Of all the body fluids which one might reasonably expect to drain from the chest cavity, chyle is among the most exotic, though it cannot share the pedestal with such oddities as urinothorax because chyle legitemately belongs in the chest cavity. It is the content of the thoracic duct which has the misfortune of being wrapped around structures of surgical interest. As such, it is frequently brutalised on the way to the oesophagus, pericardium, hiatus hernia or hilar lung tumour.
The first time, this subject came up in Question 10.2 from the second paper of 2013. The college presented the candidate with a picture of some creamy chest fluid, and invited them to reach the conclusion that this 75 year old man suffered some thoracic duct damage in the course of their elective oesophagectomy. This was probably viewed by many as an unsatisfying treatment of a fascinating complication. As if to address this injustice, the college made chylothorax the subject of a whole 10-mark SAQ in Question 5 from the first paper of 2019.
Definition of chylothorax
For 10% of the marks, Question 5 from the first paper of 2019 asked the trainees to define a chylothorax. The college answer defines it as "an accumulation of chyle (lymphatic fluid of intestinal origin) in the pleural space". Officially, the definition of a chylothorax is a diagnostic issue, and at a minimum requires the identification of chylomicrons in the pleural fluid.
Causes of chylothorax
An excellent article from 2010 lists several causes of chylothorax beyond the immediately obvious "the surgeon did it". Sure, he just had an oesophagectomy, but the surgery was a total high-five success; couldn't this chyle leak be caused by sarcoidosis?
In brief, the causes of chylothorax are as follows:
- Thoracic surgery
- Specifically, oesophagectomy appears to carry a 3.4% risk of chylothorax
- Neck surgery
- Radiation to the mediastinum
- Thoracic knife or bullet wounds
- Blunt thoracic trauma
- Forceful emesis or cough
- Associated with infection
- Associated with disease states:
- Haemangiomatosis or lymphangiomyomatosis
- Retrosternal goitre
- SVC obstruction
- Transdiaphragmatic movement of chylous ascites
Clinical manifestations of chylothorax
The first diagnostic hint to point towards chylothorax is the appearance of fluid in the chest drain which does not resemble anything normally seen in a chest drain. To borrow a description from Cellan-Jones et al (1940), "We decided to aspirate a moderate-sized right-sided pleural effusion, and to our surprise a quantity of fluid which resembled pale tomato soup was withdrawn". Its colour will vary: usually some level of pallor or creamyness is to be expected. Watts (1921) described is as "milk and eggs" because milk and eggs was the physician-prescribed postoperative diet for R.A, a 35 year old male admitted to the Western State Hospital for the Insane in 1918. "The next morning at breakfast he secreted a dull case knife and was found half an hour later with his throat cut. He had shoved the knife into the suprasternal notch as far as he could, moved it crosswise and up and down, said he put it in three different times". Anyway, the spectrum of chyle will certainly vary from person to person, and given the relative value of a picture as compared to a throusand words, one would be well justified in adding one's own experience to the published pallette of chyle :
It does not have to be creamy. When you've fasted the patient for many days, the lack of emulsified fat makes the drain content appear serosanguinous.
Diagnosis of chylothorax
Chylomicron content of pleural fluid is the gold standard. Chylomicrons are large (~1000 nm) lipid globules and they belong in chyle, and so to find them in pleural fluid is profoundly abnormal. Differential lipoprotein content of pleural drain fluid can be assessed by electrophoresis- the chylomicrons form a large discrete band on the gel. Studies of pleural fluid looking for other chacteristic features of chyle leak have used chylomicron presence as the unique identifying feature diagnostic of chyle.
A Sudan III stain for fat is a more rapidly available test than the gold standard electrophoresis; the dye colours only oil, and will reveal the pleural fluid to be chyle if brightly coloured chylomicrons can be identified on microscopy. In case one is wondering, the appearance is quite visually pleasing for the microscopist. Here's a bunch of fat globules from a chylous peritoneal fluid aspirate (Mishin et al, 2010)
Other pleural fluid features have also been suggested as diagnostic of chylothorax. These are not as reliable. The linked study followed the pleural fluid results of 74 patients with chylothorax.
- Milky colour, as already mentiond, is fairly unreliable as only 44% of these patients had milky chest drain fluid
- Protein and LDH were usually high, but varied wildly. In 14% of patients, Light's criteria would have declared the fluid a transudate.
- Pleural fluid triglyceride level was high in most, but not all patients- again, 14% had a reasonably normal pleural triglyceride level.
- Overall, authors of the abovelinked study had concluded that "the use of traditional triglyceride cutoff values used in excluding the presence of chylothorax may miss the diagnosis in fasting patients".
- Fasting test and fat challenge is the answer to such concerns. Chylomicrons are only found in the circulation within 3 hours of eating; they disappear in the fasted state. Chyle transportation is maximal after a high fat meal. One is therefore able to test for a chyle leak by observing the chest drain output after some sort of "fat challenge", typically with ice cream.
- CT and lymphangiography may be helpful but practically speaking this could be difficult.
Management of chylothorax
Management can be separated into conservative and surgical. Surgical ligation of the duct is always an option, and some say it should be the first option, but this is nt without risk. In general, experts (eg. Bender et al, 2015) suggest that if the chyle leak is small in volume (less than 500ml/day), conservative measures may be enough.
Conservative measures consist of:
- Low fat diet - or at least a diet rich in medium-chain triglycerides (hoping with reduced chyle flow the defect will close on its own)
- TPN may be required - but is not an essential part of management. If one were to use TPN, one may wish to use medium-chain fatty acids - i.e. anything with a chain length shorter than laurate (12 carbon atoms).
- It seems octreotide is an effective treatment option, as is the older somatostatin
- Taking advantage of the fact that the thoracic duct is essentially a tubular structure made up of smooth muscle, some people have started using sympathomimetic drugs to vasoconstrict it, reducing the flow of chyle. Guillem et al (2004) used etilefrine, an α- and β-adrenergic indirect sympathomimetic given as an infusion. The paper does not mention why they chose this agent instead of another sympathomimetic; certainly there does not seem to be anything uniquely duct-selective about it.
Surgical options are numerous and would have to be tailored individually. Historically approaches to the management of chylothorax have included:
- Thoracic duct ligation (just proximal to the aortic hiatus, in case you don't know where the leak is coming from)
- Talc pleurodesis
- Percutaneous lymphography and embolisation, where the thoracic duct is cannulated and has either a liquid glue sealant or coils shot up into it (Itkin et al, 2010).
- Pleuroperitoneal shunt - where, for whatever reason, thoracic duct ligation is impossible, a shunt may be formed to convert a chylothorax into a chyloperitoneum.
Why not just leave it?
Sure, to reopen a chest full of chyle seems to be a distatestful admission of defeat, especially if it was your own surgical skill which caused it. Indeed, what are the arguments for aggressive repair? What are the consequences of chyle leak and chylothorax?
- There is a serious mortality rate, reported up to 50%.
- The chyle collection is not a benign resident of the chest caivity.
- The overlying lung will collapse and become infected.
- The chyle itself can become infected, and turn into an empyema.
- Chyle is fat on its way to the systemic circulation. Loss of chyle into the chest drain will result in the loss of numerous useful substances:
- Caloric intake will be impaired
- Fluid loss will occur
- Chyle is full of lymphocytes; the loss of lymphocytes will occur (and it is diffuclt to replenish them)
- Fat-soluble vitamins will be depleted, and their replacement will be ineffective