Chylothorax is a condition characterized by the accumulation of chyle in the thoracic cavity where the heart and lungs are located.
Chyle is a type of digestive fluid that is composed of lymph and fats. It is formed in the intestines and introduced into the circulation via the thoracic duct, which is the main trunk of the lymphatic system.
Chyle from the thoracic duct is emptied into the cranial vena cava (a large vein above the heart) which is an important portal of entry to the circulatory system. When a dog suffers from Chylothorax, this fluid typically builds up between the lung and the chest wall causing severe cough and chest pain.
Since affected pets suffer from debilitation and compromised respiration, experts view chylothorax as a potentially devastating medical condition. The condition is relatively rare in dogs, but it has gained relative importance because of the poor prognosis.
Potential Causes of Chylothorax
There is quite a long list of potential causes of chylothorax in dogs. It should be noted that almost all of these conditions obstruct or impede the flow of chyle from the thoracic duct to the cranial vena cava.
An obstruction in the flow causes accumulation of chyle and build up of pressure in the thoracic duct. When this happens, chyle eventually leaks out into the thoracic cavity.
Some medical conditions that have been associated with chylothorax in pets include trauma, heart disease, blood clots in the thoracic duct, tumors in the chest cavity, and even nodular lesions caused by fungal disease. Some experts believe that there may also be a genetic element in the development of the disease.
In fact, a high incidence of chylothorax has been observed among Afghan hounds. However, there are many cases of chylothorax in dogs in which the underlying cause has not been fully established and is termed “idiopathic”.
An experiment that involved the ligation of the cranial vena cava caused lymphangiectasia (excessive dilation) of the thoracic duct that eventually resulted in a high incidence rate of chylothorax in cats and dogs.
This led to experts speculating that lymphangiectasia may pave the way for the escape of chyle through the wall of the lymphatic vessel. The result of this experiment suggests that any blockage of the cranial vena cava can also be a mechanism for the development of chylothorax.
The symptoms of chylothorax in dogs are brought about by the significant loss of large quantities of proteins, fats, lymphocytes, and fat-soluble vitamins into the pleural cavity. These symptoms include the following:
- Rapid shallow breathing or labored breathing — As chyle accumulates in the pleural cavity, the expansion of the lung is significantly obstructed leading to breathing difficulties.
- Non-productive cough
- Dehydration and electrolyte imbalance which is brought about by water and electrolyte losses
- Lethargy and weakness
- Loss of appetite
- Severe metabolic disorders and malnutrition are associated with loss of fats, proteins, and fat-soluble vitamins
- Impaired immune system function — brought about by the loss of antibodies, lymphopenia, and malnutrition
- Weight loss
- Inflammation of the pleura (the lining of the lungs) and pericardium (heart lining) brought about by chronic exposure to chyle which is an irritant.
Pathogenesis of Chylothorax in Dogs
Chyle is produced in the intestine from digested food. From the intestine, chyle reaches the cisterna chyli via the lymphatic system. The cisterna chyli is a lymphatic reservoir in the abdomen.
From the cisterna chyli, chyle passes through the thoracic duct which is the extension of the cisterna chyli into the chest. The thoracic duct carries chyle into the thoracic cavity and empties into the cranial vena cava, a major blood vessel near the heart.
Chylothorax happens when there is an abnormality in the thoracic duct that causes chyle to leak into the thoracic cavity.
The manifestation of characteristic clinical symptoms, radiographic findings, and results of fluid analysis are vital for a chylothorax diagnosis confirmation.
A thorough physical examination including auscultation of the thorax may detect a heart murmur or muffled heart and lung sounds brought about by the presence of fluid in the thoracic cavity. Heart disease is an important underlying cause of chylothorax in dogs.
A radiographic test can help confirm the presence of fluid in the thoracic cavity. The evaluation of the pleural fluid involves aspirating a sample of thoracic fluid using a small gauge needle that is inserted into the thoracic cavity. The process is called thoracocentesis.
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While it is a relatively simple procedure, there may be a need to sedate the dog. Chylothorax is highly suspected when the fluid that is aspirated appears milky.
Once these procedures reveal the presence of chylothorax, there is a need to determine the underlying cause. The following procedures or measures are usually undertaken by veterinarians:
The pet owner will be questioned about the possibility of a recent trauma or injury.
Test for heartworms (Dirofilariasis)
The presence of heartworm is evaluated using specific tests such as microfilaria concentration techniques or serum adult antigen detection tests.
To rule out neoplasia, chyle will undergo cytologic examination to find out if there are neoplastic cells present.
Thoracic duct imaging
Radiograph, ultrasound examination and echocardiography.
After the pleural cavity has been completely drained of the effusions, your veterinarian may have radiographs taken of the thoracic cavity to find out if there are masses or growths especially in and around the cranial mediastinum.
Ultrasound examination, computed tomographic scan, and echocardiography of the mediastinum may also be indicated to determine the presence of heart disease or thoracic cancer. These procedures are much more sensitive than radiographs.
If a cause for chylothorax is not fully established or identified, it is often diagnosed as ‘idiopathic chylothorax’.
There are two general approaches in the treatment and medical management of chylothorax in dogs: non-surgical intervention and surgery.
The following measures are aimed at draining fluid from the thoracic cavity and reducing chyle formation.
Evacuation of the thoracic fluid
There is a need to remove chyle from the thoracic cavity so the lungs can fully expand. This can go a long way in providing relief from any breathing difficulty or persistent bouts of coughing.
Fluid is evacuated either by intermittent thoracentesis or by placing a tube in the chest. Your veterinarian may deem it necessary for the dog patient to be hospitalized for the procedure.
To reduce the fat content of chyle, a low-fat diet is recommended. This can significantly decrease the triglyceride content of chyle. Dogs diagnosed with chylothorax should be provided with aggressive nutritional support.
Benzopyrone (Rutin) is a nutriceutical oral supplement that is often prescribed for dogs with idiopathic chylothorax.
It is thought to stimulate the breakdown and removal of protein in lymphatic vessels. However, there is still a need to prove the efficacy of this over-the-counter nutraceutical in the treatment of chylothorax in dogs.
In addition to rutin, dogs with chylothorax should be supplemented with fat-soluble vitamins, such as vitamin A, D, E, and K.
Medical management is often the initial step in treating chylothorax in dogs because there is a possibility that the problem may resolve with time. Also, the surgical procedures that are indicated for chylothorax treatment require a high level of expertise, not to mention the monetary aspect that is involved.
If your pet’s condition warrants a need for surgical intervention, you should inquire about referral to an ACVS board-certified veterinary surgeon. There are several indications for surgical intervention of chylothorax in dogs.
- Idiopathic chylothorax — Medical management of idiopathic chylothorax cases are rarely successful.
- Medical management failed to significantly reduce the flow of chyle after 5-10 days of evacuating chyle from the thoracic cavity.
- The dog is suffering from protein calorie malnutrition and hypoproteinemia.
- Loss of chyle exceeds 20 ml/kg/day
The surgical options for chylothorax in dogs include the following:
- Thoracic duct ligation and mesenteric lymphangiography
- Cisterna Chyli Ablation (CCA)
- Passive pleuroperitoneal shunting
- Active pleuroperitoneal or pleurovenous shunting
- Video-assisted thoracoscopy
Thoracic Duct Ligation (TDL)
The principle behind ligation of the thoracic duct is to promote the formation of new lymphatic connections to that part of the venous system in the abdomen. This can divert the flow of chyle away from the thoracic duct system. TDL has been shown to effectively prevent chyle leakage from the thoracic duct into the thoracic cavity. The reported success rates of TDL in alleviation of chylothorax in dog ranges between 40-60%.
A common cause of failure of TDL is not being able to ligate all the branches of the caudal thoracic duct. In order to prevent this, it is recommended that en masse ligation of all structures located in the caudal mediastinum dorsal to the aorta should be made.
To help ensure that the thoracic duct has been completely ligated, intra-operative lymphangiogram is recommended. However, this can significantly increase the operative time and requires equipment that is capable of taking intra-operative radiographs.
Another downside to the procedure is that complete ligation of the thoracic duct branches does not guarantee a 100% prevention of the recurrence of the chylous effusion.
Prolonged exposure to chyle can cause inflammation of the linings of the lung (pleura) and the heart (pericardium). A thickened pericardium can possibly lead to a slight increase in venous pressures, which the thoracic duct may perceive as an obstruction.
This can eventually result in the formation of numerous lymph channels in the cranial part of the thorax. Pericardectomy involves the removal of the thickened heart lining (pericardium).
Thoracic duct ligation + Pericardectomy
Recent procedures have combined TDL with pericardectomy. The success rate of the combined procedures are higher compared to TDL alone. Post-operative management involved placing a tube in the chest for aspiration.
To confirm the success of TDL after surgery, some veterinary surgeons perform abdominal surgery that allows contrast material injection into a lymph node or lymphatic vessel in the intestine. It has been observed that there is a higher success rate (80-100%) with combined TDL and pericardectomy in dogs with chylothorax.
Cisterna Chyli Ablation (CCA)
Ablation destroys the cisterna chyli which is an important reservoir of lymphatic fluid in the abdomen. When this happens, the body responds by creating alternative pathways for the lymph to enter the blood circulation, which can significantly relieve pressure on the thoracic duct.
Compared to thoracic surgery, video-assisted thoracoscopy is minimally invasive. The procedure has been used to perform thoracic duct ligation, pericardectomy, and cisterna chyli ablation in dogs.
Minimally invasive alternatives have also been used for performing lymphangiography. These techniques have been shown to significantly reduce the time for the procedure while easing the need for abdominal surgery.
Passive peritoneal shunting
The procedure involves putting in place a fenestrated silastic sheet in the dog’s diaphragm so there will be drainage of chyle into the abdomen where visceral and peritoneal lymphatics can reabsorb the fluid.
The ultimate objective of passive peritoneal shunting is to alleviate the respiratory distress and reduce the need for subsequent thoracocentesis. However, some experts don’t recommend this technique.
Aside from being ineffective, there is an increased possibility of neoplastic transformation in tissues as a result of chronic irritation brought about by the sheet.
Active pleuroperitoneal or pleurovenous shunting
Active pleuroperitoneal or pleurovenous shunting is a viable option for chylothorax treatment when all other therapeutic techniques have failed. The procedure involves placing a shunt catheter into the pleural space to pump fluid from the thoracic cavity to the abdomen.
Advantages of pleurovenous or pleuroperitoneal shunting of chylothorax in dogs include:
- It may allow more complete drainage of chyle from the thorax compared to passive peritoneal drainage.
- It can help overcome problems associated with inadequate peritoneal absorption that tend to occur with pleuroperitoneal shunting
- Shunts are expensive
- Shunts can be blocked with fibrin
- Some dogs won’t tolerate the regular compression of the pump chamber
- It requires a high degree of owner compliance and dedication
Mechanical (surgical) pleurodesis aims to stimulate formation of generalized adhesions between the parietal and visceral pleura. In some species of animals, pleurodesis has been induced by introducing an irritating substance into the pleural cavity.
While the technique has been recommended for chylothorax treatment in dogs and cats, many experts don’t advocate it especially in dogs with chronic chylothorax because a thickened visceral pleura can prohibit the normal expansion of the lungs.
Omentalization for the treatment of chylothorax in dogs may enhance chyle absorption from the thoracic cavity but it fails to resolve the effusion. Many veterinary surgeons don’t recommend the technique because dogs with chylothorax are at risk for potential complications, especially fibrosing pleuritis.
Potential Complications of Chylothorax Surgery
Since a dog with chylothorax is likely to be metabolically compromised, the animal has an increased anesthetic risk.
Surgery is time-consuming
Potential complications include persistent accumulation of chyle within the thoracic cavity, infection, hemorrhage, and damage to the nerves that control the diaphragm.
The general prognosis for chylothorax in dogs is “guarded to poor” even with various options for medical and surgical management of the problem.
Prolonged exposure of the linings of the lungs and heart to chyle can lead to inflammation. Chronic cases can also lead to constrictive pleuritis that may require further surgery (decortication) to remove the restrictive fibrous tissue covering the lungs.
After surgery, pain medications are generally administered. The chest tube is aspirated regularly. Oxygen supplementation may also be provided after surgery if the patient’s condition requires it.
Once the production of fluid on the chest tube resolves or is significantly reduced to allow the removal of the chest tube, most patients can be discharged. In successful cases, chyle effusion generally resolves within several days to weeks after surgery.
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by Veterinarian Alex Crow.