Chylothorax is the accumulation of excess chyle in the chest cavity. Chyle is a white colored liquid made of absorbed fat and lymph from the intestine that normally drains into the cisterna chyli of the thoracic duct. The lymphatics are the circulatory system of vessels that drain tissue fluid (fluid outside and between the body cells) into the lymph nodes then drain the lymph nodes into the thoracic duct. Lymph nodes house important immune cells, lymphocytes, which use the lymphatic system to traverse the body. Lymph is also especially important in the digestion of nutrients, particularly fatty nutrients. Fats are absorbed from the intestine and passed into the lymph vessels. These vessels drain into the main lymph receptacle in the abdomen called the cisterna chyli. This receptacle drains into the thoracic duct which passes through the diaphragm then through the chest to empty into the left jugular vein in the anterior chest.
Chylothorax in pets occurs when the thoracic duct drainage is blocked releasing the chyle into the chest cavity. This can be a fatal consequence as the lungs become collapsed and the pet is forced to breathe with only part of the normal lungs. Drainage of the chest fluid is essential at this point.
Ben is a 6-year-old Corgi who presented with difficulty breathing and radiographs showed collapse of the lungs from excessive fluid in the chest. The chest was drained of fluid and the fluid looked like milk.
The fluid was analyzed and found to have 50% of the cells to be lymphocytes and an exceedingly high triglyceride content (much higher than plasma) at 1976 mg/dl. These results indicate that Ben’s diagnosis is chylothorax from a ruptured or malformed thoracic duct. A chest tube was placed to drain the chyle. This is a special tube with a port under the skin to allow intermittent drainage of the chyle fluid without undue stress to Ben.
Ben was then scheduled for a CAT scan to see where the problem was with the thoracic duct. A dye is injected into the popliteal lymph node in the back leg and that dye is traced along the thoracic duct to plan for surgery to remove the source of the chyle in the chest. The thoracic duct is supposed to empty into the jugular vein or subclavian vein on the left side of the chest. This thoracic duct however had many convoluted branches and a tortuous path that was leaking chyle into the chest cavity.
The CAT scan shows the location of the thoracic duct with relation to the other vital structure in the chest. The thoracic duct is awfully close to the aorta which is the largest artery of the body. In addition, the nerves that control the automatic functions for the heart, aorta, lungs, esophagus, diaphragm, and intestinal organs – The Sympathetic Chain – lie just under the vertebrae and next to the thoracic duct. Prior to the surgery Ben is fed heavy cream every hour for 4 hours. This cream goes into his small intestine and then into the intestinal lymphatics then into the cisterna chyli then to the thoracic duct so that during surgery the normally clear thoracic duct is white in color.
This helps us distinguish it from the aorta and esophagus but makes it look like the nerves of the Sympathetic Chain. The CAT scan cross sectional image shows where the thoracic duct lies in relation to these vital structures.
The surgery is performed between the 8th and 9th ribs and this location will allow direct access to the thoracic duct without interference from the diaphragm or the heart. The objective is to remove a section of the thoracic duct after it is ligated so that it cannot find an alternate route to deliver the chyle into the chest. In dogs this surgery is best done from the left side of the chest and in cat it is done from the right.
The pericardial sac surrounds the heart and in chylothorax this sac becomes inflamed and thickened restricting the ability of the heart to pump. The Figure 10 image is filtered to show the pleura (coverings of the lungs) and the pericardial sac. It shows the inflammation present and dictates that the pericardial sac should also be removed from around the heart to allow normalization of the blood pressure in the chest.
Ben’s surgery was a success with the identification of the thoracic duct just behind the aorta and under the sympathetic chain. Stainless steel clips were placed above and below the segment removed and it was submitted for confirmatory histopathology. Figure 8 shows a microscopic image of this section of thoracic duct to confirm that the proper tissue was removed. The pleural linings of the lung and chest wall had adhered to the pericardial sac. The pericardial sac was very thickened. This reaction is common in pets that have long term chylothorax. The thickened and adhered pericardial sac was resected and removed to allow the heart to contract without restriction. This procedure has been shown to improve the outcome of cases of chylothorax in dogs.
One week after the surgery fluid was removed using the indwelling thoracic catheter and port to test for the presence of triglycerides and lymphocytes which are the hallmark of chyle. The sample shows macrophages, red blood cells and inflammatory cells but no lymphocytes and the fluid was negative for triglycerides. So, we feel that we can now say that Ben is cured, and we can take his chest catheter and port out. Ben’s parents say the old Ben is back, he will not stop running through the house and playing with his toys. He never has to stop to catch his breath as he did before this surgery.