What is chylothorax?
The lungs are surrounded by two membranes known as the pleurae. The parietal pleura is the outer membrane, which is attached to the chest wall, the inner one is known as the visceral pleura and is attached to the lung. In between these two membranes is the pleural cavity. The pleural cavity contains a small amount of 'pleural fluid', which lubricates the pleural surfaces.
Some diseases cause a build up of fluid which collects in the pleural space resulting in compression of the lungs. This build up of fluid is known as 'pleural effusion'. Different types of fluid can accumulate in the pleural space. Blood (hemothorax), pus (pyothorax), serous fluid (hydrothorax) and chyle (chylothorax).
Chylothorax is an accumulation of a milky fluid known as chyle within the pleural cavity. Chyle is a highly fatty lymphatic fluid draining from the intestine. Normally, chyle is channelled through the tiny ducts from one lymph node to another and collects in the thoracic duct near the heart. The thoracic duct empties into the vena cava, the large blood-carrying vein that enters the heart. 
What causes chylothorax?
Trauma can result in chylothorax. Other causes include obstruction of the thoracic duct due to heart disease, infectious disease, heartworm disease, cancer, diaphragmatic hernia or idiopathic (unknown cause).
What are the symptoms of chylothorax in cats?
- Weight loss
- Shortness of breath due to pleural effusion
- Blue-tinged mucous membranes
- They may be anorexic
How is it diagnosed?
Your veterinarian will perform a complete physical examination of your cat and obtain a medical history. He will also wish to find out the cause of chylothorax if possible. Some tests may include:
- X-ray - Fluid in the pleural space will be visible.
- A needle will be used to aspirate some of the fluid. Chyle is milky white in appearance.
- This fluid will be sent for analysis.
- Ultrasound - This may show a mass and occasionally heartworms can be seen.
- Once chylothorax is confirmed, the underlying cause is sought.
- Remove fluid and repeat x-rays to look for any masses or evidence of heart disease
- Heartworm testing.
- FeLV blood test.
How is it treated?
Where possible, treat the underlying cause.
Thoracentesis - Remove the fluid from the pleural cavity to relieve pressure. This may be performed via needle or drainage tubes may be placed in the chest for frequent drainage.
Rutin is a drug from the benzopyrone family and is readily available at health food stores may be of assistance.  The exact mechanism is unknown at this stage but it is proving to be of benefit to cats with chylothorax. For more information on rutin, read here
Surgical options may be necessary for cats with idiopathic chylothorax who fail to respond to medical management.
Surgery of the thoracic duct (thoracic duct ligation)
 Cat Health Encyclopedia - Edited by Lowell Ackermann.
 The Feline Patient - Gary D. Norsworthy, Mitchell A. Crystal, Sharon K. Fooshee, Larry P. Tilley.