How Common Is May Thurner Syndrome And Why Its Missed?

May-Thurner syndrome is more common than most people realize — and far more often missed than diagnosed. The condition happens when the right iliac artery presses on the left iliac vein in the pelvis, narrowing the vein that drains blood from the left leg. Because many people have the compression without any symptoms, it sits quietly in the body for years. When it does cause problems, it is frequently mistaken for a pulled muscle, a normal pregnancy change, or a clot that came out of nowhere.

What Is May-Thurner Syndrome?

May-Thurner syndrome is an anatomical problem, not a disease you catch. The right common iliac artery crosses over the left common iliac vein near the lower spine. In some people, that crossing creates persistent pressure on the vein.

The pressure itself is not always the issue. Over time, the vein wall can respond to the constant rubbing by building up fibrous tissue inside. This is sometimes called a “spur.” The result is a narrowed vein that struggles to move blood back up from the left leg.

Doctors also call this condition iliac vein compression syndrome. The name May-Thurner comes from two physicians who described the anatomy in the 1950s. The underlying anatomy had actually been noted decades earlier.

Here is the part that surprises many people: the compression is a normal variant of human anatomy. It appears on imaging in a large share of healthy adults who have no symptoms at all. The compression only becomes a “syndrome” when it starts causing real problems — swelling, pain, or a blood clot.

How Common Is May Thurner Syndrome And Why Is It Missed?

This is the central question, and the answer has two parts. The anatomy is common. The diagnosis is not.

Studies that looked at pelvic imaging in people without leg symptoms have found the compression in a substantial portion of them. Estimates vary widely by study and by how strictly compression is defined, but the general finding is consistent: a large minority to a majority of adults show some degree of left iliac vein compression on scans, and most of them feel fine.

Symptomatic May-Thurner syndrome is far less common than the anatomy itself. It is diagnosed more often in women than men, and it tends to show up between the ages of 20 and 50. It is also seen more often in people who have had multiple pregnancies, though the reasons are not fully pinned down.

So why is it missed so often?

• The symptoms are vague. Left leg swelling and heaviness overlap with dozens of other conditions.
• Standard leg ultrasound does not look at the pelvic veins. It images the leg, not the source higher up.
• It often appears during pregnancy, when leg swelling is expected and dismissed.
• Many clinicians are trained to look for clots, not for the anatomical cause behind them.
• Confirmation usually requires specific pelvic imaging that is not part of a routine workup.

The result is a condition that is genuinely underdiagnosed. That does not mean every swollen leg is May-Thurner. It means the diagnosis is easy to overlook when the presentation is not dramatic.

What Symptoms Does It Cause?

Many people with the compression have no symptoms at all. When symptoms do appear, they usually affect the left leg, because the left iliac vein is the one being compressed.

Common symptoms include:

• Swelling in the left leg or ankle
• A heavy, full, or aching feeling in the left leg
• Pain that worsens with standing or activity
• Visible surface veins on the left leg or thigh
• Skin that looks darker or discolored over time

The right leg is usually unaffected. That left-sided pattern is one of the few clues that points toward this condition specifically.

Symptoms often get worse during pregnancy, after long periods of standing, or during the menstrual cycle in some women. They may ease with rest and leg elevation. This pattern is not unique to May-Thurner, which is part of why it blends in with other diagnoses.

What Is the Connection to Blood Clots?

The most serious consequence of May-Thurner syndrome is deep vein thrombosis, or DVT — a blood clot in a deep vein. When the left iliac vein is narrowed, blood flow slows and can become turbulent. Sluggish flow is one of the conditions that allows clots to form.

This matters because a clot in the pelvic veins can travel. If it reaches the lungs, it becomes a pulmonary embolism, which is a medical emergency. This is not a common outcome, but it is the reason the condition is taken seriously when it is found.

May-Thurner is also found in a meaningful share of people who develop a DVT in the left leg without an obvious cause. In younger people and in those without typical risk factors, the proportion is higher. This is one reason some clot specialists now consider iliac vein compression when a left-leg DVT appears without explanation.

If you have sudden swelling, pain, warmth, or redness in one leg — especially the left — seek medical care promptly. Those can be signs of a clot.

How Is It Diagnosed?

Diagnosis starts with a careful history and physical exam. A clinician who suspects May-Thurner will usually order imaging of the pelvic veins, not just the leg.

The most common confirmatory test is a procedure called venography, sometimes combined with an intravascular ultrasound. In venography, contrast dye is used to see the vein directly. Intravascular ultrasound uses a tiny probe inside the vein to measure the narrowing from the inside, and many specialists consider it the most accurate way to assess the compression.

Other imaging, such as CT or MR venography, can also show the anatomy. A standard leg ultrasound may detect a clot but will not show the pelvic compression that caused it.

There is no single cutoff that defines how much narrowing counts as significant. Clinicians weigh the imaging findings against symptoms, and the interpretation can vary. That variability is one more reason the condition is not always recognized.

How Is It Treated?

Treatment depends on whether there are symptoms and whether a clot is present. This is an area where practice varies, and it is worth being honest about that.

If a clot is present, the first priority is treating the clot. This typically involves blood thinners. In some cases, a clinician may recommend a procedure to remove the clot or to place a stent in the narrowed vein.

For people with symptoms but no clot, options include:

• Compression stockings to help manage swelling
• Leg elevation and avoiding long periods of standing
• Blood thinners in selected cases
• A stent placed in the vein to hold it open

Stenting has become a common approach for symptomatic narrowing, and many patients report symptom relief. However, the evidence base is not as strong as it is for some other vascular procedures. There is no large body of randomized trials comparing stenting to conservative care, and long-term outcomes are still being studied. Some clinicians recommend it readily; others are more cautious. If you are offered a stent, it is reasonable to ask about the expected benefit, the risks, and how much is based on strong evidence versus clinical experience.

There is no medication or lifestyle change that reverses the underlying anatomy. The compression is structural.

Who Should Be Evaluated?

May-Thurner is not something to screen for in the general population. Most people with the anatomy never have a problem, and screening would lead to unnecessary testing and anxiety.

Evaluation makes more sense when there is a reason to suspect it. That includes:

• A left-leg DVT without a clear cause, especially in a younger person
• Persistent left-leg swelling or pain that does not have another explanation
• Symptoms that began or worsened during pregnancy and did not resolve
• Recurrent left-leg clots

If you recognize yourself in that list, it is reasonable to raise the question with a doctor who deals with vascular conditions. Bringing it up does not mean you have it. It means you are asking a fair question about a condition that is easy to miss.

Why the Diagnosis Gap Matters

The gap between how common the anatomy is and how rarely it is diagnosed is not a scandal. It reflects a real clinical challenge. A condition that is often silent, produces vague symptoms, and requires specific imaging to confirm will always be underdiagnosed.

What helps is awareness — not fear. Knowing that left-leg swelling has a specific anatomical cause worth considering is useful. Knowing that most people with the compression never develop a problem is equally useful.

The honest summary is this: the anatomy is common, the syndrome is less common, and the diagnosis is missed because the clues are quiet. If something about your left leg does not add up, that is worth a conversation.