Chylothorax, characterized by the accumulation of chyle within the thoracic cavity, is a relatively uncommon disease that affects dogs and cats. Chyle has a characteristic milky appearance (Figure 1) and it contains small molecules of fat. After eating, food is digested by your pet and the fatty component of the meal is further broken down into small molecules termed chylomicrons. The intestinal lymphatic system that travels to a structure called the cisterna chyli (CC), which is located in the front portion of the abdomen, near the kidneys, absorbs these small molecules. The CC is a lymphatic reservoir that receives chyle from the intestine but also receives lymphatic fluid from the rest of the abdomen and pelvic limbs. The thoracic duct (TD) is the extension of the CC into the chest, which carries chyle into the thoracic cavity and eventually empties its contents into the cranial vena cava (CrVC) close to the heart (Figure 2). In pets affected with chylothorax there is an abnormality in the TD that causes it to leak chyle into the thoracic cavity. These pets have difficulty breathing as the chyle that builds up in the chest prevents their lungs from fully inflating with air. The lymphatic fluid that is also a main component of chyle contains protein, white blood cells, and vitamins. The loss of large amounts of chyle into the thorax can weaken your pet’s immune system and create severe metabolic disorders. Chyle is also an irritant and chronic exposure to the lining of the lungs (pleura) and heart (pericardium) can lead to inflammation of those surfaces with further deleterious consequences.
Any disease process that obstructs or impedes the outflow of chyle from the TD into the CrVC can potentially lead to chylothorax. Cancer, fungal disease, heart disease, and blood clots within the CrVC have all been reported as causes of chylothorax in dogs and cats. It is suspected that all of these diseases prevent normal outflow of chyle from the TD into the CrVC. As a result, chyle backs up into the TD and eventually leaks out into the thoracic cavity. Ultimately, in veterinary patients, an underlying cause is rarely found and chylothorax is deemed idiopathic, or of unknown origin.
The clinical signs that a pet with chylothorax displays are not specific to the accumulation of chyle in the thorax since any substance (blood, air, pus) that impedes lung expansion can cause breathing difficulties. Your pet may develop a non-productive cough or breathing difficulties. With the loss of large amounts of chyle into the thorax, your pet can lose vital nutrients and they can become lethargic or lose their appetite. Weight loss can also occur over a period of time. Should you notice any of these signs, your pet should be evaluated by your primary care veterinarian.
The first thing your veterinarian will perform when they evaluate your pet is a physical examination. Auscultation of the thorax may reveal reduced or muffled heart and lung sounds as a result of the fluid in the thoracic cavity. A heart murmur, if detected, is an important finding as heart disease is a possible cause of chylothorax.
If there is a suspicion of fluid in the thoracic cavity (pleural effusion) often the first diagnostic test performed is radiographs. Radiographs can only confirm the presence of fluid in the thorax and cannot provide a diagnosis of chylothorax (Figure 3).
After confirming pleural effusion, obtain a sample of the fluid to characterize the disease process further. Sampling of thoracic fluid (thoracocentesis) is performed by placing a small gauge needle into the thoracic cavity and aspirating until fluid is obtained. This is a relatively uncomplicated procedure; however, it may be necessary to sedate your pet. If thoracocentesis obtains a milky fluid, chylothorax can be highly suspected. An example of chylous pleural effusion obtained from a thoracocentesis is shown above (Figure 1). Your veterinarian will likely submit a sample of the fluid along with a blood sample to a laboratory for confirmation of chylothorax.
All potential underlying causes of chylothorax should be investigated. Your veterinarian may recommend having a cardiac and thoracic ultrasound or even a computed tomographic (CT) scan performed on your pet to determine if there is heart disease or thoracic cancer that could lead to chylothorax. These tests are much more sensitive compared with radiographs.
Remember that in most cases an underlying cause of chylothorax is usually not identified.
Your pet will likely recover in the intensive care unit (ICU) of the hospital where they will be administered medications to relieve pain and have the chest tube aspirated intermittently. Oxygen supplementation may also need to be provided in the immediate post-operative period. Your pet can be discharged after surgery once their chest tube production resolves or is reduced enough allowing for chest tube removal and intermittent thoracocentesis should their clinical signs return. In successful cases, the chylous effusion resolves several days to weeks after surgery.
Reported success rates for the alleviation of chylothorax in dogs and cats undergoing TDL is variable (40-60%). The combination of TDL and pericardectomy has improved success rates (80-100%). Should surgical methods fail to resolve chylothorax, alternative surgical techniques can be discussed with your surgeon in an attempt to prevent the accumulation of chyle in the thorax.
Chylothorax is a poorly understood disease and it is important to realize that treatment (medical or surgical) may not be successful.