By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
First, The Basics
Let us consider the chest cavity for a moment. The chest can also be called the thorax and the chest cavity is sometimes called the thoracic cavity. The thorax houses the lungs and heart as well as some other structures such as nerves, large blood vessels, and other conduits (such as the esophagus transporting food through the chest and into the stomach on the abdominal side of the diaphgragm) but the main residents of the chest are the heart and lungs.
The lungs are the focus of this topic. The diaphragm and rib muscles extend and contract to draw breath into the lungs and expel it again. In many ways the lungs are like sponges filled with air. Life depends on the ability of their small chambers to expand with new air and contract to expel used air. There is not a lot of extra room inside the chest cavity so when something (such as fluid) starts taking up space in the chest, the lungs do not have room to inflate to their natural capacity. Breathing becomes hard work and the patient’s focus becomes expanding the lung against limited space, sort of like trying to blow up a balloon that is too stiff. This fluid is not actually inside the lungs and generally does not lead to coughing or sputtering; it is instead surrounding the lungs making lung expansion difficult, leading to shallow rapid breaths and recruitment of the abdominal muscles to lend strength to the act of breathing.
See a cat with a chylothorax display the typical shallow breathing that goes with fluid surrounding the lungs.
Types Of Fluids
There are many types of fluids that can accumulate in the chest cavity. When the fluid is blood, the problem is called hemothorax. When the fluid is pus, the problem is called pyothorax. When the fluid is actually air, the problem is pneumothorax. When the fluid is lymph, the problem is called chylothorax. With chylothorax, the fluid is milky when it is drained from the chest, and its whiteness is from fat. Chylothorax represents a specific problem and requires specific therapy.
The effort the patient is making to breathe will probably be the first sign of trouble and upon seeing this, your veterinarian is likely to recommend radiographs. On those, fluid in the chest will be obvious. From there, the chest will need to be tapped with a needle and the fluid drained. This will create some relief for the patient as the lung will once again be able to expand. The fluid will most likely need to be sent to the lab for testing to determine the type. If the patient is too distressed for radiographs, the chest may be immediately tapped to see if there is fluid pressure that can be relieved. After the fluid has been removed from the chest, new radiographs are often taken to see if there are any structures (such as masses or heart chamber enlargements) that have become visible now that overlying fluid is gone.
What Is Lymph?
Chylothorax in a cat
Chest radiograph from a cat with chylothorax. The "white out" is fluid (chyle) filling up the chest cavity. On a radiograph, any fluid in the chest will look white. When the fluid is withdrawn and inspected directly, the fluid may be any number of colors. As it happens, chyle actually IS white. Photo by MarVistaVet.
We all know what blood is. We all know that blood circulates in veins and arteries and is moved by the pumping action of the heart. In fact, there is another circulatory system in the body: the lymphatic system. Lymph fluid, also called lymphatic fluid, represents extra fluid draining between the cells of the body, gradually channeling into lymph vessels. The fluid is moved in these vessels by the natural movement of the body's muscles and on its way it picks up assorted cellular debris and carries it along its route like drift wood. Lymph fluid (and its cellular drift wood) circulates through the lymph nodes, where cells of the immune system are exposed to the drift wood. In this way, the immune system sees the remnants of infection, tumor cells, foreign organisms etc. and can react appropriately. Some immune cells circulate in the lymphatic fluid, facilitating the immune reaction and participating in the body's defense.
Part of the circulatory path of lymphatic fluid involves certain lymph vessels of the GI tract called lacteals, which are involved in the absorption of dietary fat. Because of this influx of fat, lymphatic fluid is milky white. Lymphatic fluid (also called chyle) contains fat, water, and lymphocytes (cells of the immune system).
When something goes wrong with the circulation of the lymph fluid and back pressure is created, lymph fluid can leak out and accumulate in the chest.
When The Diagnosis Is Chylothorax
If there is a reason for the chyle build up, it is important to find that reason. Often (especially in cats), the reason is heart disease. Heart disease generally interferes with lymphatic drainage and poor drainage leads to the chyle build up but there can be other reasons for chyle build up besides heart disease. Any sort of mass or growth in the chest could also be responsible. If a cause can be found, then it should be addressed if possible. If no cause can be found then the condition is termed idiopathic and is simply managed either medically or surgically. An echocardiogram/ultrasound of the chest is almost always needed to rule out chest masses and assess the patient for heart disease. Most cases of chylothorax are idiopathic.
Breeds that seem predisposed include Siamese and Himalayan cats, and Afghan hound and Shiba Inu dogs.
Cats are diagnosed with chylothorax approximately four times as often as dogs.
When The Diagnosis Is Idiopathic Chylothorax
There are several options for treatment though they have pros and cons. The most conservative method is medical management. This means that whenever the patient seems to be having some distress, the chest fluid is drained (see video of a fluid drain). How often this is necessary is highly individual but every few weeks is a common interval. In time, after many taps, scarring can build up to cause the fluid to loculate, which means that small pockets of fluid form rather than one drainable area. This makes tapping more difficult over time. Other problems with periodic tapping is the potential to introduce an infection with the needle stick and the fact that chyle is an inflammatory fluid that can, over a long time, create some problematic scarring between the chest wall and the lung. This is called restrictive or fibrosing pleuritis and is definitely something to avoid (see below).
A supplement called rutin may help. Rutin is available in vitamin stores and acts to stimulate cells called macrophages to carry away some of the fat in the chyle. In some individuals this supplement is helpful in reducing the amount of chyle build up.
Another treatment involves somatostatin, a chemical normally produced in the brain to regulate intestinal hormone and enzyme secretion. One of its effects is to reduce chyle flow through the thoracic duct. A commercial product can be used in pets but its use should be considered somewhat experimental.
A low-fat diet (approximately 6 percent fat on a dry matter basis) is generally also used in conjunction with the above. Medical management such as this is often recommended before surgery as some cases of chylothorax will spontaneously resolve.
In many cases, a cause of chylothorax cannot be defined. In this situation the chylothorax is said to be idiopathic and it must be treated without the benefit of treating its underlying cause. There are several options for treatment, though they have pros and cons. The most conservative method is medical management. This means that whenever the patient seems to be having some distress, the chest fluid is drained. How often this is necessary is highly individual but every few weeks is a common interval. In time, after many taps, scarring can build up to cause the fluid to loculate, which means that small pockets of fluid form rather than one drainable area. This makes tapping more difficult over time.
A supplement that may help is called rutin. Rutin is available in vitamin stores and acts to stimulate cells called macrophages to carry away some of the fat in the chyle. In some individuals this supplement is helpful in reducing the amount of chyle build up.
A low-fat diet (approximately 6% fat on a dry matter basis) is generally also used in conjunction with the above. Medical management such as this is often recommended before surgery as some cases of chylothorax will spontaneously resolve.
More permanent solutions require surgery.
Thoracic Duct Ligation And Pericardiectomy
The thoracic duct is the largest lymph vessel in the body and it runs along side the aorta (largest artery in the body) through the chest. Lymph fluid flows through it on the way to the subclavian artery where it dumps into the bloodstream. When the thoracic duct is tied off, lymph fluid must find other channels for circulation and flow of lymph through the chest is greatly reduced. Ligation (tying off) of the thoracic duct resolves the chylothorax in approximately 50% of dogs and less than 40 percent of cats. See a video of thoracic duct ligation in a cat.
Success is hugely increased by stripping the pericardium from around the heart.
The pericardium is the fibrous sac containing the heart. When it is bathed in chyle, it becomes thickened and may be slightly constricting the low pressure right side of the heart. This causes increased pressure on the right side of the heart which in turn causes increased pressure in the lymphatics. Stripping the pericardium relieves this pressure and when thoracic duct ligation is combined with pericardiectomy, chylothorax resolved in 100% (10 out of 10) dogs studied and in 80% of cats studied. This is now the surgery of choice for chylothorax in cats and dogs.
Cisterna Chyli Ablation
The cisterna chyle is a structure where lymph collects just before it flows into the thoracic duct. The removal or destruction of the cisterna chyli further diverts lymph flow away from the chest. This procedure is generally reserved for patients who did not find success with thoracic duct ligation/pericardiectomy.
It can take up to 50 days to realize the benefits of surgery for chylothorax. Some animals simply will not achieve adequate resolution and some (as many as 30% of dogs) will resolve their chylothorax only to develop a non-chylous fluid effusion. The simple use of prednisone as an anti-inflammatory measure can resolve such fluid in as many as 60% of dogs though it takes 4-6 weeks. If this is ineffective or a more rapid solution is needed, a port can be surgically placed under the skin to allow for easy fluid drainage or a pump can similarly be placed to allow the owner to pump fluid from the chest into the abdomen.
Beware Of Fibrosing Pleuritis
Chyle in the chest is irritating to the local tissue; the lungs can develop scarring from being in contact with chyle. Scarring prevents the lungs from expanding normally even after the chyle is removed. The only treatment is to surgically remove the scar tissue using a procedure called decortication, a process fraught with complications if the lungs are diffusely affected. If both lungs must be decorticated, it is common for a life-threatening pulmonary edema to occur as the lungs try to re-expand. Sometimes air leaks out of the lung and fills the chest with air (pneumothorax). Before opting for any surgical treatment of idiopathic chylothorax, this potential complicating factor should be discussed with the surgeon.
Surgical treatment of chylothorax is something that not all veterinarians are comfortable performing. Discuss with your veterinarian whether referral to a specialist would be best for you and your pet.