May-Thurner Syndrome (Iliac Vein Compression): Causes, Risks and Outlook

May-Thurner syndrome is a condition in which a large vein in the lower tummy, the left common iliac vein, gets squeezed by the artery that crosses over it. This can slow the flow of blood out of the left leg and pelvis. For many people the squeeze causes no problems at all. For others it leads to a swollen, aching left leg, a blood clot, or widened veins in the pelvis. This guide explains how it happens, what to look out for and when it needs treatment.

What is May-Thurner syndrome?

Blood from each leg returns to the heart through an iliac vein on each side of the pelvis. The two iliac veins join to form the main vein in the abdomen, the inferior vena cava. Arteries run close by, carrying blood the other way.

In May-Thurner syndrome, the right common iliac artery presses the left common iliac vein against the lower spine. The vein is pinched flat, a bit like a garden hose under a foot. You may also see it called iliac vein compression syndrome, May-Thurner disease or Cockett syndrome.

It is named after two doctors, May and Thurner, who described the anatomy in the 1950s. It is more often recognised in women, typically between their twenties and fifties, although it can affect anyone.

May-Thurner anatomy: why the left side?

The main artery in the abdomen divides into two iliac arteries just in front of the lower spine. The right iliac artery has to cross over the left iliac vein to reach the right leg. Where it crosses, the vein sits in a tight space between the artery in front and the bone behind.

Over years, the constant pulsing of the artery can irritate the inside of the vein. Some people develop small bands of scar tissue inside it, sometimes called “spurs”, which narrow it further. This is why problems are much more common on the left side than the right.

Compression is common, and usually harmless

It is important to know that some squeezing of this vein is very common. In a well-known American study, researchers looked at CT scans from 50 people who had no leg vein symptoms. About 24% had the left iliac vein narrowed by more than half, and two-thirds had it narrowed by more than a quarter.

So a scan report that mentions iliac vein compression does not, on its own, mean you have a disease or need treatment. Doctors talk about May-Thurner syndrome when the narrowing is linked to symptoms, a clot or pelvic vein problems.

What symptoms can it cause?

When the narrowing does matter, the effects usually show up in the left leg or the pelvis:

• Swelling of the left leg, often worse at the end of the day
• A heavy, aching or tight feeling in the left leg, sometimes worse when walking
• Varicose veins in the left leg, or varicose veins that come back after treatment
• Skin changes around the left ankle, and in long-standing cases, slow-healing sores
• A deep vein thrombosis (DVT) in the left leg, which is often how the condition is first found

In women, blood that cannot drain easily through the narrowed vein may detour through veins in the pelvis. These can widen and cause a dull, dragging pelvic ache or varicose veins on the vulva or upper thigh, much like pelvic congestion syndrome. Read more about the warning signs of May-Thurner syndrome.

May-Thurner syndrome and blood clots

Because blood flows more slowly through the narrowed vein, a clot can form there and spread down the leg. Clots are more likely when other risks are added, such as pregnancy, the contraceptive pill or hormone treatment, surgery, a long flight, illness or being immobile.

DVT warning signs: get help the same day

See a doctor urgently, the same day, if you notice:

• Swelling in one leg, usually the calf or thigh, that came on over hours or days
• Throbbing or cramping pain in the leg, especially when standing or walking
• Warm skin over the painful area, or skin that looks red or darker than usual
• Hard, swollen veins that are sore to touch

Signs of a clot in the lungs: call 995

A piece of clot can break off and travel to the lungs (a pulmonary embolism). This is an emergency. In Singapore, call 995 if you have sudden breathlessness, chest pain or upper back pain that is worse when you breathe in, coughing up blood, or you feel faint, especially if you also have a painful, swollen leg.

How is it diagnosed?

Diagnosis starts with your history and an examination. Tests may include:

• Duplex ultrasound. Usually the first scan. It is good for finding clots in the leg, but the iliac veins sit deep in the pelvis and can be hard to see clearly.
• CT or MR venography. These scans map the veins and show where the artery crosses the vein. They can also rule out other causes of swelling.
• Catheter venography with intravascular ultrasound (IVUS). A thin tube is passed into the vein, contrast dye shows the flow on X-ray, and a tiny ultrasound probe looks at the vein from the inside. IVUS is the most accurate way to measure how narrow the vein is, and it is usually done at the same time as treatment.

Because compression is common in people without symptoms, a good specialist will look at whether the scan findings actually match your symptoms. Our guide to CT and MR venography explains these scans in more detail.

How is it treated?

Treatment depends on whether you have symptoms and whether there is a clot.

• No symptoms. Usually no treatment is needed. Staying active, not sitting still for long periods and not smoking help lower the general risk of clots.
• A new DVT. Blood-thinning medicine (anticoagulants) is the main treatment. For a large, recent clot, a specialist may use a catheter to dissolve or remove it, and then deal with the narrowing so the clot is less likely to return.
• Ongoing symptoms from the narrowing. The vein can be opened with a balloon and held open with a metal mesh tube called a stent. This is done through a small needle puncture, usually in the groin or behind the knee, often as a day case or with one night in hospital. A balloon alone does not work well here, because the artery keeps pressing on the vein.

After a stent, you will usually take blood-thinning or anti-platelet medicine for a period, and have follow-up scans to check the stent stays open. Some people have a dull backache for a week or two. Possible problems include a clot in the stent, re-narrowing and, rarely, the stent moving. Open surgery is now rarely needed. See iliac vein stenting for more detail.

A Singapore vascular surgeon’s patient guide to May-Thurner syndrome gives another overview of the tests and treatment.

May-Thurner syndrome in pregnancy

Pregnancy makes blood more likely to clot, and the growing womb presses on the pelvic veins. Clots in pregnancy affect the left leg much more often than the right, and left iliac vein compression is thought to be one reason.

If you have been told you have May-Thurner syndrome, or you have had a DVT before, tell your obstetrician early in pregnancy. They may suggest blood-thinning injections during pregnancy or after the birth, depending on your overall risk. Never start or stop a blood thinner without medical advice.

What is the outlook?

Most people with iliac vein compression live normal, healthy lives. For those who need treatment, a stent often eases leg swelling and heaviness, although results vary and long-term follow-up is needed. Our page on living with May-Thurner syndrome covers activity, sleep and long-term outlook. May-Thurner is one of several vein compression problems. Another, which affects the left kidney vein, is nutcracker syndrome.

When to see a doctor

See a doctor if you have ongoing swelling, heaviness or aching in your left leg, varicose veins that keep coming back on the left side, or a dull pelvic ache with vulval or upper thigh varicose veins. You can use our pelvic symptom check to organise your thoughts before an appointment.

Get urgent medical help for sudden swelling and pain in one leg. Call for emergency help if you have sudden breathlessness, chest pain, coughing up blood or fainting. In Singapore, call 995 in an emergency.

This article is general information, not personal medical advice. Please speak to a doctor about your own symptoms. How we create our content.