Chylous Effusions subclavian vein superior vena cava Pleural chylous effusions develop due to transec- tion of the duct or block- age by tumor (typically lymphoma) thoracic duct Peritoneal chylous effusions develop due to cirrhosis, or portal vein thrombosis cysterna chyli lymph nodes Chylous effusions are rich in triglyc- Chylous vs. Pseudochylous erides and chylomicrons, with normal levels of cholesterol; lymphocytes Effusions may also be found. The thoracic duct runs along the spinal column, carrying lymph from the legs, gut, and chest. It Accumulations of fluid in body cavities are broadly neutral fat using stains such as Oil Red O or Sudan III. is a fluid rich in lymphocytes and chylomicrons. grouped into the two categories of transudates and ex- Chylous effusions occur in the pleural and peritoneal Chylous effusions of the peritoneal and pleural udates based on total protein, albumin, and LDH levels cavities. Causes of chylous ascites include obstruction cavities develop when the flow is obstructed. (see page 78). Many additional parameters, such as of lymphatic drainage by cirrhosis or portal vein throm- glucose, cell count and differential, and microbiologic bosis, and rare cases of congenital lymphatic anomalies culture, are often evaluated as well. Transudates are may be the cause in children. Chylous pleural effusions usually benign and are commonly the result of conges- are usually due to thoracic duct transection by trauma Pseudochylous Effusions tive heart failure or cirrhosis with hypoalbuminemia. or surgery, or obstruction of the thoracic duct by a ma- Exudates often signify serious abnormalities such as in- lignancy, often lymphoma. fection or malignancy. Most lipid-rich effusions, particularly those from Lymph from the lower extremities, abdomen, and the pleural and peritoneal cavities, are chylous. How- thorax is returned to the circulation by the thoracic ever, there is another group of cloudy to opalescent duct system. This duct is formed by the confluence of effusions that are important to recognize as they have lymphatics draining the abdomen and lower extremi- very different origins and clinical significance. These ties just below the aorta. It then ascends along the “pseudochylous” or “chyliform” effusions are often vertebral column, receiving lymph originating in the post-inflammatory, but they can be a feature of any thorax, and terminates at the left subclavian vein, re- long-standing effusion. They are more common in ef- turning lymph to the circulation. Lymph flow is greatly fusions originating in the pericardial, joint, and bursal enhanced during the absorptive phase of digestion, spaces, and are rich in cholesterol, with low triglyc- and this lymph is rich in chylomicrons, giving it a milky eride content and no chylomicrons. Formed elements turbid appearance. On the other hand, thoracic duct present commonly include cholesterol crystals, but lip- lymph in the post-absorptive state is clear. id-laden macrophages or, rarely, malignant cells may Chylous effusions are an uncommon cause of an ex- be found as well. These effusions are not of primary Chronic long-standing effusions contain udative effusion. Chylous effusions are described as tur- lymphatic origin but arise from breakdown products breakdown products of cells which are rich in Pseudochylous effusions are rich in cho- bid or milky, but if composed solely of post-absorptive of any chronic effusion. Thus, clinical considerations cholesterol (a major component of cell mem- lesterol, with normal levels of triglycerides lymph, may be clear. They are frequently cellular and include rheumatoid arthritis or other collagen vascular branes). and no chylomicrons; lipid-laden macro- are rich in triglycerides, with a normal cholesterol level. diseases, tuberculosis, or a nonspecific chronic bursitis, phages may be seen. Chylomicrons can be identified if a smear is stained for such as “housemaid’s knee.” 268 Crystals Crystals 269.