Lymphatics

Question 1

Where do the lymphatic channels eventually converge and drain into?

Answer 1

At the level of the venous angle, where the subclavian and jugular vein converge.

Question 2

Why does back up of lymph lead to edema?

Answer 2

Because the lymphatic system is the only way of returning proteins, such as albumin, back to blood from the interstitium.

Question 3

What are 2 biggest lymphatic draining tracts?

Answer 3

• Right lymphatic duct = drains the head and the right thoracic limb
• Thoracic duct = drains most of the body, left thoracic limb and everything caudal to it

Question 4

Discuss the pathogenesis and treatment of chylothorax.

Answer 4

What is chylothorax?

Chyle = a milky fluid that contains a high concentration of fat (trigycerides)

Chylothorax is the accumulation of chyle within the chest cavity. The majority of chyle comes from the intestines following absorption of fat from an ingested meal. The fat enters the lacteals (small lymph vessels) in the wall of the intestine and then is transported to the regional lymphnodes, passes through larger lymph vessels and joins the largest lymph vessel in the abdomen called the cisterna chyli. The location of the cisterna chyli is along side the largest artery in the abdomen called the aorta at the level of the left kidney. From there the lymph fluid moves through the chest via the thoracic duct, which finally joins a large vein in the chest called the vena cava. Thus the lymph fluid is delivered into the blood stream.

Chylothorax result from leakage of lymphatic fluid in the chest cavity through leaky lymph vessels in the chest. Obstruction to flow of lymph fluid can be due to increased pressure in the thin-walled lymph vessels. The obstruction of the lymphatic flow can be due to heart disease, cancer with the chest, fungal infections in the chest, heartworm disease, congenital defects, trauma, chronic vomiting, coughing, diaphragmatic hernia, twisted lung lobe, and a blood clot in the veins in the chest. Although rupture of the thoracic duct logically seems like it should be the most common cause of chylothorax, studies have shown that the thoracic duct is grossly intact in these cases. Even experimental tearing of the thoracic duct results in only transient chylothorax that spontaneously resolves in about 2 weeks. In most patients that have a work-up for chylothorax, a primary cause of this problem is not found therefore it is called idiopathic chylothorax.

Signs

Sex nor age seem to be a factor in the development of chylothorax. A number of breeds are predisposed to this condition and include cats, Afghans, and Mastiffs. Shallow, rapid respirations, and coughing are typically the most common presenting signs. Other signs may include loss of appetite, weight loss, gagging, lethargy, regurgitation, vomiting and exercise intolerance.

Diagnostic testing

Chest x-rays will always show the accumulation of fluid in the chest cavity and collapse of lung lobes. Fluid collected from the chest with a needle is analyzed and will have a high level of trigycerides (fat) and can have a mixture of white and red blood cells (photo right shows typical appearance of chyle). Other testing that commonly is done includes repeat chest x-rays after draining the fluid from the chest, ultrasound of the abdomen and chest, and blood tests (complete blood count and chemistry profile, heartworm test, fungal titers if indicated).

Medical therapy

Before surgery is considered, a low fat diet and a medication called Ruitin may be tried. If the problem resolves…great. The diet and Ruitin should be continued indefinitely. Intermittent removal of fluid from the chest with a needle will be necessary until the medical therapy becomes effective. Unfortunately, successful medical therapy is uncommon. If a patient has an underlying cause of chylothorax, treatment of the primary disease process (fungal infection, cardiomyopathy, etc) should resolve the accumulation of chyle in the chest.

Surgery

A variety of techniques have been recommended over the years for chylothorax. Historically, thoracic duct ligation alone has been recommended as a sole technique, but has poor clinical results.

The pericardectomy has been recommended as it was believed to reduce the venous pressure, thus allowing the lymphatic flow to be improved. However, a recent study has not really shown a true decrease in venous pressure. In addition, this technique in our experience give good success for this condition.

The cisterna chyli is a dilated portion of the lymphatic system within the abdomen near the kidneys. The cisterna chyli then continues through the diaphragm and then becomes the thoracic duct. Cisterna chyli ablation combined with thoracic duct ligation is the surgery that we currently use to treat chylothorax. This surgery is performed through an incision in the right side of the abdomen and involves removal of the cisterna chyli within the abdomen. As a result chyle will accumulate in the abdomen, but eventually resorbs. The result of this surgery is that new connections of the lymphatic system to the venous system form, thus bypassing the high demand of lymphatic flow within the thoracic duct.

Question 5

What are some possible causes of chylothorax?

Answer 5

Most commonly idiopathic…. Causes to rule out:

• Trauma to thoracic duct- rare, usually self limiting (resolves in 2 weeks)
• Increased lymphatic flow (2o to increased hepatic lymph production)
  
  • Thoracic lymphangectasia (dilated and tortuous vessels which leak)
  • Intestinal or generalized lymphangectasia
• Decreased lymphatic drainage
  
  • Cranial mediastinal mass (lymphoma, thymoma)
  • Fungal granulomas
  • Venous thrombi
  • Congenital abnormalities of thoracic duct
• Increased systemic venous pressures
  
  • Heart disease- cardiomyopathy, heartworm disease, pericardial effusion, tetralogy of Fallot, tricuspid dysplasia, cor triatriatum dexter
• Lymphangiosarcoma
• FeLV, FIV
• Diaphragmatic Hernia
• Lung lobe torsion