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.

Clinical Presentation

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).

Physical Examination Findings

Most animals with chylothorax have a normal body temperature unless they are extremely excited or severely depressed. Additional findings may include muffled heart sounds, depression, anorexia, weight loss, pale mucous membranes, arrhythmias, murmurs, and pericardial effusion.

Diagnostic Imaging

The radiographic signs of pleural effusion are provided on. Animals that have collapsed lung lobes that do not appear to reexpand after removal of chyle or other pleural fluid should be suspected of having underlying pulmonary parenchymal or pleural disease, such as fibrosing pleuritis. Although the etiology of the fibrosis is unknown, it apparently can occur subsequent to any prolonged exudative or blood-stained effusion. Diagnosis of fibrosing pleuritis is difficult. The atelectatic lobes may be confused with metastatic or primary pulmonary neoplasia, lung lobe torsion, or hilar lymphadenopathy. Radiographic evidence of pulmonary parenchyma that fails to reexpand after removal of pleural fluid should be considered possible evidence of atelectasis with associated fibrosis. Fibrosing pleuritis should also be considered in animals with persistent dyspnea in the face of minimal pleural fluid.

CT lymphangiography may be able to quantify branches of the thoracic duct more accurately than standard radiographic lymphangiography. In a recent study, CT lymphangiography was performed by percutaneously injecting 1 to 2 ml of nonionic contrast material into the mesenteric lymph nodes of 4 dogs with chylothorax using ultrasound guidance (Johnson et al, 2005). Helical thoracic CT images were acquired before and after injection of the contrast media. The technique documented the location and character of the TD and its tributary lymphatics and may prove useful for surgical planning in animals with chylothorax.

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. The most diagnostic test is comparison of serum and fluid triglyceride levels. Chylous effusions have a higher triglyceride concentration than simultaneously collected serum.

Medical Management

If an underlying disease is diagnosed, it should be treated and the chylous effusion managed by intermittent thoracentesis. If the underlying disease is effectively treated, the effusion often resolves; however, complete resolution may take several months. Surgical intervention should be considered only in animals with idiopathic chylothorax or those that do not respond to medical management. Chest tubes should be placed only in animals suspected of having traumatic chylothorax (very rare) with rapid fluid accumulation, or occasionally after surgery. Electrolytes should be monitored; hyponatremia and hyperkalemia can occur in dogs with chylothorax undergoing multiple thoracentesis. A low-fat diet may reduce the amount of fat in the effusion, which may improve the animal's ability to resorb fluid from the thoracic cavity. Benzopyrone drugs have been used for the treatment of lymphedema in human beings for years. Whether these drugs might be effective in reducing pleural effusion in animals with chylothorax is unknown; however, preliminary findings suggest that some animals treated with rutin (50-100 mg/kg, PO. TID) have complete resolution of effusion 2 months after initiation of therapy. Whether the effusion resolves spontaneously in these animals or is associated with the drug therapy is unknown.

Somatostatin is a naturally occurring substance that has an extremely short half-live. It inhibits gastric, pancreatic, and biliary secretions (i.e., glucagon, insulin, gastric acid, amylase, lipase, and trypsin) and prolongs gastrointestinal transit time, decreases jejunal secretion, and stimulates gastrointestinal water absorption. In recent years analogues of somatostatin have been used to successfully treat chylothorax in humans with traumatic or postoperative chylothorax. In these patients, reduced gastrointestinal secretions may aid healing of the TD by decreasing TD lymphatic flows. It has also been reported to result in early decreased drainage and early fistula closure in dogs with experimental transection of the TD. The mechanism by which non-traumatic chylothorax may benefit from this treatment is unclear; however, resolution of pleural fluid (chyle and postoperative serosanguineous effusion) in both dogs and cats has occurred after administration of octreotide. Octreotide (Sandostatin; 10 mcg/kg subcutaneously three times a day for 2 to 3 weeks) is a synthetic analogue of somatostatin that has a prolonged half-life and minimal side effects. Soft stools that resolve after withdrawal of the drug may occur. Prolonged treatment should be discouraged because people treated for longer than 4 weeks are at risk for gallstones.

Surgical Treatment

Surgical intervention is warranted in animals that do not have underlying disease and in which medical management has become impractical or is ineffective. Properly performed, TD ligation results in over 80% of dogs and cats resolving their effusion (Fossum et al, 2004). Formation of a nonchylous effusion (from pulmonary lymphatics) may occur in some animals after surgery. Mesenteric lymphangiography may be particularly difficult to perform in cats. While mesenteric lymphangiography is not essential, catheterization of a mesenteric lymphatic and injection of methylene blue makes identification of the TD and its branches much easier. Thoracic duct ligation has been performed using thoracoscopy (Radlinsky et al, 2002).

Many animals with chylothorax have either a thickened pericardium or tissue overlying the pericardium is thick. When the pericardium or overlying tissues are thickened or abnormal in animals with derangements in lymphatic flow (be it chylothorax or continued serosanguineous flow after TD ligation), pericardiectomy may act to lower right-sided venous pressures. Therefore, pericardiectomy is recommended in conjunction with TD ligation.

Surgical Techniques

Thoracic Duct Ligation

Perform an intercostal thoracotomy (right side for dogs, left side for cats) at the eighth, ninth, or tenth intercostal space or make an incision in the diaphragm. Locate the thoracic duct and use hemostatic clips and/or silk suture (2-0 or 3-0) to ligate it. Visualization of the thoracic duct is greatly aided by injecting methylene blue into the lymphatic catheter. Perform a subtotal pericardectomy by reaching forward under the rib cage.

Note: Wear magnification and use a headlight to help visualize the TD and its small branches!

References

1.  Fossum TW, Mertens MM, Miller MW, et al: Thoracic duct ligation and pericardectomy for treatment of idiopathic chylothorax, J Vet Intern Med 18: 307, 2004.

2.  Radlinsky MG, Mason DE, Biller DS, Olsen D: Thoracoscopic visualization and ligation of the thoracic duct in dogs, Vet Surg 31, 138, 2002.