Chylothorax in Cats

What is chylothorax?

Chylothorax is a relatively rare condition in cats in which lymphatic fluid (called chyle) accumulates in the pleural cavity. The pleural cavity lies between the lungs and the inner lining of the chest wall. Normally, only about a teaspoon (5 mL) of clear fluid is present in this space, keeping the lungs’ surface lubricated so they do not adhere to the chest wall. When chylothorax is present, up to a quart (1 L) of fluid may be present in the pleural cavity.

When a diagnosis of chylothorax is made, two abnormalities have developed:

1. A pleural effusion is present. An abnormal amount of fluid has accumulated in the pleural space surrounding the lungs. This accumulation limits the cat's ability to expand the lungs completely, forcing it to breathe rapidly and shallowly. In some cases, pleural effusion leads to respiratory failure and death.

2. The fluid is milky-white rather than clear. The normally clear fluid in the pleural space has been replaced by chyle, a milky-white fluid from the thoracic duct (an area of the lymphatic system that returns lymph to the blood). This unique fluid forms from lymphatic drainage of the intestinal tract and is high in triglycerides, which gives it a distinct color and chemical composition. Normally, the thoracic duct transports chyle to a large vein called the cranial vena cava for further processing by the body. With chylothorax, the chyle drains into the pleural space instead of the cranial vena cava.

Are some cats more likely to get chylothorax?

Purebred cats, especially the Siamese and Himalayan breeds, may be at increased risk of developing chylothorax. Male and female cats appear equally affected.

What are the clinical signs of chylothorax?

The main clinical sign of chylothorax is labored or difficult breathing. Some cats appear to be holding their breath because there is a delay between inspiration and expiration. Coughing is often the first sign of chylothorax in some cats. Coughing is not normally reported with other causes of pleural effusion, so it can be an important indicator of chylothorax; there are relatively few causes of coughing in the cat as compared to dogs or humans. In some situations, clinical signs of the underlying disease, such as a tumor or heart failure, may overshadow those of the pleural effusion.

Occasionally, owners note no abnormalities, other than depression or exercise intolerance. The lack of signs is due to a slow accumulation of fluid in the pleural space: the cat can adapt or compensate until the condition becomes life-threatening.

What causes chylothorax?

In some cases, chylothorax may be caused by trauma or by increased pressure within the thoracic duct or vena cava. Trauma may cause the thoracic duct to rupture or become “leaky.” The most common causes of traumatic chylothorax include automobile injuries and falls that cause injury to the chest cavity. Increased pressure within the thoracic duct or vena cava may occur due to congestive heart failure, heartworm infection, fungal disease, or chest tumors and may cause chylothorax. If the exact cause cannot be identified after appropriate diagnostic procedures, it is called idiopathic chylothorax.

How is chylothorax diagnosed?

Your veterinarian may suspect your cat has chylothorax if there is a history of chest trauma or other chest disease and your cat is coughing or having difficulty breathing. When your veterinarian listens to the chest with a stethoscope (auscultation), the accumulated fluid will muffle heart and lung sounds.

To reach a definitive diagnosis of chylothorax, the following tests are usually performed:

Thoracic radiography (chest X-ray): This test confirms the presence of fluid in the chest cavity but does not characterize the type of fluid.

Fluid analysis: A small amount of fluid is removed from the chest cavity via a “chest tap” or thoracocentesis.

• First, the color of the fluid is noted. Chyle is typically white or light pink in color, while effusion is often clear or amber.
• Next, chemical tests on the fluid are done to determine its triglyceride (fat) content; if the fat content is high, the fluid is most likely chyle.
• Finally, when examined under the microscope, chyle typically contains large numbers of lymphocytes - a type of white blood cell commonly found in lymphatic fluid.

Once chylothorax is confirmed, additional tests will be performed to look for an underlying cause. Such tests may include blood work (with tests for feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), and heartworm disease), additional chest and abdominal radiographs, thoracic ultrasound, echocardiography (a cardiac ultrasound), and testing of the fluid for evidence of bacterial or fungal infections.

How is chylothorax treated?

Treatment is first directed at stabilizing the cat by restoring normal breathing. Fluid can be drained from the pleural space with a syringe and needle. In almost all cases, the fluid will re-form within 24 to 72 hours and a chest drain will need to be surgically placed to make daily drainage easier. This drain may be left in place until chyle accumulation stops.

A medication called octreotide (Sandostatin®) may be helpful, as it decreases the flow of chyle in the thoracic duct; however, success from using this drug can be variable.

A supplement called Rutin may be a beneficial treatment in some cats with chylothorax. It is thought to stimulate cells called macrophages to remove fat in the chyle, reducing the amount of fluid accumulation. Rutin is available at pharmacies and health stores. A low-fat diet may also be recommended to lower triglyceride levels and improve the cat’s ability to reabsorb chyle from the chest cavity.

Feline chylothorax is a disease under active research. Several new treatments may be on the horizon. Surgical procedures are sometimes used to treat the condition, such as ligating (tying off) the thoracic duct. Regardless, the most successful therapy for your cat will be directed toward treating the current underlying disorder.

What complications can occur from chylothorax?

Chyle is irritating to the heart, the lungs, and the tissues covering the inside of the chest cavity. If chyle has been present in the chest for several weeks, adhesions or scar tissue may form around the lungs and heart; this is called restrictive pleuritis. This scar tissue contracts and compresses the lungs, making it impossible for them to expand properly, causing a condition called fibrosing pleuritis.

Radiographs reveal lungs that are rounded instead of pointed, and inflammatory cells are prominent in the chest fluid. An ultrasound examination can reveal fluid between the chest wall and the lung tissue. Scar tissue, as mentioned above, may be visible in more severe cases. Many cats experience one or two episodes of idiopathic chylothorax and then experience spontaneous resolution without relapse.

What is the prognosis?

Chylothorax is a serious and potentially fatal disease. However, the prognosis is generally good if four conditions are met:

1. Breathing is stabilized. The cat must be able to withstand the initial diagnostic procedures and placement of a chest drainage tube. The accumulated fluid severely compromises respiration. Unfortunately, some cats die before sufficient fluid can be removed to improve their respiratory status, or shortly thereafter due to irreversible damage caused by the fluid.

2. The underlying disease is identified and successfully treated. In many cases, an underlying disease cannot be identified. In cases of idiopathic chylothorax, the prognosis is usually good if the fluid production can be controlled.

3. The underlying disease does not recur. Chylothorax is likely to recur if the underlying disease cannot be identified and treated. In this case, the cat must be closely monitored for return of respiratory difficulty.

4. The chylothorax is resolved before restrictive pleuritis develops. Once restrictive pleuritis is present, the prognosis is grave, even if the underlying disease has been successfully treated.