Chylous Pleural Effusion
The thoracic duct carries chyle, which is a mixture of ingested triglycerides, chylomicrons, and cholesterol, from the intestine to the venous system. It follows the course of the aorta or azygos vein from the retroperitoneal space to the upper mediastinum. The thoracic duct may be damaged by trauma or malignancy, resulting in leakage of chyle into the pleural space.
Identifying chylothorax is important in determining the etiology of pleural effusion. Chylous effusions are typically described as exudative lymphocytic pleural effusions with a milky appearance. The appearance of a white ring at the top of pleural fluid after storage at 4 degrees C for 12 hours confirms the presence of chylomicrons. The traditional biochemical criterion for chylothorax is a pleural fluid triglyceride level greater than 110 mg/dL and cholesterol less than 200 mg/dL (Mayo Clin Proc 1980;55:700-4).
Twenty nine years later, investigators from the Mayo Clinic published a more recent biochemical analysis of pleural fluid obtained from 74 adults with a diagnosis of chylothorax (Mayo Clin Proc. Feb 2009;84(2)129-33). Most chylous effusions (86%) were classified as exudative effusions. Gross appearance of the fluid was not a sensitive diagnostic criterion. Only 44% of cases had the classic milky appearance attributed to chylothorax.
This study validated the original criterion. The mean+/-SD triglyceride value for transudative chylothoraces was 192+/-105 with a median of 195 mg/dL while the mean+/-SD triglyceride value for exudative chylothoraces was 855+/-816 with a median of 601 mg/dL. However, 14% of patients had triglyceride values less than 110 mg/dL, primarily due to perioperative fasting and malnourishment.
In cases of suspected chylous effusion with triglycerides less than 110 mg/dL, the specimen can be sent to a reference laboratory for lipoprotein electrophoresis. The presence of chlyomicrons in the fluid supports the diagnosis of chylothorax.
Pseudochylothorax is the accumulation of cholesterol crystals in a chronic effusion. It may be seen in patients with rheumatoid pleurisy or tuberculosis. Pleural fluid cholesterol >200 mg/dL and triglyceride <50 mg/dL is diagnostic of pseudochylothorax.
