By Ashley Villatoro, DVM, DACVS-SA

Chylothorax is a complex disease process that affects both dogs and cats. It occurs when chylous effusion accumulates within the thoracic cavity. Chylous fluid consists of lymphocytes, protein, and triglycerides.^1-3

Chylothorax has many underlying possible etiologies that ultimately cause decreased drainage of the thoracic duct, increased lymphatic burden, or both. It can be secondary to infection (parasitic, fungal), mediastinal masses, congenital conditions, lung lobe torsion, heart-based tumors, iatrogenic trauma, and thrombosis, among others.^3-5 When a definitive cause cannot be identified, the condition is considered primary, or idiopathic.

Treating chylothorax typically involves either medical or surgical management.

Medical management aims to decrease the amount of lymphatic fluid excretion while increasing the absorption and removal of accumulated pleural fluid. Medications commonly used include rutin, a supplement that increases lymphatic fluid absorption^6,7 and octreotide, which reduces lymphatic fluid excretion. A nutritional change to a lower-fat diet may decrease the amount of chylous fluid production and transport through the thoracic duct.³ Drainage of pleural effusion via thoracocentesis or placing temporary thoracostomy tubes is used to aid in patient respiration and clinical signs secondary to the pleural effusion.

Surgical management of secondary chylothorax targets treatment of the underlying condition. In cases deemed idiopathic, surgical intervention is recommended after failed medical management. The mainstay of treatment is ligation of the thoracic duct.^1,2,8 Additional procedures can include pericardiectomy and cisterna chyli ablation.^1,2 Adjunctive procedures to aid in removal of pleural effusion include thoracic omentalization,⁹ pleuro-peritoneal shunt creation,¹⁰ and pleural port placement.^11,12

Surgical outcome for chylothorax in small animal patients is highly variable, with the prognosis more guarded in cats than in dogs.^1,13 Traditional surgical interventions for idiopathic chylothorax are highly invasive, typically requiring open chest surgery (Figures 1 and 2) with the possibility of added abdominal laparotomy.^3,4 Owners may be averse to such highly invasive procedures, with a possibility of nonresolution or continued need for pleural effusion evacuation. The use of minimally invasive surgery in small animals has made advances in the last several decades, and its use has been applied for surgical conditions, such as chylothorax.^13-15

Advantages of minimally invasive surgery in small animals have been well described in the literature, including quicker healing time, smaller incision size with inherent decreased incisional complications (Figures 3 and 4), quicker time to discharge, and less post-operative pain.^16-19 Advances in using high-level imaging concurrently with minimally invasive techniques have improved pre- and intra-operative planning for chylothorax and acceptable outcomes. Summarized are the four most common components of surgical management of chylothorax via minimally invasive surgery.

Thoracic duct ligation

The thoracic duct is the primary lymphatic vessel that allows for lymphatic drainage from the majority of the body and into the venous system. The thoracic duct originates from the cisterna chyli, a lymphatic network, located ventral to the lumbar vertebrae residing in the retroperitoneal space. The thoracic duct then travels cranially through the aortic hiatus in the diaphragm and into the thoracic cavity. The thoracic duct is typically located on the right dorsolateral surface of the aorta in dogs and on the left side in cats. The thoracic duct then empties into the left external jugular vein or jugulosubclavian angle.²⁰

Thoracic duct ligation is the primary treatment for chylothorax. The thoracic duct is ligated within the thoracic cavity to decrease leakage in the pleural cavity. Ligation of the thoracic duct allows for new anastomoses to be created outside the thoracic cavity between the lymphatic and venous systems. Thoracic duct ligation can be performed via a traditional open approach, using a right-sided 10^th intercostal thoracotomy approach in dogs or a left-sided 10^th intercostal thoracotomy approach in cats. Ligation of the thoracic duct is performed via ligation of all the structures dorsal to the aorta and ventral to the sympathetic trunk using suture, hemoclips, or a vessel sealing device.

Prior to surgery, advanced imaging in the form of a CT scan combined with lymphangiography can be performed to better understand each patient's individual anatomy of the thoracic duct. Recent literature has described variations in the configuration of the thoracic duct among patients.^21,22 Higher success rates may be achieved with patient-specific surgical planning, as additional branches and their location can be better identified (Figure 5). For the thoracoscopic approach to the thoracic duct, pre-operative imaging and planning are crucial for patient positioning and ideal port placement.

The first report of the use of thoracoscopy for visualization and ligation of the thoracic duct in dogs was published in 2002.²³ Since this time, several additional papers have investigated the use of thoracoscopy for thoracic duct ligation.^13-15 For thoracic duct ligation, the patient can be positioned in either sternal recumbency or lateral recumbency.

Typically, three ports are placed in the caudal thorax, allowing access to the side of the thoracic duct in that region. In dogs, the typical port placements are the 8^th, 9^th, and 10^th intercostal spaces on the right side.²⁰ In cats, the port placement is typically the 8^th, 9^th, and 10^th intercostal spaces on the left side.²⁰ Thoracoscopy is then performed to visualize the region containing the thoracic duct (Figure 6). Typically, one-lung ventilation is not needed due to the ventral location of the lung lobes with positioning. Suction of chylous fluid can be performed for better visualization. Exploratory thoracoscopy can be performed on the hemithorax entered for any gross abnormalities (Figure 7).

Although pre-operative imaging is useful in approach planning, the thoracic duct itself is not readily visualized intra-operatively. Advances in methods for visualizing the thoracic duct intraoperatively have occurred over the past decade.^24,25 Traditionally, methylene blue was used for visualization of the thoracic duct.^26,27 Many different injection sites have been described, such as the popliteal lymph node, diaphragmatic crus, intra-hepatic, or cecocolic lymph node. Recently, using indocyanine green (ICG) for direct near-infrared fluorescence lymphography (NIRF-L) has flourished for a variety of purposes. The use of ICG for visualization of the thoracic duct has been described with better visualization than methylene blue (Figure 8).²³

Once the thoracic duct has been identified, dissection immediately dorsal to the aorta is then performed to isolate the thoracic duct (Figure 9). A longitudinal incision is created dorsally to the aorta, carefully avoiding the intercostal arteries to prevent hemorrhage (Figure 10). If an intercostal artery is damaged, ligation can be performed using a vessel sealing device if necessary.

Dissection then continues to clear the adventitia from the aorta ventral to the thoracic duct and to clear the dorsal mediastinum ventral to the hypaxial musculature and sympathetic trunks. Once the thoracic duct is freed and all associated branches visualized, ligation can then be performed. Figure 11 illustrates endoclips being applied to the thoracic duct. Alternatively, a vessel sealing device can be used solely, or as in Figure 12, concurrently, to ligate and divide the thoracic duct between endoclips. An en bloc encircling suture can also be performed to ligate the structures between the aorta and the sympathetic trunk.

Pericardiectomy

Pericardiectomy is often performed in patients with chylothorax in conjunction with thoracic duct ligation. The pericardium of dogs with chronic effusion, such as those with chylothorax, thickens over time or becomes fibrotic. Fibrosis of the pericardium can lead to constrictive pericarditis, due to a decrease in cardiac preload and elevation of right atrial and central venous pressure.²⁸ Increased pressure on the lymphatic system from increased venous pressure can lead to leakage of chylous fluid from the thoracic duct. Therefore, pericardiectomy is theorized to aid in preventing constrictive pericarditis.²⁸

Restrictive pericarditis may not be present in all dogs with chylothorax; however, additional procedures outside of echocardiogram, such as cardiac catheterization, are required for diagnosis.²⁹ Therefore, it is possible pericardiectomy may not be required in all cases; however, it is still reported as a common concurrent surgical procedure for thoracic duct ligation with favorable outcomes.

A subtotal pericardiectomy or pericardial window can be performed with an intercostal or paraxyphoid approach. First, the mediastinum is broken down for access to both hemithoraces (Figure 13). Additional ports can be placed for additional instrument placement or visualization (Figure 14). The pericardium is grasped as shown using a thoracoscopic grasper. Laparoscopic Metzenbaum scissors or use of a vessel sealing device is used to enter the pericardial sac. Using the vessel sealing device, either a pericardial window or subtotal pericardiectomy is then performed (Figure 15). The pericardium is saved for bacterial and fungal culture and submitted for histopathological examination.

Cisterna chyli ablation

The cisterna chyli is located medial to the hilus of the left kidney and serves as a reservoir for chyle. It is proposed that in cases with thoracic duct ligation, the increase in pressure within the thoracic duct may allow for alternate pathways of lymphatic drainage, leading to failure.³⁰ By ablating the cisterna chyli, this may allow reduction of the pressure and a decrease in recurrence of the chylothorax. Recent studies have shown high resolution rates without cisterna chyli ablation, but can be considered during initial surgical intervention or for cases of non-resolution or recurrence. Laparoscopic approach to the cisterna chyli can be performed and has been described in recent literature.^30,31 Patient positioning can be performed in sternal or lateral recumbency, with the laterality determined best via pre-operative imaging and lymphangiography.^30,31 Single or multiport placement has been described with ablation performed via tearing of the cisterna chyli with laparoscopic instruments.^30,31 Reported complications can include post-operative chyloabdomen requiring abdominocentesis.³¹

Pleural port placement

Patients with chylothorax typically have some degree of continued pleural effusion, either short-term, only immediately post-operatively, or for life. Pleural effusion can cause varying levels of respiratory compromise and may lead to sequelae, such as pericardial tamponade and fibrosing pleuritis. Thoracocentesis is often required with chronic pleural effusion if the underlying cause cannot be treated or is not completely attenuated. Complications with repeated thoracocentesis include possible iatrogenic pulmonary injury, infection, pain, and the need for sedation.

A permanent port for drainage of pleural effusion is now available in dogs and cats in the form of a modified vascular port. The port includes a multi-fenestrated silicone tube placed within the thoracic cavity which exits the chest and is connected to a subcutaneous port. This port is accessed through the skin using a special Huber needle, which allows for repeated insertion without damage to the port. This port is advantageous due to its ability to facilitate repeated thoracic drainage without incurring high costs, decreasing the chance of infection, and allowing for use of intra-thoracic treatments, such as intra-cavitary chemotherapy (Figure 16). Potential complications of pleural ports include possible obstruction, infection, or migration.¹¹

Outcome and prognosis

A minimally invasive surgical approach to idiopathic chylothorax has been reported to yield outcomes ranging from 80 to 100 percent^13,14,33 success in dogs and 50 to 100 percent in cats.^15,34 These percentages are comparable to and improved compared with traditional open approaches.^1,2 This may be due to more recent advances in imaging and the use of intra-operative lymphangiography to better identify the thoracic duct and any additional branches.

Possible complications of the thoracoscopic approach to idiopathic chylothorax include non-resolution requiring continued medical management or additional surgical procedures,³⁵ delayed recurrence of effusion after initial resolution, need for intra-operative conversion, and anesthetic-associated death.^13-15,33

Although favorable outcomes have been reported for minimally invasive surgical treatment of idiopathic chylothorax, further investigation involving larger study populations is warranted. Additionally, a longer time frame for follow-up post-operatively would be beneficial to better evaluate delayed complications and overall long-term outcomes. Continued research remains essential to further understand the complex pathophysiology of idiopathic chylothorax and the best surgical intervention.

Ashley Villatoro, DVM, DACVS-SA, achieved board certification from the American College of Veterinary Surgeons in small animal surgery in 2024. She is currently an assistant professor of general surgery in the Department of Clinical Sciences at the Colorado State University College of Veterinary Medicine and Biomedical Sciences. Dr. Villatoro is currently a minimally invasive fellow candidate in small animal soft tissue surgery. She has an interest in minimally invasive soft tissue surgery, interventional radiology, surgical oncology, wound management, and reconstruction techniques.

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