Some people have a narrowing of their big left pelvic vein (= left common iliac vein) that can put them at risk for a blood clot (deep vein thrombosis = DVT) in that area and in the left leg. The narrowing is due to pressure onto the vein by the overlying big pelvic artery (= right common iliac artery), shown on image 1 . This condition is called May Thurner syndrome. It is an anatomic variant people are born with, but it is not inherited from the parents. While May-Thurner syndrome causes DVT in some people, in many it causes no problems and is irrelevant.
Normally, the blood that comes from the legs flows through the deep veins of the legs and the pelvis into the big vein in the abdomen (= inferior vena cava) and then to the heart (figure 1). Normal anatomy is that the artery that goes to the right leg (= right common iliac artery) lies on top of the vein coming from the left leg (= left common iliac vein), shown in image 1. This close proximity leads, in some people, to pressure of the artery onto the vein, so that the vein is pinched between the artery in front and the vertebral body behind it. This may lead to varying degrees of narrowing of the vein and is called “May Thurner syndrome”. It is not a disease, but a congenital anatomic variant. Mild and moderate degrees of narrowing are typically asymptomatic. More severe degrees can lead to an impairment of blood flow from the leg. The narrowed vein can also clot, resulting in left leg DVT.
The syndrome is named after the physicians R. May and J. Thurner, who first described this phenomenon in 1957. It has also been termed the “iliac compression syndrome”. It is likely the reason why more DVTs, particularly during pregnancy, occur in the left leg than in the right.
How common is it?
Some degree of narrowing of the iliac vein appears to be very common. However, this has not been studied well. A small radiologic study indicated that nearly 2/3rds of people have a mild narrowing, and half have nearly 50 % narrowing of the vein (reference 1). However, how common severe narrowing, i.e. more than 90 % narrowing is, is not known.
Many people with May-Thurner syndrome have no symptoms, even if they have pronounced narrowing of the iliac vein. This is because other, smaller veins that bypass the narrowed area enlarge and drain the blood from the leg vein very effectively. These bypassing veins are called “collaterals”. Some people with severe narrowing have leg swelling and pain, symptoms that are often called chronic venous insufficiency. And, finally, some people are at increased risk for DVT from the narrowing, particularly if other risk factors (such as pregnancy, contraceptives or clotting disorders) are also present.
A routine Doppler ultrasound of the legs that is typically done to evaluate for DVT can not discover May-Thurner, as the veins in the pelvis can typically not be well seen by a Doppler ultrasound – they are too far above the groin (above the “inguinal ligament”, in medical terms) and too deep to be optimally imaged. To make a diagnosis a CT venogram (CTV) or MR venogram (MRV) are needed, or an invasive contrast venogram study. Intravascular ultrasound (IVUS) has also come in use in recent years to detect May-Thurner syndrome (references 2,3).
(a) No treatment is needed or indicated if the narrowing is coincidentally discovered and is not causing any symptoms. (b) In the patient who has chronic leg swelling, pain or even a skin ulcer (i.e. symptoms of venous insufficiency or postthrombotic syndrome) and who is found to have May Thurner syndrome, stenting of the narrowed stretch of vein (image 2) can lead to significant symptom improvement. (c) It has not been studied and is not known whether stenting is beneficial (i.e. whether it reduces the future risk of another clot) in the patient who had a DVT or PE and is found to have May-Thurner syndrome, but has recovered from the DVT and has no leg symptoms.
Since 1995 venous stents have been placed into the narrowed area of the veins, to widen them and keep them expanded (ref 4-9). Unfortunately, only few studies have investigated the long-term success of the procedure, i.e. how often the stents improve symptoms and remain patent. Stents appear beneficial at least in the short-term improvement of symptoms, within the first 1-2 years of stent placement (references 4-9). However, nearly 2/3rds of stents may close up within the first 5 years of their placement (ref 9). When that happens, repeat radiological procedures to re-open the stents can often be successfully performed. Thus, overall, nearly ¾ of all stents are open 5 years after their placement (ref 9). Unfortunately, it is not known (a) whether patients who had a stent placed should remain on long-term (life-long) blood thinners, such as warfarin (coumadin©), and (b) whether aspirin has any benefit in keeping these stents open if blood thinners like warfarin are not given.
In the past, surgical procedures were done to take the pressure off the vein by moving the overlying artery somewhere else: (a) venous bypass surgery of the narrowed area, (b) cutting of the iliac artery and repositioning of the artery behind the iliac vein, and (c) construction of a tissue sling or flap to lift the artery off the iliac vein. These surgeries are not commonly done any more, as stenting has become the main treatment.
If a patient has a fair amount of leg pain and swelling and a localized narrowing in the left common iliac vein, i.e. May Turner syndrome, I typically recommend stent placement. Once a stent has been placed I recommend at least 3 months of warfarin, target INR 2.0-3.0. Thereafter, the decision to come off or stay on warfarin depends on the patient’s risk factors for recurrent clots (DVT), such as (a) what triggered the first clot, (b) how many episodes of DVT the patient has had previously, (c) whether the patient has a strong clotting disorder, (d) whether the patient has left-over clot in his/her legs, and (e) whether a D-dimer blood test is positive or negative. If a decision is made to discontinue the blood thinner (such as warfarin), then I typically recommend long-term aspirin 81 mg per day. However, it is not known whether aspirin has any benefit in keeping venous stents open.
- Kibbe MR et al. Iliac vein compression in an asymptomatic patient population. J Vasc Surg 2004;39:937-943.
- Murphy EH et al. Device and imaging-specific volumetric analysis of clot lysis after percutaneous mechanical thrombectomy for iliofemoral DVT. J Endovasc Therapy 2010;17:423-433.
- Raju S et al. High prevalence of nonthrombotic iliac vein lesions in chronic venous disease: a permissive role in pathogenicity. J Vasc Sug 2006;44:136-144.
- Berger A et al: Iliac compression syndrome treated with stent placement. J Vasc Surg 1995;21:510-514.
- Neglen P et al.: Balloon dilatation and stenting of chronic iliac vein obstruction: technical aspects and early clinical outcome. J Endovasc Ther 2000;7:79-91.
- O’Sullivan GJ et al.: Endovascular management of iliac vein compression (May-Thurner) syndrome. J Vasc Intervent Radiol 2000;11:823-836.
- AbuRahma AF et al.: Iliofemoral deep vein thrombosis: conventional therapy versus lysis and percutaneous transluminal angioplasty and stenting. Ann Surg 2001;233:752-760.
- Heijmen RH et al.: Endovascular stenting in May Thurner syndrome. J Cardiovasc Surg 2001;42:83-87.
- Kwak HS et al. Stents in common iliac vein obstruction with acute ipsilateral deep venous thrombosis: early and late results. J Vasc Interv Radiol. 2005;16:815-822.
- Knipp BS et al. Factors associated with outcome after interventional treatment of symptomatic iliac vein compression syndrome. J Vasc Surg 2007;46:743-749.
For health care professionals
This same topic, discussed for health care professionals, can be found here.
Disclosure: I have no financial conflict of interest to this blog entry.
Last updated: Jan 10th, 2011