Chylothorax results from the leakage of chyle from the thoracic duct or one of its lymphatic branches into the pleural space. Common causes include neoplasm and iatrogenic injury. Life-threatening metabolic derangements may occur if chylothorax is not recognized and treated promptly. Numerous maneuvers can be undertaken as part of a conservative approach to treating chylothorax, but surgical intervention is usually the most effective method of achieving definitive results.

The thoracic duct was first described in humans by Veslingus in 1634. The first successful thoracic duct ligation was performed by Lampson in 1948 for a traumatic chylothorax.¹ Before this time, chylothorax was treated conservatively and associated with a mortality rate of almost 50%. An accurate understanding of the anatomic course of the thoracic duct and common sites of variant anatomy can help to minimize iatrogenic chylothorax and ensure successful surgical intervention through direct repair or mass ligation.

The thoracic duct is a continuation of the cisterna chyli as it passes from the abdomen into the thorax (Fig. 133-1). There are many variations in the course and number of divisions of the thoracic duct. In most individuals, the thoracic duct starts at the cisterna chyli and enters the chest through the aortic hiatus, coursing behind the esophagus between the aorta and azygos vein. The duct runs cephalad in the right hemithorax along the spinal column. At the level of the fifth or sixth thoracic vertebra, the duct crosses to the left hemithorax and continues cephalad along the left side of the esophagus into the superior mediastinum behind the aortic arch and posterior to the left subclavian artery. The duct terminates at the angle formed by the left internal jugular and left subclavian veins, where it drains into the venous system. Of surgical importance is the consistency in location and paucity of branching of the thoracic duct between the cisterna chyli and the level of the eighth vertebral body in the lower right hemithorax.

Chyle is a milky fluid that consists primarily of proteins, triglycerides, and lymphocytes. Under normal conditions, the thoracic duct transports up to 4 L of chyle per day. Factors that influence flow rate include diet, drug intake, intestinal function, and physical mobility. The flow of chyle is maintained by changes in intraabdominal and intrathoracic pressure along with intrinsic alterations in the muscular tone of the thoracic duct.² Valves irregularly spaced throughout the system assist in unidirectional circulation. Although valves may be present throughout the thoracic duct, there is one valve constant in location that lies within 1 cm of the lymphaticovenous junction (see Fig. 133-1, inset A). Drainage of chyle into the rapidly flowing venous system can be explained by the Bernoulli effect, the law governing the behavior of a fluid as it passes from a region of low pressure to a region of high pressure.

Most of the fluid that composes chyle comes from the gastrointestinal tract and liver. It contains high concentrations of digestive products as well as lymphocytes and immunoglobulins.² Between 60% and 70% of dietary fat is absorbed via the lymphatic system in the form of chylomicrons. Short- and medium-chain fatty acids (i.e., <10 carbon atoms in length) are taken in directly via the portal system. The absorption of dietary fat gives chyle its milky-white color. This explains the high concentration of triglycerides in chyle relative to plasma. Lymphocytes make up 95% of the cellular component of chyle (90% are T-lymphocytes), whereas the concentrations of electrolytes, antibodies, and enzymes approximate those of plasma.³ Loss of these constituents as a result of injury to the thoracic duct can be quite debilitating from both a nutritional and an immunologic standpoint.

Congenital chylothorax is the most common cause of pleural effusion in the newborn. The condition may result from lymphovascular malformations, complete thoracic duct atresia, or trauma during birth. In the adult, the principal causes of chylothorax are trauma or iatrogenic injury, followed by obstruction or disruption secondary to a neoplastic process. Penetrating injuries to the neck, chest, or abdomen may result in direct ductal damage, whereas blunt trauma may create increased wall tension and distention injury as a result of crushing blows or spinal column hyperextension (Table 133-1).

Iatrogenic injuries occur four times more frequently than blunt or penetrating trauma.⁴ These injuries are more common after esophagectomy, but also may be associated with neck dissections, lung resection, aortic surgery, and removal of mediastinal tumors.⁵ Postoperative injuries include transection, resection, tangential laceration, and tears of lateral ductal branches. Injury to the thoracic duct also may occur after interventions such as left-sided central line insertion, translumbar angiography, and sclerotherapy for esophageal varices.

Malignant ductal obstruction can arise secondary to an intrinsic disease such as intraluminal involvement from neoplastic cells or from extrinsic compression. Seventy percent of malignant duct leaks occur in association with lymphoma.⁴

Chylothorax is a rare event, with exception of the 5% to 8% incidence of chylothorax associated with esophagectomy. By comparison, the incidence of chylothorax after lung resection is only about 0.5%.⁶ The diagnosis of chylothorax is rarely expected or obvious and requires a healthy index of suspicion (Table 133-2). After esophagectomy, chest tube output approaching or exceeding 1 L/day immediately should raise suspicion of a chyle leak, especially if the fluid is milky white. However, high chest tube output simply may be the result of extensive operation, postoperative oozing, or volume overload in the immediate postesophagectomy period. Also, a clear fluid may masquerade as chyle in an unfed patient. For the patient that presents with chylothorax in the nonpostoperative state, shortness of breath or an abnormal chest radiograph will be the presenting symptom or sign, and a number of clinical tests are helpful in making the diagnosis.

Pleural fluid analysis is an aid to diagnosis. A sampling of pleural fluid should be sent for Gram stain, protein and triglyceride determinations, and cell count with differential. Chylothorax is suspected immediately if the fluid is white but neutrophils and bacteria are absent on Gram stain.

The triglyceride level may be even more helpful as a diagnostic aid. A high triglyceride concentration (>110 mg/dL) has a high specificity for chyle leak, whereas concentrations of less than 50 mg/dL are less than 5% likely to be due to a chyle leak. A cholesterol:triglyceride ratio of less than 1 is diagnostic of chyle.⁷ Many consider a lymphocyte count of greater than 90% to be equally diagnostic. The presence of chylomicrons in pleural fluid is diagnostic for chyle; however, electrophoresis is necessary to establish their presence. The diagnosis can be confounded if the patient has been fasting or is on an elemental diet. In the proper clinical setting, a fatty-food bolus (e.g., heavy cream) and subsequent increase in chest tube output also can be diagnostic of chylothorax.

Lymphangiography can confirm a chylothorax as well as identify the site of the leak. This type of localization study can prove useful in planning an operative approach for repair, especially when coupled with postlymphangiogram CT imaging.⁸ Lymphangiography can demonstrate accessory ducts and the course of the main duct in over 97% of patients.⁹

A large, prolonged chyle leak can have several adverse effects. Patients are often already malnourished and/or immunosuppressed as a result of prior surgery or illness. In the presence of a chyle leak, the nutritional and immunologic status of the patient is further compromised. Not only is there a significant loss of fluid and electrolytes (at times, >1.5 L/day), but also a significant number of T-lymphocytes can be lost through the drainage of chyle fluid, resulting in lymphopenia and immunosuppression. Finally, there is depletion of nutritional stores through loss of fats and protein. Accumulation of chyle in the pleural cavity can quickly cause compression of underlying lung, respiratory compromise, and even tension chylothorax. The first priority in managing these patients is adequate drainage of chyle from the thorax, reexpansion of the lung, and assurance of pleural apposition. This is usually best accomplished through placement of a posteriorly directed chest tube. Replacement of IV fluids and electrolytes is initiated. Drainage of chyle should drop significantly with cessation of oral intake; total parenteral nutrition should be instituted immediately if this route is chosen.