Chylothorax is a collection of chyle in the pleural space. Chylothorax remains a complicated condition that is poorly understood and difficult to treat. In most animals, abnormal flow pressures within the thoracic duct are thought to lead to exudation of chyle from intact, but dilated, thoracic lymphatic vessels. Increased lymphatic flow, decreased lymphatic drainage, or both may lead to dilation of the thoracic lymphatic vessels and subsequent exudation of chyle. Possible causes of chylothorax include anterior mediastinal masses (mediastinal lymphosarcoma, thymoma), heart disease (cardiomyopathy, heartworm disease, congenital cardiac diseases, etc.), fungal granulomas, congenital abnormalities of the thoracic duct, and diffuse lymphatic abnormalities. Trauma is an uncommonly recognized cause of chylothorax in dogs and cats since the thoracic duct heals rapidly following injury and within a few weeks the effusion resolves without treatment. Despite extensive diagnostic workups, in the majority of the animals, the underlying etiology is undetermined (idiopathic chylothorax).
Any breed dog or cat may be affected. A breed predisposition has been suspected in the Afghan hound and Shiba Inu. Among cats, Oriental breeds such as Siamese and Himalayan appear to have an increased prevalence. Coughing is often the first abnormality noted by owners until the animal becomes dyspneic. Other clinical signs are signs consistent with animals with pleural effusion. These include dyspnea and muffled lung and heart sounds. Chylothorax is diagnosed on the basis of aspirating milky-appearing fluid form the pleural space, which contains chylomicrons, as determined on the basis of microscopic examination, and has a cholesterol:triglyceride ratio of <1:1 or triglyceride concentration that is higher than the serum triglyceride concentration. The physical characteristics of the fluid is consistent with a modified transudate. The total nucleated cell count is usually less than 10,000 and consists primarily of small lymphocytes or neutrophils. The protein content is variable and may be inaccurate due to interference of the refractive index by the high lipid content of the fluid.
Diagnostic evaluation of dogs and cats with chylothorax, using radiography, ultrasonography, fluid analysis, echocardiography, and other tests, is intended to rule out primary diseases that may cause effusion. If the animal is not overtly dyspneic, thoracic radiographs should be taken to confirm the diagnosis of pleural fluid. Delay thoracic radiographs until after thoracocentesis in animals with pleural effusion that are severely dyspneic. Radiographic signs associated with pleural effusion include undefined cardiac silhouette, interlobar fissure lines, rounding of lung margins, and separation of lung borders from the thoracic wall. If the animal is stable and ultrasonography is available, ultrasonography should be performed fluid removal because the fluid acts as an acoustic window enhancing visualization of thoracic structures. Ultrasonography is used to evaluate cardiac function, valvular lesions and function, congenital cardiac abnormalities, the presence of pericardial effusion, pleural effusion, and mediastinal masses.
If an underlying disease is diagnosed it should be treated and the chylous effusion managed by intermittent thoracocentesis. However, in a large percentage of animals, a primary cause is not identified. Many treatments have been recommended for animals with idiopathic chylothorax. Non-surgical methods include pleural drainage, dietary management and vitamin supplementation, and pharmaceutical control. Palliative drainage of the thorax is important for alleviating signs of respiratory distress. However, chest drainage alone only allows for temporary resolution of effusion. Chyle contains many beneficial substances such as protein, electrolytes, and fat-soluble vitamins. Long-term drainage of chyle from the thorax may lead to dehydration, hypoproteinemia, malnutrition, electrolyte disturbances, and possible immune deficiency. Long term medical management of chylothorax may lead to chronic inflammation causing restrictive pleuritis that can be a serious, life-threatening sequelae of chronic chylothorax. Dietary management includes the use of a low-fat diet to decrease flow through the thoracic duct, which in turn reduces the volume of effusion. However, studies have shown that the use of low fat-diet reduces the lipid content of the effusion but does not seem to effectively decrease the volume of effusion. By decreasing the amount of fat in the effusion, it may improve the animal’s ability to resorb fluid from the thoracic cavity. Supplementation of oils high in medium chain triglycerides (MCT) has been recommended but its benefit is uncertain. MCT reportedly will bypass intestinal lymphatic vessels and be absorbed directly into the portal system. But current studies have shown that MCT-containing oils is absorbed through intestinal lymphatic vessels and thus, may be of little benefit for animals with chylothorax. With regards to pharmaceutical control, benzopyrone drugs have been used for the treatment of lymphedema in humans for years. Reportedly, these drugs act by stimulating macrophages to break down proteins in lymph which promotes reabsorption. Whether there might be effective in decreasing pleural effusion in animals with chylothorax is unknown and requires further study.
Surgical intervention is often attempted when medical treatment is not effective or when animals have severe respiratory complications. Surgical methods include thoracic duct ligation, pleuroperitoneal or pleurovenous shunts, pleurodesis (not recommended), and omentalization of the thoracic cavity. Thoracic duct ligation has long been used as a treatment for dogs and cats with chylothorax. This procedure is intended to stop flow of chyle into the pleural space by diverting lymphatic flow to alternate lymphaticovenous pathways. The thoracic duct is approached via a right sided thoractomy in dogs and via left sided thoracotomy in cats. The dust is usually ligated in the caudal aspect of the mediastinum near the diaphragm. An alternate method of occlusion of the thoracic duct is via embolization. Isobutyl 2-cyanoacrylate can be injected through a catheter placed in a mesenteric lymphatic vessel which leads to complete obstruction of the thoracic duct. Results of studies have shown that thoracic duct ligation in successful in 20-60% of dogs and cats with chylothorax. Recurrence is possible, therefore, results that initially appear favorable should be viewed cautiously, especially for animals that have not yet had long-term follow-up monitoring. Pleuroperitoneal or pleurovenous shunts are devices that allow movement of fluid from the thoracic to abdominal cavity where it can be absorbed by serosal surfaces and omentum, or to the venous circulatory system. Active shunts or drains are those that remove fluid from a cavity using negative pressure, as opposed to those relying on gravity to provide passive drainage. Potential complications include clotting of the tubing, sepsis, and adherence of the liver or omentum to the implant. Omentalization of the thoracic cavity uses the physiological properties of omentum to control the pleural effusion. The omentum has a large surface area with absorptive lymph-draining capability and early case reports on transplanting the omentum into the thoracic cavity in small animals with chylothorax have shown some promise but has only been used in a limited number of cases.
Chylothorax remains a complicated condition that is poorly understood and difficult to treat. Although results of recent studies have improved our understanding of the pathophysiologic mechanism of the disease, treatment failure remains common. Additional research is needed to better delineate the pathophysiologic mechanism of idiopathic chylothorax and to improve treatment results.
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Author: Michael B. Mison, DVM, Diplomate ACVS Seattle Veterinary Specialists