CHAPTER 29 Chylothorax
Chylothorax is the collection of an excessive amount of chyle in the pleural space. The continued loss of chyle—which can amount to 2 to 3 L/day after a thoracic duct injury1—leads to significant depletion of fats (up to 70% of dietary intake), proteins, and T lymphocytes.2 As a consequence, marked disturbances in the immunologic and nutritional profile occur in these patients, along with a mass effect created by dislocation of intrathoracic structures by the enlarging fluid collection. Indeed, the flow rate of chyle within the thoracic duct can be as high as 110 mL/hr.1 If left untreated, chylothorax may yield an overall mortality rate of 50%.3
CLASSIFICATION AND ETIOLOGY
The term traumatic is often used to include both iatrogenic and postinjury chylothoraces,2 which usually represent the most common causes of significant chyle accumulation in the chest. Neoplastic etiology can account for up to 20% of chylothoraces.2 In a recent report from the Mayo Clinic, the etiology was surgery or trauma in 50% of the patients, medical conditions in 44%, and unknown in 6%.4 This unusual distribution compared with the commonly reported series5 was explained by the high volume of surgical cases being performed each year at that institution.
In the pediatric group, congenital chylothorax appears early after birth, possibly because of a combination of thoracic duct malformation and sudden elevation of venous pressure.6 Neonatal chylothorax has been reported in conjunction with several syndromes, such as Noonan’s syndrome and Down syndrome.2,6 In addition, the incidence of chylothorax after cardiothoracic procedures in children is reported to be as high as 3.8%.7 The detection of a pleural effusion in this age group should immediately arouse suspicion of chylothorax.6 Tuberculosis with significant mediastinal adenopathy may still be responsible for “spontaneous” bilateral chylothoraces in children because of the obstruction to centripetal flow.8
Excessive chyle collection in the pleural space may occur in association with benign and malignant tumors. Reportedly, almost 50% of patients with chylothorax have cancer. Of these, 70% have lymphoma.5 Conversely, chylothorax was reported in 10% of the patients with lymphangioleiomyomatosis (LAM) treated at the Mayo Clinic over a 24-year period.9
Intraoperative injuries may ensue from surgical procedures conducted in the proximity of the thoracic duct anatomic course.2,10,11 Even minor maneuvers, such as raising a pleural flap over the thoracic aorta or dividing the inferior pulmonary ligament, can cause this complication.12 In 1999, the Mayo Clinic reported that after 11,315 general thoracic surgical procedures, 47 patients (0.42%) had postoperative chylothorax.1
Chylothorax has been reported to complicate the postoperative course in 1% of patients who underwent esophagectomy,13 requiring surgical reexploration in almost 90% of those cases.3 Although it occurred in less than 1% of patients subjected to pulmonary resection, only 38% of those underwent reoperation for final treatment.1 A detailed list of possible causes of chylothorax was reported by De Meester14 in the 4th edition of this book, and it was recently modified by Nair.2
ANATOMIC CONSIDERATIONS
A certain variability in the distribution of the thoracic duct and its tributaries is not uncommon. At some point in embryogenesis, the thoracic duct is a bilateral structure,15 and it can be double or triple in up to 40% of the population.15 A single thoracic duct is found in about 65% of individuals. Chylothorax can result from leak of lymph from major collectors—the most sizable being the thoracic duct—or from multiple lymphatic channels that make up a network of tributaries consistently demonstrated by several anatomic studies.11
The centripetal flow toward the left subclavian vein is regulated by three factors16: (1) the vis a tergo created by the continuous enteral absorption of chyle constituents, which pushes the chyle from the cisterna chyli to the left subclavian vein, (2) the aspiration effect given by the negative effect of the intrathoracic pressure facilitating the cephalad flow, and (3) lymphatic vessel contractions, generated by smooth musculature to empty the duct into the subclavian vein.
PATHOPHYSIOLOGY
A chylothorax can result from a chyle leak (by direct injury or obstruction of the major lymphatic vessel) or from generalized transdiaphragmatic flow from chylous ascites.9 The distinction between idiopathic (sometimes called spontaneous) and secondary causes of chylothorax depends on the presence of an identified etiology. Secondary causes include neoplastic and inflammatory conditions, as chylothorax may result from obstruction to centripetal flow or from increased flow rate with extreme dilation of lymphatic vessels. As a consequence, lymphatic vessels are thought to preserve—at the beginning—their structural integrity at the expense of increased permeability into the pleural space.2,9 Constrictive pericarditis, superior vena cava obstruction, and mediastinal fibrosis resulting from cancer treatment can generate chylothorax according to this pathophysiologic model.5 Thus, patients with liver cirrhosis can develop spontaneous chylothorax because of a twofold or threefold increase in diameter of the thoracic duct from an unusual backflow and pressure.17
Another factor implied in the onset of chylothorax is valve competency in the lymphatic vessels, as demonstrated by the rarity of such complication after pulmonary resection and extensive mediastinal nodal dissection. Valve insufficiency–induced backflow from the thoracic duct into the areas of nodal dissection and injury of the lymphatic network may explain the chylous effusion11 after pulmonary resections with concurrent nodal dissection.
Clinically, chylothorax resulting from pleural carcinomatosis or from tubercular involvement may have a gradual onset and development. This can be explained by progressive lung trapping caused when the visceral pleura are thickened by the persistent chemical irritation of chyle components.12 In time, fibrotic visceral pleura may impose a restrictive physiology on the affected lung by significantly reducing its compliance. This scenario is more often the case with the so-called pseudochylothorax (see later).
Induced or secondary causes of chylothorax include traumatic and iatrogenic causes, which have in common the interruption of the vessel by direct injury, inadvertent division, or blunt trauma to the thoracic duct or its tributaries. Rupture of the thoracic duct may also occur after sudden hyperextension of the spine (e.g., seat belt injury),18 with vertebral fractures or dislocation, or after protracted and vigorous vomiting or coughing.6
