is shown in Figure 5.

Management of nontraumatic chylothorax

This is shown in Figure 6.

We present two representative cases; Case 1 is shown in Figure 7, and Case 2 is shown in Figure 8.