May-Thurner syndrome is a condition where the right iliac artery (a major artery in your pelvis) presses down on the left iliac vein, partially blocking blood flow out of your left leg. This compression can cause leg swelling, pain, and in some cases, blood clots. The anatomy that creates this compression is surprisingly common: one study of people with no symptoms found that 24% had more than 50% compression of the vein, and 66% had more than 25% compression. Most of these people never develop problems, but for those who do, effective treatments exist.
How the Compression Works
In your lower abdomen, just above the pelvis, the right common iliac artery crosses over the left common iliac vein on its way to the right leg. In everyone, these two vessels sit close together. But in some people, the artery presses the vein firmly against the spine behind it, narrowing the vein like a garden hose pinched between two hard surfaces.
Over time, this constant pulsing pressure can do more than just narrow the vein. The inner wall of the compressed vein may develop small fibrous growths called “spurs” that further obstruct blood flow. These changes happen gradually, which is why symptoms often appear slowly over months or years rather than all at once. The restricted blood flow makes it harder for your left leg to drain properly, and in the worst cases, the sluggish flow creates conditions ripe for a blood clot (deep vein thrombosis, or DVT) to form.
Who Is Most Affected
May-Thurner syndrome is diagnosed more often in women than men, and it tends to show up in younger adults, typically between the ages of 20 and 50. Factors that increase pressure in the pelvis or slow venous blood flow can push an otherwise silent compression into symptomatic territory. These include pregnancy, use of hormonal birth control, prolonged immobility after surgery, and conditions that make blood more likely to clot. But because the underlying anatomy is so common, many people carry the compression their entire lives without ever knowing.
Symptoms to Recognize
Many people with May-Thurner syndrome have no symptoms at all. When symptoms do appear, they almost always affect the left leg. The most common signs are pain, swelling, and a feeling of heaviness or tightness in the leg. Some people notice the tightness improves overnight while they sleep, only to return during the day.
As the condition progresses without treatment, more visible changes can develop. These include darkening of the skin on the lower leg (hyperpigmentation), small clusters of visible veins near the surface, and in advanced cases, open sores or ulcers that are slow to heal. None of these symptoms are unique to May-Thurner syndrome, which is one reason it often goes undiagnosed or gets attributed to other venous problems.
The most serious complication is a DVT in the left leg. If you develop sudden, significant swelling in one leg along with pain and warmth, that warrants urgent medical attention regardless of whether you’ve been told you have May-Thurner syndrome.
How It’s Diagnosed
Standard ultrasound can detect blood clots and evaluate blood flow in the leg, but it is not reliable for diagnosing the compression itself. The vein sits deep in the pelvis where ultrasound has limited visibility. For this reason, doctors typically rely on more advanced imaging.
CT venography, a CT scan done with contrast dye to highlight the veins, has proven to have high sensitivity and specificity for confirming May-Thurner syndrome. It can show exactly where the artery compresses the vein and whether a blood clot is present. MR venography (an MRI-based approach) offers similar detail without radiation exposure. In one case series, MR venography confirmed the diagnosis in all nine patients and was even able to detect vein abnormalities that standard ultrasound had missed entirely.
The most detailed view comes from intravascular ultrasound (IVUS), where a tiny ultrasound probe is threaded inside the vein itself. This gives doctors a real-time picture of the vein’s internal shape and the degree of narrowing. IVUS is typically used during a procedure rather than as a standalone diagnostic test. In one study, it confirmed May-Thurner syndrome in 62% of patients evaluated and directly influenced treatment decisions in about half the cases.
Treatment With Venous Stenting
Endovascular stenting has become the first-line treatment for May-Thurner syndrome. The procedure is minimally invasive: a catheter is guided through a vein (usually accessed at the groin or behind the knee) to the compressed area, and a small metal mesh tube (stent) is placed to hold the vein open. If a blood clot is present, it’s typically cleared first using clot-dissolving medication or mechanical devices before the stent goes in.
The results are encouraging. In one study, 93% of patients with blood clots and 95% of those treated for swelling alone reported complete or partial symptom relief. At three years, stent patency (meaning the stent remained open and functional) was 91% for both groups, with secondary patency reaching 95% after any needed touch-up procedures.
Long-term outcomes do vary depending on whether a blood clot was involved. A separate study tracking patients over three years found patency rates of 93.6% at one year and 87.5% at three years for patients without clots. For those who had clots before stenting, the numbers were lower: 85.4% at one year and 66.7% at three years. Re-intervention was uncommon in both groups, with most needed procedures happening within the first six months.
Blood Thinners After Stenting
After a stent is placed, you’ll typically be prescribed blood-thinning medication to prevent clots from forming inside the stent. The optimal type and duration of this therapy is still an open question in medicine, and practices vary between doctors.
The general approach depends on what triggered the problem. If a clear temporary risk factor was involved, such as recent surgery or estrogen-based medication, a course of about six months of blood thinners is common. For people without an obvious trigger, doctors may continue treatment for 6 to 12 months, particularly in younger patients. Studies suggest that longer courses of blood thinners (more than six months) are associated with better stent patency at one year: 89% compared to 78% for those who stopped at six months.
Conditions That Look Similar
Because the symptoms of May-Thurner syndrome overlap with many other conditions, getting the right diagnosis can take time. Chronic venous insufficiency, where leg vein valves weaken and allow blood to pool, produces very similar swelling, skin changes, and heaviness. Pelvic congestion syndrome, caused by enlarged veins in the pelvis, can create overlapping pelvic and leg pain. Nutcracker syndrome, another compression problem where the left kidney vein gets squeezed between two arteries, causes blood in the urine and abdominal pain rather than leg symptoms, but it sometimes coexists with May-Thurner syndrome.
The key distinguishing feature of May-Thurner syndrome is that symptoms are isolated to the left leg. If both legs are equally affected, the cause is more likely something systemic. If swelling is limited to the left side, especially in a younger woman with an otherwise unexplained DVT, May-Thurner syndrome deserves a closer look with appropriate imaging.