The Case of the Laboring Lab
1. Referral:
“Sam” is a five year-old neutered male Lab cross* that presented for a two week history of exercise intolerance and increased effort of breathing. No coughing or nasal discharge had been noted. Pleural effusion was noted on thoracic radiographs. Thoracentesis was performed which yielded a milky fluid. Sam was referred for a potential chylothorax.
2. Exam:
The physical exam on Sam was unremarkable except that he was tachypneic and had dull heart and lung sounds on auscultation.
3. Procedures:
Additional thoracic radiographs were obtained. A thoracentesis was performed and submitted to the lab for fluid analysis. It consisted of a milky, white fluid that contained a large population of neutrophils and lymphocytes. Chylomicrons were also noted. Triglyceride levels in the fluid were significantly higher than that in the blood. Additional diagnostics included a CBC, biochemical profile, urinalysis, heartworm test, echocardiogram, and abdominal and thoracic ultrasound. All of these tests were normal.
4. Diagnosis:
Based on the laboratory findings, most notably the elevated triglyceride level and presence of chylomicrons in the pleural effusion, and the lack of any other significant findings, a diagnosis of idiopathic chylothorax was made.
5. Treatment:
After a thorough discussion with the owner Sam was taken to surgery. A right flank approach was made to isolate lymphatic vessels at the ileocecocolic junction. A lymphangiogram was performed to identify the thoracic duct and its branches. The lymphangiogram showed an abnormal thoracic duct, especial its terminus in the cranial mediastinum. Methylene blue was injected into the thoracic duct which permitted direct visualization of the thoracic duct. A separate right T9-10 intercostal thoracotomy was performed. After dissection of the mediastinal pleura, the thoracic duct could be visualized intimately associated with the aorta. The duct was isolated and ligated with silk and hemaclips. A pericardectomy was performed ventral to the phrenic nerves. A post- ligation lymphangiogram showed no further patent ducts. A thoracostomy tube was placed to facilitate evacuation of any remaining chyle, fluid, and air. The tube was removed 7 days postoperatively as the chyle production was significantly diminished.
6. Discussion:
Chylothorax can be caused by many different disease processes, including heartworm, neoplasia, any disease that causes obstruction of the thoracic duct in the cranial mediastinum (e.g., an abscess or granuloma), heart disease, and surgical trauma. Rupture of the thoracic duct has not been reported. In Sam’s case, no inciting cause was identified so idiopathic chylothorax was diagnosed, the most common diagnosis associated with chylothorax. Ligation of the thoracic duct and pericardectomy are the two procedures most commonly used to treat the disease. Other procedures include embolization of the thoracic duct, cisterna chyli ablation, and omentalization of the thorax, among others. Pericardectomy is usually included as part of the surgical procedure because there may be restrictive pericardial disease, secondary to the inflammatory changes induced by the chyle. This can cause mild pressure changes in the thoracic lymphatics causing them to leak.
Though ligation of the thoracic duct is thought to be the best treatment at this time, it’s clear that the treatment does little to address the underlying pathophysiology, and is therefore not always successful. Currently, success as measured by resolution of the effusion, is probably 50-70%, though different studies vary in their success rates. Chylothorax is likely a very complicated disease for which the cause remains unknown.
7. Recovery:
Fortunately Sam responded well to treatment and is disease free over a year post surgery.
by Stephen A. Bilbrey, DVM, MS,
Diplomate ACVS
*Photo of Labrador Retreiver is not “Sam.”