Embolizing the Thoracic Duct

a.k.a. "misadventures in thoracic surgery"

Rarely, the thoracic duct is damaged during thoracic surgery, and can be detected as a left-sided chylous pleural effusion (chylopericardium has also been reported). The traditional treatment for thoracic duct injury with left chylothorax has been clipping the thoracic duct followed by pleurodesis, but in 1998 percutaneous access of the thoracic duct was first attempted, and a 45-71% success rate of percutaneous embolization has lead some to try it as a first pass to avoid surgery.

Transection of the thoracic duct is not the only way to cause a chylothorax, however.  It has been noted that compression or stenosis of the thoracic duct leads to the development of small, fragile collaterals running in parallel.  These thin collaterals are easily damaged and can rupture into the pleural space.


First, a little bit about the thoracic duct...  The lymphatics begin as end-bulbs (lacteals in the intestine) and merge into progressively larger lymphatic channels. The lymph moves centrally due to the action of one-way valves, spontaneous contraction of lymphatic channels, and muscular or organ activity. These channels eventually converge around the L1-L2 vertebra. If the confluence is centralized and fusiform, it's termed a cisterna chyli. The thoracic duct arises from this level and rises between the esophagus and aorta, to empty into the left internal jugular or left subclavian vein... thus returning the lymph to the vascular blood stream.  The thoracic duct can have multiple or duplicated channels within it.  Lymph at the periphery is clear and colorless, but lymph originating at the intestine (from the lacteals) is often cloudy since it is a suspension containing fat (about 60-70% of fat makes its way to the bloodstream through the lymphatic system).













So how do you access the thoracic duct?  Well... that's the trick.  It would seem tempting to try to cannulate it in a retrograde manner through its cephalad venous confluence, but this is not only very technically difficult, but if there is a complete transection of the skinny thoracic duct, then you also won't be able to embolize the outflow channel.

So instead, you have to go antegrade... but how do you find the upstream entrance to the thoracic duct?  The trick is to perform a pedal lymphangiogram first as a road map. First lymphazurin is injected into the subcutaneous soft tissues (at the interdigital spaces of the first, second, and third toes). A small incision is then made and the colored lymphazurin ("lymph" + "azure") highlights the foot lymphatic channels in the subcutaneous tissues. A lymphatic channel is then selected and cannulated with a 30 gauge catheter, after which lipiodol is slowly infused (below).  Multiple lymphatic channels can be cannulated to try to increase the amount of contrast in the lymphatic system.

Lipiodol contrast extending up the lymphatics of the lower extremity toward the pelvis, and from there to the abdomen and cisterna chyli.

Hemodynamics: Collateral Flow

The concept of collateral flow in the vascular circuit is not too difficult to appreciate and has a wide variety of applications in arterial physiology.  The concept is similar to flow in electrical circuits.  If in parallel, vascular resistances are additive as reciprocals, that is...

1/Rt = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn

For a vascular circuit like the one above, this means that flow through all resistances (Rt) is greater than flow through any one resistance. This makes intuitive sense.

The resistance of any one tube can be estimated by the Pouiselle law (see post from 10/28/12), so:

laminar resistance in a tube (R) = (8 η L) / (π r^4 )

May-Thurner Syndrome

The left common iliac vein is in a tight spot, caught as it is between the right common iliac artery and the lumbar/sacral spine. Like the "Nutcracker" syndrome (see post "10/3/2012"), some people are more prone to AP compression of the abdomen and pelvis for whatever reason.  Oddly enough, the demographics for May-Thurner syndrome, like the Nutcracker Syndome is mostly female (3:1), younger (10s-30s), and it also occurs more frequently in patients who have had multiple pregnancies.

This relative narrowing at the downstream common iliac vein is theorized to lead to an increase in left-sided deep venous thrombus. May-Thurner is really just a subset of central venous occlusion syndromes... and as with central venous occlusion, lower extremity venous stasis is not appreciably helped by change in position (such as with stasis from valvular incompetence). Nor would the Doppler waveform change appreciably with Valsalva.... nor would it resolve with compression stockings or leg exercise.

Compression of the left iliac vein can result in a rage of presentations: from asymptomatic (with a pressure gradient across the compression of < 2 mmHg), so the development of venous "spurs" (described below), to the development of extensive pelvic collaterals with or without pelvic and lower extremity thrombosis.(May-Thurner syndrome).

Stenosis at the characteristic May-Thurner location.  The left iliac vein is also smaller than its counterpart, presumably due to slow flow.

Hemodynamics - Pouiselle's Law

Although a very simplified version of blood flow through a vessel -- Pouiselle's model for flow in a cylindrical tube is useful for arranging the relationships between variables in the hemodynamic circulation. In particular, the incredible importance of blood flow on the radius of the vessel (directly proportional to the fourth power), which is conceptually applicable in all sorts of clinical situations (vasodilators/vasocontrictors/atherosclerosis/etc.)