May Thurner syndrome is a condition where the left iliac vein gets compressed by the right iliac artery crossing over it. This compression can reduce blood flow out of the left leg and lead to deep vein thrombosis (DVT). It is not a rare condition, but it is often missed because many people do not have obvious symptoms until a clot forms. The primary cause is anatomical — the way your blood vessels are positioned from birth — and the main risk factors include being female, being between 20 and 50 years old, and having had multiple pregnancies.
What Exactly Happens in May Thurner Syndrome?
To understand May Thurner syndrome, you need to picture the major blood vessels in your pelvis. The right iliac artery and the left iliac vein cross paths. In most people, this crossing causes no problems. But in some, the artery presses hard against the vein, pinching it against the spine.
Think of it like stepping on a garden hose. The water flow slows down. In the same way, blood flow from the left leg slows. Over time, the vein wall can thicken from the pressure. This makes the narrowing worse. The sluggish blood is more likely to clot. That clot is a deep vein thrombosis, which can cause pain, swelling, and in serious cases, travel to the lungs.
The compression itself does not always cause symptoms. Many people walk around with this anatomy and never know it. The problem starts when blood flow is slow enough that a clot forms. That is when most people get diagnosed.
What Causes May Thurner Syndrome and Who’s at Risk?
The cause is anatomical. You are born with the blood vessels in this position. It is not something you develop later in life or catch from an infection. The right iliac artery sits on top of the left iliac vein. In people with May Thurner syndrome, the artery compresses the vein more than normal. The exact reason why some people have more compression than others is not fully understood. But the physical setup is present from birth.
Who gets it? The data is clear on several groups. Women are diagnosed much more often than men. Research published in the Journal of Vascular Surgery found that about 70 percent of cases are in women. Age also matters. Most people diagnosed are between 20 and 50 years old. Pregnancy is a major risk factor. The growing uterus puts extra pressure on the pelvic veins, and the hormonal changes increase the risk of clotting. Each pregnancy raises the risk further.
Other risk factors include scoliosis, which can shift the spine and change how the vessels sit against it. Being tall and thin is also linked, possibly because the vessels are stretched differently. But these are associations, not direct causes. The strongest evidence points to anatomy plus pregnancy history.
What Are the Symptoms of May Thurner Syndrome?
Many people have no symptoms at all. The condition is often found only after a DVT develops. When symptoms do appear, they are almost always in the left leg. That is a key clue. Right leg symptoms are rare with this condition.
The most common symptoms include:
- Swelling in the left leg, especially the calf and ankle
- Pain or a heavy feeling in the left leg
- Skin that feels warm to the touch on the left leg
- Visible veins on the left thigh or lower abdomen
- Leg fatigue that gets worse with standing or walking
If a clot has formed, the leg may suddenly swell more, turn red, and feel very painful. This is a medical emergency. The CDC reports that DVT affects up to 900,000 Americans each year, and May Thurner syndrome is a cause in a significant number of left-leg cases. If your left leg swells suddenly, do not wait — get to a hospital.
How Is May Thurner Syndrome Diagnosed?
Doctors do not typically screen for May Thurner syndrome in healthy people. It is usually found when someone has a left-leg DVT and the standard risk factors for clots are not present. That is when a doctor starts looking for anatomical causes.
Ultrasound is the first step. It can show a clot and measure blood flow. But ultrasound does not always see the compression clearly. The iliac veins are deep in the pelvis, and ultrasound has limits there. If the ultrasound suggests May Thurner syndrome, the next step is often a venogram. This is an X-ray where dye is injected into the vein. It shows exactly where the narrowing is and how severe it is.
Another tool is intravascular ultrasound (IVUS). This is a small ultrasound probe on a catheter that goes inside the vein. It gives a very clear picture of the compression. Research from the Society for Vascular Surgery shows that IVUS finds more compressions than regular venograms. It is becoming the gold standard for diagnosis.
What Treatment Options Are Available?
Treatment depends on whether a clot is present. If there is an active DVT, the first step is to remove the clot. This is done with a procedure called catheter-directed thrombolysis. A doctor threads a thin tube into the clot and delivers clot-busting drugs directly. Sometimes a device is used to break up and remove the clot mechanically.
Once the clot is gone, the compression still remains. The standard treatment is to place a stent in the left iliac vein. A stent is a small metal mesh tube that holds the vein open. It keeps the artery from pinching it closed. The American Heart Association notes that stenting for May Thurner syndrome has high success rates, with most people having good blood flow afterward.
After stenting, people take blood thinners for several months to prevent the stent from clotting. The length of time varies. Some people stay on them for 6 months. Others take them for a year or longer. Your doctor will decide based on your specific case.
For people who do not have a clot yet but have symptoms like swelling, the decision is less clear. Some doctors recommend stenting to prevent future clots. Others prefer to monitor and treat only if a clot develops. There is no strong evidence yet that stenting in people without clots is always the right move. It is a conversation you need to have with a vascular specialist.
What Are the Long-Term Outcomes and Risks?
For people who get treated, the outlook is generally good. Studies show that stenting keeps the vein open in over 80 percent of people at five years. Symptoms like swelling and pain often improve. The risk of another DVT in the same leg drops significantly.
But stenting is not without risks. The stent can move or break, though this is rare. The biggest risk is that the stent itself clots. This happens in about 5 to 10 percent of cases. When it does, you need another procedure to clear it. Taking blood thinners as prescribed lowers this risk.
Another long-term issue is post-thrombotic syndrome. This is chronic leg pain, swelling, and skin changes that happen after a DVT. Even with successful stenting, some people develop this. It is more common if the clot was large or if treatment was delayed. Compression stockings can help manage symptoms.
Frequently Asked Questions
Frequently Asked Questions
Can May Thurner syndrome go away on its own?
No. The anatomical compression is permanent unless treated. Symptoms may come and go, but the physical narrowing does not resolve without intervention.
Is May Thurner syndrome genetic?
There is no known direct genetic cause. The anatomy is present at birth, but no specific gene has been linked to it. Family history of DVT may increase risk though.
Can men get May Thurner syndrome?
Yes, but it is less common. Studies show about 30 percent of cases are in men. The risk factors are different — men are less likely to have pregnancy-related compression.
Does May Thurner syndrome always require a stent?
Not always. If no clot has formed and symptoms are mild, monitoring may be enough. Stenting is standard when a DVT has occurred or symptoms are severe.
