May’s sign is not a widely recognized standalone medical eponym, and if you’ve come across this term, it most likely refers to the hallmark finding associated with May-Thurner syndrome: compression of the left iliac vein by the right iliac artery, which can be detected through imaging or suspected based on characteristic physical signs in the left leg. This vascular condition affects roughly 2 to 5 percent of people investigated for lower extremity venous disorders, and it often goes undiagnosed until complications develop.
What May-Thurner Syndrome Is
May-Thurner syndrome is an anatomical problem where the right iliac artery, which carries blood down to the right leg, presses against the left iliac vein where both cross paths in the pelvis. This compression narrows the vein and restricts blood flow returning from the left leg to the heart. Over time, the reduced flow can cause blood to pool, increasing the risk of deep vein thrombosis (a blood clot in a deep vein) and chronic venous insufficiency.
The condition is named after Robert May and J. Thurner, who described this anatomical compression pattern in the 1950s. Because the compression happens internally and can exist without symptoms for years, many people never know they have it until a clot forms or leg swelling becomes persistent enough to prompt investigation.
Typical Symptoms and Who Gets It
Many people with May-Thurner syndrome have no symptoms at all. When symptoms do appear, they concentrate in the left leg and can range from mild to severe. The most common presentation is a younger woman with left lower extremity swelling that improves with rest and elevation. Subtle early signs include a feeling of tightness in the left leg that resolves during sleep, mild swelling, visible small veins (telangiectasias), or skin discoloration.
As the condition progresses, symptoms can worsen to include:
- Pain and swelling in the left leg, sometimes with sudden onset
- Skin changes such as darkening (hyperpigmentation) over the lower leg
- Venous claudication, a cramping or heavy sensation in the leg during walking
- Venous ulceration, open sores that develop from chronic poor circulation
The clinical presentation varies considerably from person to person. Some patients experience a gradual worsening of venous insufficiency over months or years, while others present with the sudden onset of left leg pain and swelling. Acute episodes are more common following periods of immobility, after surgery, or during pregnancy and the postpartum period, all situations where blood flow naturally slows and clot risk increases.
How It’s Detected
Because the symptoms overlap with many other causes of leg swelling, May-Thurner syndrome is easy to miss on a standard physical exam. A swollen left leg with skin changes might initially be attributed to varicose veins or a simple blood clot without anyone investigating the underlying compression.
The key to diagnosis is imaging that can visualize the pelvic veins. Ultrasound is often the first step, though it can be limited in viewing deep pelvic structures. More definitive imaging includes CT venography or MR venography, which create detailed pictures of the veins and can show exactly where the artery is compressing the vein. In some cases, intravascular ultrasound (a tiny ultrasound probe threaded into the vein itself) provides the most accurate measurement of how much narrowing is present.
Why the Left Leg Is Almost Always Affected
The anatomy of the pelvis explains why May-Thurner syndrome is overwhelmingly a left-sided problem. The right iliac artery naturally crosses over the left iliac vein as both travel through the lower pelvis. This crossing point creates a pinch between the artery in front and the spine behind. While right-sided and bilateral variants exist, they are rare. If you’re experiencing unexplained swelling or discomfort specifically in the left leg, this anatomical quirk is one of the reasons your doctor might consider May-Thurner syndrome.
Treatment and What to Expect
Treatment depends on the severity of symptoms and whether a blood clot has already formed. For people with a clot, the first priority is dissolving or removing it, typically through catheter-based procedures where a thin tube is guided into the vein to deliver clot-dissolving medication directly to the blockage.
The underlying compression itself is most commonly treated with a venous stent, a small mesh tube placed inside the vein at the point of compression to hold it open. This is a minimally invasive procedure performed through a small puncture, usually in the groin or behind the knee. Most patients go home the same day or the next. After stenting, you’ll typically take blood-thinning medication for several months to prevent new clots from forming around the stent while the vein heals.
For people with mild symptoms and no clot, compression stockings and monitoring may be sufficient. However, if symptoms progress or a clot develops, intervention becomes more urgent. Long-term outcomes after stenting are generally good, with most patients seeing significant improvement in swelling and pain. Stent patency rates (the percentage of stents that remain open) are high, though periodic follow-up imaging is standard to ensure the stent continues to function properly.
Risk Factors Worth Knowing
Beyond the anatomical predisposition, several factors raise the likelihood of May-Thurner syndrome becoming symptomatic. Prolonged immobility from long flights, bed rest, or sedentary work increases clot risk in an already compressed vein. Hormonal factors play a role too: oral contraceptives, hormone replacement therapy, and pregnancy all increase the blood’s tendency to clot. Scoliosis or other spinal conditions that alter pelvic alignment can worsen the compression. Dehydration, recent surgery, and conditions that make blood “stickier” (such as inherited clotting disorders) compound the risk further.
If you’ve had an unexplained deep vein thrombosis in your left leg, especially if you’re younger and don’t have typical clot risk factors, May-Thurner syndrome is worth discussing with your doctor. It’s one of those conditions that, once identified, has a straightforward treatment path, but it requires someone thinking to look for it in the first place.