What is May Thurner Syndrome?
May Thurner Syndrome (MTS) is an anatomic variant in which the right common iliac artery compresses the left common iliac vein against the bone of the spine. Depending on the amount of compression, blood flow in the left common iliac vein will slow down or even stop, leading to a blood clot or deep vein thrombosis (DVT). Other conditions associated with MTS include left leg swelling that is mistaken for lymphedema, chronic pelvic pain or pelvic venous insufficiency (i.e. pelvic congestion syndrome), and low back pain.
Is May Thurner Syndrome a new condition?
No. Left leg swelling due to left iliac vein compression was first described in 1908 by Dr. James Playfair McMurrich, a professor of anatomy at the University of Michigan. Later in 1956, Drs. May and Thurner defined it anatomically based on cadaver autopsies. In 1965, Drs. Cockett and Thomas made the clinical association between iliac vein compression syndromes and leg swelling.
How is May Thurner Syndrome diagnosed?
First, MTS is a clinical diagnosis, not an imaging diagnosis. Compression of the left iliac vein can be seen on many type of imaging including ultrasound (US), CT, or MRI. Only by a thorough physical exam and history of symptoms can the diagnosis of MTS be made. The imaging helps to support the clinical diagnosis NOT make the diagnosis. The reason for this is that a large percentage of the population will have compression (sometimes severe compression) of the left iliac vein on imaging and have no symptoms their entire life. This does not mean that they all have MTS. In order to have MTS, one needs to have a “syndrome” which includes “symptoms”. If there are no symptoms, than there is no syndrome.
Secondly, compression see on imaging may not be “real” compression. There is a wide range of what is considered “normal compression” on imaging. In fact, certain conditions such as dehydration can make an iliac vein look smaller and narrower or “more compressed”. With rehydration, this narrowing can significantly improve.
Third, as technology has advanced, vascular experts have learned that US and CT/MRI give a “one moment in time” snapshot of the iliac vein. Instead, intravascular ultrasound or IVUS is a technology that allows an ultrasound probe to be placed INSIDE the vein so that the physician can see in real time whether there is compression of the vein. The procedure that is done to evaluate the vein with IVUS is known as a venogram.
In May Thurner Syndrome, why does the artery compress the vein?
Blood coming from the heart into the arteries is under very high pressure. If a normal blood pressure is 120/80, there is 120 mmHg of pressure inside the arteries. Veins, on the other hand, are very low pressure vessels with approximately 5 to 15 mmHg of pressure inside them. The arteries are only a few millimeters away from the veins and so the high pressure right common iliac artery, whose walls pulsate and bulge out with each heartbeat, compresses the low pressure left common iliac vein crossing under it.
How is May Thurner Syndrome treated?
MTS is treated by placing a stent inside the left common iliac vein. This stent is permanent and cannot be removed. The stent prevents the right common iliac artery from compressing the left common iliac vein which now has a rigid piece of metal inside it. In very rare instances in which there is hesitancy to place a stent due to the young age of the patient (i.e. teenager), a physician may decide to do balloon angioplasty alone. While this may help to break up some of the clot and scar tissue in the vein, this technique often fails in the long run as it does not fix the constant compression of the iliac vein by the artery. In such cases, patients should be educated about the high risk of DVT recurrence and that a stent may be needed in the future.