Chyle is delivered to the venous circulation via the thoracic duct. The presumed accumulation within the thoracic cavity is due to an obstruction or relative obstruction of the duct or its lymphaticovenous anastomosis. Physical obstruction (e.g., cranial mediastinal masses, thrombosis) or increased venous pressure can cause leakage of chyle into the pleural space because of increased hydrostatic pressure. Imbalances between thoracic duct flow and ability of lymph to cross the lymphaticovenous junction would also lead to chylous effusion. Mild increases in hydrostatic pressure have been hypothesized, but absolute pressure alterations, or altered central venous pressure, have not been documented to cause chylothorax. Indeed, surgery may be aimed at altering central venous pressure (i.e., pericardectomy, cisterna chyli ablation) in an effort to improve lymphatic flow and formation of new anastomoses. Idiopathic chylothorax is the most common form of the condition, and if lymphangiography is done, multiple, dilated, and tortuous lymphatic vessels are seen in the cranial thorax. The underlying cause of the condition is usually undetermined, but the appearance of the lymphatics resembles lymphangiectasia.

Medical management may include feeding a low-fat diet in an attempt to minimize the fat in the fluid and increase fluid absorption from the pleural space. Benzopyrones have been used to treat edema in humans and have been attempted in veterinary patients with chylothorax. Their efficacy has not been shown, but treatment will not likely be harmful. Rutin is typically dosed at 50 to 100 mg/kg orally every 8 hours. Octreotide therapy has also been anecdotally reported to result in cure in some veterinary patients and can be dosed at 10 μg/kg subcutaneously every 8 hours for 2 to 3 weeks. Side effects include loose stools and increased risk of gallstone formation with prolonged (>4 week) treatment.^7-9 Also, octreotide treatment requires careful monitoring of fluid and electrolyte balance to avoid dehydration, hyponatremia, and hyperkalemia. In addition, pathological changes such as malnutrition, inflammatory cell loss, and possible immunocompromise need to be assessed during treatment.¹⁰ Medically managed cases need to be carefully monitored for the development of fibrosing pleuritis, which may be more common in cats. The condition occurs because of an imbalance of fibrin deposition and resorption on the visceral pleura.¹¹ Animals that fail 2 to 4 weeks of medical management are candidates for surgery.

Primary and secondary chylothorax have been treated surgically. It is strongly recommended that animals with diagnosed secondary chylothorax also undergo treatment appropriate for the primary cause; otherwise, a low success rate is anticipated.⁶ The pleural effusion can be managed during therapy by repeated thoracocentesis. Extreme care should be taken to perform the procedure with aseptic technique to avoid inoculation of a nutrient-rich fluid with bacteria.

Patient positioning depends on the procedures to be performed during the anesthetic episode. Possible procedures include thoracic duct ligation (TDL), cisterna chyli ablation (CCA), popliteal dye infusion, mesenteric lymph node infusion with dye, mesenteric lymphatic catheterization for coloration, and pericardectomy. The thoracic duct is usually approached from the right in dogs and the left in cats and can be done in lateral or sternal recumbency.¹⁵ Laparoscopic CCA has been described with a left-sided approach in sternally recumbent dogs and would likely be performed similarly in cats.¹⁶ Furthermore, most surgeons prefer a subtotal pericardectomy, which requires the patient be placed in dorsal recumbency.^6,17,18 Thus, patient repositioning is required if more than one procedure is done.

The extent of surgical preparation depends on the approach and whether lymphatic coloration will be performed. The author prefers to color the lymphatics in every case to ensure complete ligation of all duct branches and to simplify cisterna chyli ablation. The author prefers to position the patient in sternal recumbency; however, thoracoscopic thoracic duct ligation has also been described in lateral recumbency.¹⁹ Sternal recumbency allows for multiple procedures, without patient repositioning. However, positioning must be done carefully to allow for lymphangiography or lymphatic coloration. Lateral recumbency positioning has previously been considered standard: left lateral recumbency in dogs, right lateral recumbency in cats.