Chylothorax – the basics

In veterinary medicine there is a general feeling, a superstition if you will, that things tend to happen in threes. Recently the Sunstone Surgery service has been getting inquiries about, giving advice for, and treating cats and dogs with a condition called chylothorax. So far we have heard about or treated two cases of chylothorax in recent weeks and we thought that as we wait for the next case to present we would take this opportunity to give our clients and readers an overview of chylothorax – what it is, how it is diagnosed, treatment options, and overall prognosis.

 

Chylothorax is a condition where chyle leaks into the chest cavity; if enough fluid accumulates in the chest, the lungs cannot expand effectively making it difficult to breathe.

 

But, what is chyle exactly?

To understand what chyle is, we have to look at how fluid flows in the body and what happens to fat when it is digested and absorbed.

 

While most people understand that veins and arteries conduct blood to and from the heart, few remember that there is a third type of vessel in the body called lymphatics. The lymphatic system has three main jobs:

(1) It helps maintain fluid balance in the body by providing a way for fluid that leaks out of blood vessels to get back into the bloodstream.

(2) It plays a major role in immune function by making and transporting immune cells, and providing a way for immune cells to get into the bloodstream.

(3) It is responsible for taking up and transporting fats absorbed by the gastrointestinal tract.

 

As everyone knows, oil (fat) and water do not mix well, however in order to transport fats within the body to where they need to go, they have to! To allow fat to be transported within fluid, the small intestine packages digested fats into molecules called chylomicrons. Chylomicrons are essentially fat surrounded by protein so that the fat can be ‘dissolved in’ and easily carried by lymph fluid and blood without trying to separate out. Normal lymph fluid is clear, however after a meal the intestinal lymphatics will be full of milky appearing fluid which is simply intestinal lymph fluid with chylomicrons, or chyle.

 

The intestinal lymphatics drain into an abdominal structure called the cisterna chyli – a reservoir and gathering place for all of the intestinal lymph. From the cisterna chyli, lymph flows forward into the chest via the thoracic duct and then the thoracic duct empties directly into the major veins entering the heart.

 

So, how or why does chyle leak into the chest?

The short answer is: we usually don’t know. Chylothorax occurs when fluid leaks out of the thoracic duct before it empties into the venous system. This can occur secondary to any disease that makes it harder for the thoracic duct to empty appropriately (i.e. heart disease, pericardial disease, lung lobe torsion, abscesses/granulomas, cancer, etc.). However the vast majority of the time in dogs and cats, chylothorax is idiopathic; meaning we can’t find a definitive cause for the condition. It is poorly understood how or why some dogs and cats just start leaking chyle into their chest with no apparent trauma and no apparent disease process preventing chyle from flowing normally.

 

How would I know if my dog or cat has a chylothorax?

Again, the short answer is: you might not know. If your pet is happy, healthy, eating, drinking, breathing, and acting normally – it would be very, very, unlikely for them to have a chylothorax.

 

Animals with chylothorax can vary in their presentation, some present on emergency due to the acute inability to breathe and some will just have a vague history of decreased appetite, lethargy, and weight loss.

 

On physical exam most have animals have findings consistent with fluid within their chest cavity: changes to their mucous membrane color, decreased lung sounds or heart sounds in certain parts of their chest, an elevated respiratory rate, difficulty breathing with restraint or when being placed in certain positions. X-rays of the chest would show fluid around the lungs. However without further testing, we would not be able to tell what kind of fluid was causing the problem.

 

Thoracocentesis (chest drainage) is usually indicated in animals that are having trouble breathing and have fluid in their chest. The good thing about this is that by drawing fluid off the chest, this usually gives their lungs more room to expand, improves their ability to breathe, and allows us to get a fluid sample for analysis. While the appearance of the fluid can give us an idea of if it is blood or pus or chyle, submitting the fluid to the lab for specific testing is really the best way to definitively diagnose what type of fluid it is.

 

Chyle can range from clear to cloudy and milky or pink tinged in color. Although chyle is not the only cloudy fluid that can accumulate in the chest, and if the pet hasn’t been eating, it might look clear. There are many tests that can be performed on fluid taken from the chest that can help identify what kind of fluid it is. To definitively diagnose thoracic fluid as chyle, we take a fluid sample from the chest and a blood sample taken at the same time and compare the amount and type of fat (triglyceride and cholesterol levels) in the fluid to the levels in the blood. Chyle will have high triglycerides and low cholesterol when compared to blood.

 

My pet has fluid in their chest that was diagnosed as chyle…now what?

If your pet has a chylothorax, your vet’s first priority will be to make sure they are stable and able to breathe. Depending on how much fluid your pet is producing into their chest, this may involve repeated thoracocentesis (chest drainage). Then their next steps will be to try to determine if they can identify a cause for the chylothorax. This will usually involve baseline bloodwork (CBC, Chemistry panel, perhaps a coagulation panel), appropriate infectious disease testing (heartworm, FeLV, FIV), a complete cardiac work-up including cardiac ultrasound, and imaging of the chest (either x-rays or CT scan). If there is a primary cause identified, that should be addressed as part of the treatment plan.

 

However it is not uncommon for all the testing to come up negative, resulting in a diagnosis of idiopathic chylothorax. Given that chylothorax is a relatively rare condition in dogs and cats, the number of studies conducted and our ability to make definitive evidence-based treatment recommendations is limited. In order to manage and attempt to cure chylothorax, there are a variety of surgical and non-surgical treatment options, but most commonly a combined approach yields the best results. 

 

How do you treat chylothorax without surgery?

Non-surgical treatment of chylothorax typically consists of some combination of the following:

-       Intermittent chest drainage (thoracocentesis). Removing fluid from the chest cavity via thoracocentesis or by placing a chest tube allows for temporary improvement in respiration. This helps with the immediate symptoms of chylothorax but does not address the underlying cause. Use of chest drainage alone only has an approximately 25% success rate for resolution of chylothorax.

-       Use of low-fat diets. Feeding a low-fat to animals with chylothorax decreases the fat content in the chyle which may decrease the overall inflammatory effect that chyle has on the lungs and other tissues in the chest.  However there is little evidence to support low-fat diets decreasing total lymph flow through the thoracic duct. Based on this, use of low-fat diet as the only treatment is not recommended.

-       Octreotide is a drug that decreases flow through the thoracic duct and has been used to treat traumatic chylothorax in people. Results of treatment in dogs and cats with octreotide are variable and the available literature states a success rate of approximately 40%.

-       Rutin is an over the counter nutraceutical made from the Brazilian fava d’anta tree. We do not know exactly how Rutin works to help resolve chylothorax but some theories include: increasing fluid uptake by lymphatics, decreasing the leakiness of blood vessels, and increasing immune activity to scavenge protein from the fluid in the chest. While there have been no published studies in dogs, cats treated with rutin showed an approximately 65-70% response rate.

 

When is surgery indicated and what does it do?

Surgery is definitely indicated if a chylothorax persists with high-volume effusion for more than 4 weeks despite non-surgical treatment (these animals typically require frequent chest drainage and continue to have large volumes of fluid at each tap). However early surgical intervention is frequently recommended to try to reduce the secondary effects of chylothorax which can include: low blood protein levels, decreased immune function, dehydration, and weight loss. Additionally, chyle in the chest acts as an irritant and can result in irreversible thickening and scarring of the surface of the lungs (pleuritis) and pericardium (pericarditis) both of which in their advanced stages can be life-threatening.

 

The primary surgical procedure performed for chylothorax is thoracic duct ligation (TDL). By tying off the thoracic duct right as it enters the chest, we are attempting to prevent chyle from getting to the chest cavity and forcing new connections to form between the lymphatics and veins further upstream. Success of getting chylothorax to resolve with TDL alone is not great – current literature reports likelihood of resolution in dogs to be between 50-60% and in cats between 15-50%.

 

As such, most surgeons will combine TDL with other procedures to help re-route or decrease resistance to chyle entering the bloodstream. The most common secondary procedure to TDL is a subtotal pericardectomy or pericardial window. This procedure involves removing some or most of the sac around the heart (pericardium). By performing a subtotal pericardectomy, we are able to relieve any restrictions to expansion of the heart, decreasing pressure on the veins and improving flow of lymph back into the bloodstream. By combining TDL with subtotal pericardectomy, we have a reported improvement in our chances of resolving the chylothorax to 80% in cats, and between 60-100% in dogs.

 

Additional surgical techniques that have been used to treat chylothorax address the condition by trying to decrease pressure on the thoracic duct (cisterna chyli ablation), trying to improve absorption of fluid (thoracic omentalization), trying to eliminate space around the lungs for fluid to accumulate (pleurodesis), and trying alternate methods to block lymph flow (cisterna chyli and thoracic duct embolization). However most have not been proven to definitively improve the chances of resolving the effusion.

 

Even with surgery, some dogs and cats continue to leak fluid into their chest for several weeks to months after surgery; and some that have resolution may have recurrence of effusion several months to years later. As such, adjunctive drainage devices to help with continued evacuation of the chest may be needed to manage the fluid build-up.

 

Because successful treatment of chylothorax is highly individual, it can be a difficult and frustrating disease to manage, however surgical treatment of chylothorax is typically our best chance at achieving good short and long-term outcomes. Without treatment, or in animals that do not respond to medical or surgical treatment, humane euthanasia is often chosen to prevent an affected dog or cat from suffering through repeated respiratory emergencies and chronic illness.

 

While we at Sunstone wish no animal would have to face this condition, we are available to discuss and help guide you through your treatment options if you ever find one of your furry family members in need of our assistance.

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