Introduction

Chylous effusions, particularly chylothorax, are frequently reported in dogs and cats. Chyle is the term used to denote lymphatic fluid arising from the intestine and therefore containing a high quantity of fat. Chyle is normally transported to the venous system by a network of lymphatics in the mesentery (intestinal trunk). These lymphatics arborize in the cisterna chyli, a large dilated sac that lies adjacent to the aorta at L1-L4. The thoracic duct (TD) is the cranial continuation of the cisterna chyli and is generally said to begin between the crura of the diaphragm. In dogs (cats), the thoracic duct lies between the aorta and the azygous vein on the right side (left side) of the thorax and terminates in the venous system of the neck (left external jugular vein or jugulo-subclavian angle).

Regardless of the etiology, chylothorax is a potentially devastating disease. Chylothorax results in both compromised respiration and debilitation because of loss of large amounts of proteins, fats, fat-soluble vitamins, and lymphocytes into the pleural cavity. Electrolyte abnormalities may occur in animals with chylothorax; hyperkalemia and hyponatremia have been noted in dogs with both experimental and spontaneous chylothorax undergoing multiple thoracenteses.

Medical management

Because the surgical options for treating this condition require a high level of expertise to be successful and because the condition may spontaneously resolve with time, many animals are initially treated medically. This involves feeding a low-fat diet, performing needle thoracentesis as needed to relieve dyspnea (not on a regular basis) and supplementing the dog or cat with a benzopyrone such as Rutin (50-100 mg/kg, PO, TID). Owners should be warned of the potential development and grave prognosis associated with severe fibrosing pleuritis with chronic chylothorax. If medical management is being attempted it is important to provide for the patient's nutritional and metabolic needs until the effusion spontaneously resolves. Generally, meeting the animal's caloric needs by feeding a low-fat diet or a homemade diet is adequate. Supplementing the diet with low or medium chain triglycerides (MCT) is no longer advised because animals find these supplements to be unpalatable and their efficacy is questionable.

Surgical options

Surgical intervention may be warranted in animals that do not have underlying disease and in whom medical management becomes impractical, such as in animals that require thoracentesis more frequently than once a week, or those in which repeat thoracentesis fails to relieve the dyspnea. Surgical options in cases uncomplicated by severe fibrosing pleuritis include mesenteric lymphangiography and thoracic duct ligation, subtotal pericardiectomy, passive pleuroperitoneal shunting, active pleuroperitoneal or pleurovenous shunting, and pleurodesis. Of these, only the first two (TD ligation and pericardiectomy) are recommended by the author.

Thoracic duct ligation with mesenteric lymphangiography

Thoracic duct ligation is performed in cats from a left lateral intercostal thoracotomy or transdiaphragmatically. In dogs the procedure is performed from a right lateral intercostal thoracotomy. The mechanism by which thoracic duct ligation is purported to work is that following thoracic duct ligation abdominal lymphaticovenous anastomoses form for the transport of chyle to the venous system. Therefore, chyle bypasses the thoracic duct and the effusion resolves. Unfortunately, thoracic duct ligation results in complete resolution of pleural effusion in only 50% of dogs operated; in cats, the success rate is even less (< than 40%). Advantages of thoracic duct ligation are that if it is successful it results in complete resolution of pleural fluid (as compared to palliative procedures described below) and may prevent fibrosing pleuritis from developing. The disadvantages include that operative time is long, which is problematic in debilitated cats, there is a high incidence of continued or recurrent chylous or nonchylous (from pulmonary lymphatics) effusion and mesenteric lymphangiography is often difficult to perform in cats. Without mesenteric lymphangiography, complete ligation of the thoracic duct cannot be assured; however, an experimental paper assessing lymphangiography in cats suggested that this technique may not be uniformly successful in verifying complete ligation of the thoracic duct. Additionally, some animals may form collateral lymphatics past the site of the ligature and thus reestablish thoracic duct flow. If chyle flow is directed into the diaphragmatic lymphatics, chylothorax may continue or recur. For lymphangiography, food is withheld 12 hours prior to surgery. The appropriate side of the thorax and abdomen, or just the abdomen if a midline celiotomy is being performed, is prepared for aseptic surgery. If a thoracic approach to the thoracic duct is being used, a paracostal incision is made in order to exteriorize the cecum. Once the cecum has been exteriorized a lymph node adjacent to the cecum is located. A small volume (0.5-1 ml) of methylene blue may be injected into the lymph node to increase visualization of lymphatics. Repeated doses of methylene blue should be avoided due to the risk of inducing a Heinz body anemia or renal failure. Careful dissection of the mesentery near this node allows large lymphatic vessels to be visualized and cannulated with a 22-gauge over-the-needle catheter. Cannulation of this lymphatic is more difficult in the cat than in dogs because cats have more fat in their mesentery and their lymphatics are significantly smaller. Two sutures (3-0 silk) are placed in the mesentery and used to secure the catheter and an attached piece of extension tubing in place (the ends of the suture can be looped over the hub of the extension tubing). An additional suture may be placed around the extension tubing and through a segment of intestine to prevent dislodgement of the catheter. A three-way stopcock is attached to the end of the extension tubing and a water soluble contrast agent such as Renovist^r (ER Squibb and Sons, Princeton NJ) is injected at a dosage of 1 ml/kg diluted with 0.5 ml/kg of saline. A lateral thoracic radiograph is taken while the last milliliter is being injected. This lymphangiogram can be used to help identify the number and location of branches of the thoracic duct that need to be ligated and it can be repeated following ligation to help identify whether or not complete ligation of the thoracic duct was performed. It will also help determine the extent of lymphangiectasia present in the cranial thorax.

The thoracic duct is approached through a caudal intercostal thoracotomy (8, 9, or 10^th intercostal space) or via an incision in the diaphragm. Once the duct has been located, hemostatic clip can be used to ligate it. The advantage of using hemoclips is that they can be used as a reference point on subsequent radiographs if further ligation is necessary. However, I prefer to also place a nonabsorbable suture, such as silk, on the duct. Visualization of the thoracic duct can be aided by injecting methylene blue into the lymphatic catheter. If a catheter was not placed, the dye can be injected into a mesenteric lymph node.

Pericardiectomy

Thickening of the pleura and pericardium occurs in animals with chylothorax. Although the role of the thickened pericardium in this disease is not well understood, it has been the observation of the author that some animals benefit from pericardiectomy. If the thickened pericardium causes even a slight elevation in venous pressures, the TD may see this as an obstruction with resultant formation of numerous lymphatics in the cranial thorax. These lymphatics may likely leak transmuraly. Although experimental at this point, the author has had some success with pericardiectomy as the sole treatment of dogs and cats with chylothorax. Further studies and additional experience are necessary to recommend this technique in affected animals; however, it looks promising as sole therapy or as an adjunctive therapy to TD ligation in animals with chylothorax.

Other treatments

Passive pleuroperitoneal shunting has been recommended as treatment of chylothorax in animals, but this technique is no longer recommended by the author. The goal of placing a fenestrated silastic sheet in the diaphragm was to allow drainage of the chylous fluid into the abdomen where the fluid could be reabsorbed by visceral and peritoneal lymphatics, thereby alleviating the respiratory distress and need for subsequent thoracentesis. The author has not found this technique to be effective and chronic irritation of the sheeting may be associated with neoplastic transformation of tissues. Active pleuroperitoneal or pleurovenous shunting has been recommended for the treatment of chylothorax in dogs and cats and may be a reasonable consideration in animals in which all other therapies have failed. Commercially made shunt catheter are available and can be used to pump fluid from the thorax to the abdomen. The catheter is placed under general anesthesia. A vertical incision is made over the middle of the fifth, sixth, or seventh ribs. A pursestring suture is placed in the skin at this site and following the placement of fenestrations in the venous end of the shunt catheter, the catheter is bluntly inserted into the pleural space. A tunnel is created by blunt dissection under the external abdominal oblique muscle, and the pump chamber is pulled through the tunnel. The efferent end of the catheter is then placed into the abdominal cavity through a preplaced pursestring suture and incision located just caudal to the costal arch. The shunt must be placed with the pump chamber directly overlying a rib so that the chamber can be effectively compressed. Advantages of pleurovenous or pleuroperitoneal shunting of chyle are that it may allow more complete drainage of the thorax than passive peritoneal drainage. Pleurovenous shunting overcomes problems with inadequate peritoneal absorption which may occur with pleuroperitoneal shunting. Disadvantages are that the shunts are expensive, they may easily occlude with fibrin, some animals will not tolerate compression of the pump chamber, and they require a high degree of owner compliance and dedication. Additionally, thrombosis, venous occlusion, sepsis, and electrolyte abnormalities have been reported in human beings.

Pleurodesis is the formation of generalized adhesions between the visceral and parietal pleura. Adhesions may occur spontaneously in association with pleural effusion or in some species they can be induced following instillation of an irritating substance into the pleural cavity. This technique has been recommended for the treatment of chylothorax in dogs and cats, but is not recommended by the author. In order for pleurodesis to occur, the lungs must be able to contact the body wall; however, many animals with chronic chylothorax have some thickening of their visceral pleura which prohibits normal lung expansion (see fibrosing pleuritis above). Neither mechanical (surgical) pleurodesis or talc administration resulted in pleurodesis in experimental dogs; however, thickening of the pleura did occur in some animals. Chemical or surgical pleurodesis is unlikely to be successful in animals with chylothorax.

Omentalization has been advocated as a treatment of chylothorax. Although this procedure may enhance absorption of chyle from the thoracic cavity, it does not resolve the effusion. Because these animals are still at risk for fibrosing pleuritis, its routine use is not recommended by the author.