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.
TYPES OF FLUIDS
There are many types of fluids that can accumulate in a body 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, its whiteness being from fat. Chylothorax respresents a special problem and requires special 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 radiographs, 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 fluid 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?
If there is a reason for the chyle build up, it is important to find that reason. Often (especially in the cat), 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 masses in the chest and assess the patient for heart disease.
Cats are diagnosed with chylothorax approximately four times as often as dogs.
Siamese and Himalayan breeds of cat seem predisposed while in dogs the Afghan hound and Shiba Inu are predisposed.
DIAGNOSIS: IDIOPATHIC CHYLOTHORAX
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 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. Here are some procedures that have been recommended in the past:
THORACIC DUCT LIGATION
The thoracic duct is the largest lymph vessel and is present in the chest. In older times it was thought that chylothorax resulted from a tear in the duct, spilling lymph into the chest. This turns out generally not to be the case but ligation of the duct is successful in permanently resolving chylothorax in 50% of dogs undergoing surgery. Less than 40% of cats achieved resolution. In patients where surgery was not successful, fluid build up continued (though not always chyle, sometimes just watery fluid would return).
This technique involves placement of a sort of a drain in the diaphragm so that fluid from the chest could drain into the belly where it causes no harm. Shunts can be active (where there is a pump that can be manually compressed) or passive where normal breathing and motion moves the fluid. Complication rate is fairly high with this procedure (nearly 50%) and it has not been recommended for cats. It does yield an average of 20 months of symptom-free living in dogs.
THORACIC DUCT LIGATION WITH PERICARDECTOMY
This method appears to be the current treatment of choice. 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. When the pericardium is stripped away, lymph may be able to circulate in more normal channels that it previously could not. When this procedure was combined with thoracic duct ligation, chylothorax permanently resolved in 100% of dogs and 80% of cats. The surgery is difficult and highly invasive (as one might imagine since it involves manipulation of the heart) and should be performed by a surgery specialist only.
BEWARE OF FIBROSING PLEURITIS
Chyle in the chest is irritating to the local tissue; the lungs can actually 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.
Page last updated: 8/29/2017