Chylothorax and immune mediated pleural effusion - part I and II

Chylothorax

In most animals, abnormal flow or pressures in the thoracic duct (TD) are thought to lead to exudation of chyle from intact but dilated thoracic lymphatic vessels (a condition known as thoracic lymphangiectasia). These dilated lymphatic vessels may form in response to increased lymphatic flow (caused by increased hepatic lymph formation), decreased lymphatic drainage into the venous system as a result of high venous pressures, or both factors acting simultaneously to increase lymph flow and reduce drainage. Any disease or process that increases systemic venous pressures (i.e., right heart failure, mediastinal neoplasia, cranial vena cava thrombi, or granulomas) may cause chylothorax. Trauma is an uncommonly recognized cause of chylothorax in dogs and cats because the thoracic duct heals rapidly after injury, and the effusion resolves within 1 to 2 weeks without treatment.

Possible causes of chylothorax include anterior mediastinal masses (mediastinal lymphoma, thymoma), heart disease (cardiomyopathy, pericardial effusion, heartworm infection, foreign objects, tetralogy of Fallot, tricuspid dysplasia, or cor triatriatum dexter), fungal granulomas, venous thrombi, and congenital abnormalities of the thoracic duct. It may occur in association with diffuse lymphatic abnormalities, including intestinal lymphangiectasia and generalized lymphangiectasia with subcutaneous chyle leakage. The underlying etiology is undetermined in most animals (idiopathic chylothorax) despite extensive diagnostic workups. Because the treatment of this disease varies considerably depending on the underlying etiology, it is imperative that clinicians identify concurrent disease processes before instituting definitive therapy.

Diagnosis

Signalment

Any breed of dog or cat may be affected; however, a breed predisposition has been suspected in the Afghan hound for a number of years. Recently, it has been suggested that the Shiba Inu breed may also be predisposed to this disease. Among cats, Oriental breeds (i.e., Siamese and Himalayan) appear to have an increased prevalence. Chylothorax may affect animals of any age; however older cats may be more likely than young cats to develop it. This finding was believed to indicate an association between chylothorax and neoplasia. Afghan hounds appear to develop this disease in middle age, but affected Shiba Inus have been less than 1 year old. A gender predisposition has not been identified.

History

Coughing often is the first (and occasionally the only) abnormality until the animal becomes dyspneic. Many owners report that coughing began months before presenting the animal for care; therefore, animals that cough and do not respond to standard treatment of nonspecific respiratory problems should be evaluated for chylothorax. Coughing may be due to irritation caused by the effusion or may be related to the underlying disease process (i.e., cardiomyopathy, thoracic neoplasia).

Diagnostic imaging

CT lymphangiography may be able to quantify branches of the thoracic duct more accurately than standard radiographic lymphangiography; however, I have not found this technique to be clinically useful.

Laboratory findings

Fluid recovered by thoracentesis should be placed in an EDTA tube for cytologic examination. Placing the fluid in an EDTA tube rather than a clot tube allows cell counts to be performed. Although chylous effusions routinely are classified as exudates, the physical characteristics of the fluid may be consistent with a modified transudate. The color varies depending on the dietary fat content and the presence of concurrent hemorrhage. The protein content is variable and often inaccurate because of interference with the refractive index by the high lipid content of the fluid. The total nucleated cell count usually is below 10,000/ul and consists primarily of small lymphocytes or neutrophils with lesser numbers of lipid-laden macrophages.

Chronic chylous effusions may contain low numbers of small lymphocytes due to the body’s inability to compensate for continued lymphocyte loss. Nondegenerative neutrophils may predominate with prolonged loss of lymphocytes or if multiple therapeutic thoracenteses have induced inflammation. Degenerative neutrophils and sepsis are uncommon findings because of the bacteriostatic effect of fatty acids but can occur iatrogenically as a result of repeated aspiration. To help determine if a pleural effusion is truly chylous, several tests can be performed, including comparison of fluid and serum triglyceride levels; Sudan III staining for lipid droplets; and the ether clearance test. The most diagnostic test is comparison of serum and fluid triglyceride levels. Chylous effusions have a higher triglyceride concentration than simultaneously collected serum.

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