May-Thurner syndrome occurs when a vein in the pelvis becomes compressed by an artery. It often does not cause symptoms, but it can narrow the vein and impair blood flow, which raises the risk of deep vein thrombosis (DVT).

DVT is a blood clot that develops in a deep vein. DVT is potentially serious as it can sometimes result in a pulmonary embolism. However, most people with May-Thurner syndrome do not experience this complication.

Keep reading to learn more about May-Thurner syndrome, including the symptoms, causes, risk factors, treatments, and recovery.

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May-Thurner syndrome occurs when one blood vessel in the pelvis compresses another. These blood vessels are the right iliac artery and the left iliac vein.

The right iliac artery carries oxygen-rich blood from the heart toward the right leg. The left iliac vein carries oxygen-depleted blood from the left thigh toward the heart.

In May-Thurner syndrome, the right iliac artery presses on the left iliac vein. Some degree of compression appears to be a variation in anatomy, but in some people, this compression narrows the vein, impairing blood flow. This can lead to blood pooling in the legs and the potential risk of DVT.

DVT occurs when a blood clot forms in a deep vein. May-Thurner syndrome is the cause of 2–5% of all DVTs, but some evidence suggests that the incidence may be higher.

Although May-Thurner usually affects the left vein, there are rare reports of the condition on the right side. Other names for the condition include iliac vein compression syndrome and Cockett syndrome.

May-Thurner syndrome can be serious if it significantly blocks blood flow. At this point, the condition can be life threatening. A person may develop DVT, which sometimes results in a pulmonary embolism — the sudden blockage of one of the arteries in the lungs.

However, most people with May-Thurner syndrome do not experience this. Other blood vessels may compensate for the narrowing of the left iliac vein, meaning there is still adequate blood flow to the heart.

In other cases, the narrowing of the left iliac vein may not be severe enough to cause complications. If it is, treatment can help substantially.

Most people with May-Thurner syndrome do not have symptoms. In some cases, subtle signs or symptoms appear in the left leg. They can include:

  • tightness that disappears after a night’s sleep
  • hyperpigmentation
  • swelling
  • venous ulcerations, which are open skin sores
  • telangiectasias, which are small widened blood vessels in the skin

These symptoms are not specific to May-Thurner syndrome, so a person should not assume they have the condition if they experience these symptoms.

Doctors are not sure why May-Thurner syndrome develops in some people and not others. Initially, scientists thought it might be an anatomical difference present from birth, but there is some evidence it can also develop over time.

The two doctors who gave the condition its name, Dr. May and Dr. Thurner, theorized that the irritation from the pulsating artery causes the formation of bands, known as spurs, around the left iliac vein. They believed this caused the vein to narrow.

Scientists are still learning whether this theory is accurate, but they do know that certain risk factors appear to raise the risk of May-Thurner syndrome. They include:

  • female sex
  • recently given birth
  • dehydration
  • use of oral birth control
  • scoliosis, or curvature of the spine
  • prolonged immobility following surgery
  • conditions that cause a tendency to develop blood clots

To diagnose May-Thurner syndrome, doctors typically use medical imaging tests. This may include:

  • Ultrasound: This is the method doctors often use to diagnose DVT. However, it cannot always detect May-Thurner syndrome.
  • CT: CT scans can detect May-Thurner syndrome more effectively and has just under 95% specificity. It can also rule out other causes of compression.
  • MRI: Doctors sometimes use MRI scans to diagnose this condition, but it has some limitations. The compression of the vein can vary over time, so a single MRI may not capture it.
  • Venography with intravascular ultrasound: This is the gold standard for diagnosing May-Thurner syndrome. It provides an accurate measurement of the vein and provides the doctor with information about structural changes around the vein wall.

Doctors may not treat May-Thurner syndrome in people with no symptoms and a low risk of DVT. Instead, they may monitor a person to ensure they do not begin showing signs of these complications.

However, people who do have a risk of clotting may need a stent, which is a small tube doctors place inside a vein to keep it open. Doctors can insert a stent using a very thin catheter, making it a minimally invasive procedure.

Inserting a stent widens the left iliac vein, preventing blood from pooling in the legs and lowering the chance a person will get DVT. If a person already has DVT, doctors may recommend the following:

Catheter-directed thrombolysis

This procedure can break up a blood clot and allow doctors to place a stent at the same time.

To perform this surgery, a doctor will insert a catheter into the vein with the clot and administer a clot-dissolving drug. They will then insert the stent through the same catheter.

Percutaneous thrombectomy

This procedure involves using a device that allows surgeons to remove a blood clot through a small puncture in the skin. This means doctors do not need to use the medications involved in thrombolysis, which have risks.

To perform the procedure, a doctor inserts a catheter into a blood vessel. Medical imaging shows them where to direct it. They can then use the catheter to break up or extract the clot.

Surgical removal

At present, doctors do not often use open surgery to remove blood clots, but it may be necessary in some situations. For example, a doctor may suggest surgically removing a clot if stenting has not produced a good outcome.


Anticoagulation involves taking medications to prevent the formation of blood clots inside the narrowed vein.

Taking these drugs alone is not typically enough to treat DVT in the left iliac vein, so doctors typically combine this approach with a procedure to break up clots and widen the blood vessel.

There is a range of medications doctors can recommend, including:

Recovery from procedures for May-Thurner syndrome depends on a person’s condition and the type of treatment they received.

For example, catheter-directed thrombolysis and percutaneous thrombectomy are less invasive and less of a risk than open surgery to remove blood clots. People who undergo less invasive procedures tend to have shorter recovery times.

Below are some complications:

Post-thrombotic syndrome

A common complication of May-Thurner syndrome is post-thrombotic syndrome. This is chronic venous insufficiency that can manifest in symptoms such as:

  • leg heaviness
  • pain
  • itching
  • swelling

It does not cause death, but it can affect quality of life. Dissolving blood clots through mechanical or medication-induced means, as well as wearing compression stockings, may lower the risk of this complication.

Bleeding from treatment

The treatment of catheter-directed thrombolysis can increase the risk of bleeding, so it is not appropriate for individuals who have:

  • internal bleeding
  • had a stroke from disrupted blood flow in the brain
  • experienced head trauma within the past 3 months

Pulmonary embolism

Pulmonary embolism is a complication that can develop in people with DVT. Signs of this include:

If someone experiences these symptoms, they should dial 911 for immediate treatment.

May-Thurner syndrome is the compression of the left iliac vein by the right iliac artery. This often does not produce symptoms, but it can restrict blood flow to the heart and elevate the risk of DVT.

Researchers theorize that the cause stems from irritation from the pulsating artery, which may cause the development of bands around the vein. If a person has or is at risk for DVT, doctors typically treat May-Thurner syndrome with minimally invasive procedures that break up the clot and widen the vein.