Theresa W. Fossum, DVM, MS, PhD, DACVS
Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, TX, USA
Fluid may accumulate in the pleural space because of decreased oncotic pressure (i.e., pure transudate caused by hypoalbuminemia [serum albumin concentration usually less than 1.6 g/dl]), increased hydrostatic pressure (i.e., modified transudate caused by cardiac disease, diaphragmatic hernia, tumor, lung lobe torsion), increased permeability (i.e., infection, tumor lung lobe torsion), or hemorrhage (i.e., coagulopathy, tumor, trauma).
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. Chylothorax has been reported in a cat after ligation of the left brachycephalic vein. 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/associations of chylothorax include anterior mediastinal masses (mediastinal lymphoma, thymoma), heart disease (cardiomyopathy, pericardial effusion, heartworm infection, foreign objects, tetralogy of Fallot, tricuspid dysplasia, double-chambered right ventricle, or cor triatriatum dexter), fungal granulomas, venous thrombi, peritoneopericardial diaphragmatic hernias causing intrapericardial compression of the heart, 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.
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. Contrast enhanced CT lymphangiography may be beneficial in identifying the TD of dogs; however, its utility remains to be proven in cats. Injection of 60 mg/kg of iohexol into the popliteal lymph node enabled quality CT without extranodal leakage. CT lymphography appeared to provide better spatial resolution without superimposition of surrounding tissue compared to fluoroscopy. Injection of contrast into a mesenteric lymph node may be done percutaneously with ultrasound guidance, via a small paracostal incision, or with laparoscopy.
If an underlying disease is diagnosed, it should be treated and the chylous effusion managed by intermittent thoracocentesis. 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. Resolution of chylothorax associated with ligation of the left brachycephalic vein with medical management (rutin, low fat diet) has been reported in a cat. 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 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-life. 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 cause 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 µg/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 intervention is warranted in animals that do not have underlying disease and in which medical management has become impractical or is ineffective. Surgical options include thoracic duct ligation plus pericardectomy (with or without mesenteric lymphangiography), passive pleuroperitoneal shunting, active pleuroperitoneal or pleurovenous shunting, pericardiectomy, omental drainage, and pleurodesis. Only thoracic duct ligation, pericardiectomy, mesenteric lymphangiography, and active pleuroperitoneal shunting are recommended by the author and will be discussed during the lecture.
The route, by which the thoracic cavity becomes infected, usually is not evident (i.e., hematogenous spread; migrating foreign objects such as plant awns; penetrating wounds, particularly bite wounds; extension from diskospondylitis; extension from pneumonia; pulmonary neoplasia or abscessation; pulmonary or thoracic wall trauma; esophageal perforation; and postoperative infection). Immunosuppressive diseases (e.g., feline leukemia virus [FeLV] and feline immunodeficiency virus [FIV]) should be excluded in animals with pyothorax, but there is no evidence that development of pyothorax requires debilitation or an increased susceptibility to infection. Aspirations of oral flora may be a common mechanism of pleural space infection in cats, whereas thoracic puncture (e.g., bite wounds) may be a less common cause of pyothorax than previously believed. Direct extension from pulmonary disease may be a cause in many animals.
A number of organisms often are cultured from animals with pyothorax; however, there is a high incidence of obligate anaerobes as sole pathogens. Obligate anaerobic infections or gram-positive filamentous organisms (i.e., Nocardia and Actinomyces spp.) frequently are cultured from dogs with pyothorax; obligate anaerobes and/or Pasteurella spp. are the most common feline isolates.
There is no breed predisposition, and pyothorax may occur in animals of any age. It has been widely held that young, male cats that fight and receive chest wounds are at increased risk; however, more recently parapneumonic spread of infection after colonization and invasion of lung tissue by oropharyngeal flora is thought to be the most frequent cause of feline pyothorax. Cats from multi-cat households appear to be at increased risk. Adult, large-breed dogs (particularly hunting dogs) may be more commonly affected because they often inhale plant foreign material and suffer penetrating thoracic wounds.
A delay of several weeks between the trauma that induced the pyothorax and the onset of clinical signs is not uncommon. Most animals are presented for evaluation of respiratory distress or anorexia or both. Coughing is a common presenting complaint in cats with pyothorax.
Physical Examination Findings
Affected animals usually have a restrictive respiratory pattern (i.e., rapid, shallow respirations), and many are febrile. Additional findings in patients with pyothorax may include depression, anorexia, weight loss, dehydration, muffled heart and lung sounds, and pale mucous membranes. The chest wall may seem incompressible in cats with thoracic effusion.
Thoracic radiographs usually reveal pleural effusion. The cause of the pyothorax is seldom apparent radiographically; however, increased opacity in the thoracic cavity after thoracentesis may indicate an abscess or foreign body. Consolidated lung lobes that do not reexpand after fluid removal may indicate fibrosing pleuritis or lung lobe torsion.
Although the cause of the effusion often is not discernible, attempts should be made to find and, if possible, correct underlying diseases. Management of these animals needs to be aggressive. After diagnosis, placement of a chest drain should be considered. If available, continuous suction devices can be used; however, most animals can be managed with intermittent aspiration. Lavage should be performed two or three times daily. Isotonic fluid, such as saline or lactated Ringer's solution (warmed to room temperature), should be used at a dosage of 20 ml/kg of body weight. The fluid remains in the thoracic cavity for 1 hour and is then removed. Addition of antibiotics to the lavage fluid offers no advantage over the use of appropriate systemic antibiotics. If antibiotics are used in the lavage fluid, the systemic dose should be reduced to minimize toxicity. The use of proteolytic enzymes is controversial and is no longer recommended by most authors. However, the addition of heparin (1500 units/100 ml of lavage solution or administering 100 U/kg SC TID) appears beneficial. Lavage may be required for 5 to 7 days. Systemic antibiotic therapy should be based on the results of microbial culture and sensitivity testing.