Chyle is a mixture of lymph and chylomicrons from the intestinal lymphatics that contains 60-70% of all ingested fat. It is connected to the venous circulation by way of the mesenteric lymphatics that empty into a dilated lymphatic collecting channel at the base of the diaphragm known as cisterna chili. The cisterna chyli continues forward as the thoracic duct and is located between the aorta and the azygous vein on the left side of the dorsal thorax. The thoracic duct empties into the left external jugular vein of the neck or the jugulosubclavian angle.
A lesion or obstruction along this pathway will result in the collection of chyle in the chest and sometimes abdomen as well. Trauma, neoplasia, malformation, lymphatic obstruction, congestive heart failure are known to cause chylous effusion. Many times though, no cause can be identified and is termed idiopathic chylothorax. This effusion can compromise respiration and cause breathing difficulties. Fibrosing pleuritis or constrictive pleuritis is a frequent sequela to chylothorax. The pleura becomes fibrosed and thickened causing atelectasis and preventing normal lung expansion.

Heart disease, heartworm disease, congenital, diaphragmatic hernia, idiopathic, lung lobe torsion, lymphangiectasia, neoplasia, pancreatitis, obstruction to thoracic duct flow and traumatic rupture of thoracic duct.

Clinical findings:
Slow heart rate, heart sounds muffled, abdominal breathing, open mouth breathing, rapid shallow breaths, restrictive breathing pattern, cachexia, depression, dehydration, hypothermia, malaise
chest percussion dull, pulmonary sounds decreased, chest trauma, exercise intolerant or reluctant to move, onset gradual, chronic pale mucous membranes

Evidence of pleural effusion from thoracic radiographs or ultransonography. Thoracocentesis demonstrates chyle from the lymphatic system in the thorax. The distinguishing features are an opaque milky fluid, of moderate protein content, and moderate cellularity. Chylous and pseudochylous fluids are difficult to separate. Chylous fluids are high in triglycerides (cholesterol:triglyceride ratio less than 1), are positive for orange staining chylomicrons when stained with Sudan III, and clear when alkalinized and mixed with ether.


Conservative treatment is recommended initially for chylothorax not due to malignancy or infection. Pleural effusion should be removed from the chest by needle aspiration for diagnostic purposes and to relieve extreme dyspnea. An over-the-needle intravenous catheter 18-16 gauge is placed ventrally as the radiographs may indicate. A chest tube should be placed on one or both sides of the chest if adequate drainage is not possible from just one side. The chest tube is used for removing accumulating fluid and flushing with sterile saline solution. Heparin 150 U/L can be added to the solutions to prevent clot formation obstructing the tube. Chest lavage 3 times daily is continued until the fluid is clear and very little can be recovered (2-3 ml/kg/d). This usually takes from 5-10 days of therapy. A diet low in fat and high in carbohydrates is the initial therapeutic modality. Medium chain triglycerides (MCT oil) 1-2 ml/kg/d can supply the needed lipids and by pass the intestinal lymphatics. Hills R/D diet is a low fat diet.
Surgical intervention is recommended if conservative therapy fails to resolve the cylothorax after 14 days. Thoracic duct ligation combined with a pericardectomy is considered the definative  treatment for chylothorax and is highly effective. Bilateral chest drains are left in place and flushed for 3-7 days after surgery.  Rutin has been used to reduce the inflammation and fibrosis that accompanies chylothorax. Rutin is a benzopyrone compound extracted from the fruit of the Brazilian Fava D'Anta (Dimorphandra) tree. No significant toxicity of rutin has been reported but there are no studies confirming its benefits. Suggested dose is 500 mg/cat orally twice daily.