May-Thurner Syndrome

Blood from both legs flows back to the heart through the veins in the pelvis, called the iliac veins, which then join to form a large vein in the abdomen called the inferior vena cava. In the lower abdomen, the right iliac artery crosses directly in front of the left iliac vein. In some people, this artery presses the vein against the spine beneath it.

Ongoing pressure, combined with the pulsing of the artery, can cause the inner wall of the vein to thicken and form bands or scar tissue inside it. As a result, blood flow from the left leg is obstructed. This condition is called May-Thurner syndrome, after the two doctors who described it in 1957, and is also known as iliac vein compression syndrome or Cockett syndrome.

Many People Have No Symptoms

Compression of the left iliac vein is fairly often found on imaging tests in people who have no complaints at all. One study using abdominal CT found compression of more than half the vein's diameter in about a quarter of people without symptoms. That is why compression alone does not always need treatment. The term syndrome is used when the compression causes symptoms or blood clots.

Two Main Forms

The first form is a blood clot in the deep veins of the left leg that often extends up to the thigh and pelvis. This clot often appears when there is an additional trigger, such as pregnancy, the period after childbirth, birth control pills, surgery, or long periods without moving. Deep vein thrombosis is covered on the Deep Vein Thrombosis page.

The second form develops slowly without a clot: the left leg swells and feels heavy or painful when walking, followed by varicose veins, skin changes, and eventually ulcers, covered on the Chronic Venous Insufficiency page. In women, this compression can also play a role in chronic pelvic pain caused by widened pelvic veins.

Treatment Based on Symptoms

The 2021 ESVS guidelines on venous thrombosis and the 2022 ESVS guidelines on chronic venous disease address the management of iliac vein obstruction. Clots are treated with blood thinners, and in selected patients the clot is removed through a catheter and a stent is then placed in the compressed vein. Compression without symptoms usually only needs monitoring.

Your doctor asks about your symptoms and any history of blood clots, pregnancy, and hormone use, then compares both legs. Swelling or a clot that affects only the left leg, especially up to the thigh, is an important clue.

Duplex ultrasound is the first step to look for clots in the leg veins and assess the pattern of blood flow from the pelvis. The pelvic veins lie deep, so ultrasound is often not enough to see the compression. Your doctor may then order CT venography or MR venography to see the location and severity of the compression and rule out pressure from other causes, such as a mass in the pelvis.

If a procedure is being considered, catheter venography and ultrasound from inside the vessel (IVUS) give a more accurate measurement of the narrowing. Because compression is also often found in people without symptoms, your doctor always interprets imaging results together with your symptoms. Blood tests may be done to look for a tendency to clot easily and to check kidney function before contrast dye is used.

Make an appointment with your doctor if your left leg often swells, feels heavy, or develops varicose veins without a clear cause, especially if you have had a blood clot before. Go to the emergency room right away if your left leg suddenly becomes swollen and painful, or if you develop sudden shortness of breath and chest pain.

• Left leg swelling that persists or keeps coming back
• Leg pain when walking that eases when the leg is raised
• Varicose veins in the left leg, inner thigh, or around the genitals
• A history of clots in the left leg, especially at a young age
• Plans to become pregnant or use birth control pills after having had a clot
• New swelling or pain after a vein stent is placed