Why is May–Thurner syndrome usually on the left?

Because of anatomy. The right common iliac artery crosses in front of the left common iliac vein, pressing it against the spine. Everyone has this crossing; in some people the compression is enough to narrow the vein and obstruct drainage from the left leg.

It is suspected from the pattern — unexplained left-leg swelling, or a left-leg DVT without another cause — and confirmed on imaging. Duplex ultrasound at consultation looks at the leg and, where possible, the pelvic veins; CT or MR venography maps the compression; and intravascular ultrasound during any procedure measures its true severity, which venography alone understates.

Does every compression need a stent?

No. Many people have some degree of compression with no symptoms and need nothing. Treatment is for compression that is causing symptoms — persistent swelling, a DVT, or post-thrombotic changes — and the decision is made on how much the vein is narrowed and what it is doing to the leg.

What does treatment involve?

Deep venous stenting: through a needle puncture, a self-expanding venous stent is placed across the compressed segment to hold it open, usually as a day procedure under local anaesthesia with sedation. Where an acute clot is present, it is cleared first. A period of blood-thinning medication and ultrasound surveillance follows.

Venous disease

May–Thurner Syndrome

Compression of the left iliac vein by the overlying artery — an under-recognised cause of one-sided leg swelling and DVT. Assessed and treated in Singapore.

Last reviewed 15 Sept 2026

Reviewed by Dr Tay Jia Sheng · Vascular, Endovascular & General Surgeon

What is May–Thurner syndrome?

May–Thurner syndrome is compression of the left common iliac vein — the main vein draining the left leg — by the right common iliac artery that crosses in front of it, pinning it against the spine. It is one of the conditions I assess and treat in Singapore. The crossing is normal anatomy; what makes it a syndrome is compression severe enough to obstruct drainage, so that the whole left leg empties against resistance.

It is under-recognised because its symptoms are easily attributed to other things, and because the compression sits in the pelvis, out of reach of a standard leg ultrasound.

It is classically described in women between 20 and 50, but it is not confined to them. In a 2020 case report I co-authored, it was the cause of an extensive DVT in a middle-aged man — worth keeping in mind whenever a man presents with an unexplained left-leg clot.

How it presents

Swelling, heaviness or aching of the left leg with no other explanation, often worse through the day
A deep vein thrombosis of the left leg, particularly in a younger person or without a clear trigger — the narrowed vein slows flow and encourages clotting
Post-thrombotic symptoms that persist after a left-leg DVT has been treated
Varicose veins or venous insufficiency of the left leg that recur or fail to settle after standard treatment

Why it matters

Left untreated, symptomatic compression keeps the leg draining poorly and raises the risk of clot — and of the post-thrombotic changes that follow. Reopening the vein addresses the cause rather than the symptoms, which is why it is worth looking for in a left leg that does not behave as expected.

How I assess it

Duplex ultrasound at the consultation examines the leg veins and looks for the indirect signs of an outflow obstruction. Where compression is suspected, CT or MR venography maps the pelvic veins directly. If a procedure is planned, intravascular ultrasound — an ultrasound probe inside the vein — measures the true degree of narrowing, which venography alone tends to understate, and sizes any stent precisely. I read every scan and make every treatment decision myself.

Treatment options

Observation where compression is present but not causing symptoms
Anticoagulation where a DVT has occurred, as for any deep vein clot
Catheter-based clot removal for an acute, extensive left-leg DVT, which often uncovers the compression underneath
Deep venous stenting — a self-expanding venous stent placed across the compressed segment through a needle puncture, usually as a day procedure, followed by a period of blood-thinning medication and ultrasound surveillance

When to see a doctor promptly

One leg that is persistently more swollen than the other, a left-leg DVT without an obvious cause, or left-leg venous symptoms that have not responded to treatment — each deserves a vascular assessment that looks above the leg, into the pelvis.

Reference: May–Thurner Syndrome: A Consideration for Deep Vein Thrombosis in Males. Case Reports in Medicine 2020;2020:2324637.

Frequently asked questions

Why is May–Thurner syndrome usually on the left?: Because of anatomy. The right common iliac artery crosses in front of the left common iliac vein, pressing it against the spine. Everyone has this crossing; in some people the compression is enough to narrow the vein and obstruct drainage from the left leg.
How is it diagnosed?: It is suspected from the pattern — unexplained left-leg swelling, or a left-leg DVT without another cause — and confirmed on imaging. Duplex ultrasound at consultation looks at the leg and, where possible, the pelvic veins; CT or MR venography maps the compression; and intravascular ultrasound during any procedure measures its true severity, which venography alone understates.
Does every compression need a stent?: No. Many people have some degree of compression with no symptoms and need nothing. Treatment is for compression that is causing symptoms — persistent swelling, a DVT, or post-thrombotic changes — and the decision is made on how much the vein is narrowed and what it is doing to the leg.
What does treatment involve?: Deep venous stenting: through a needle puncture, a self-expanding venous stent is placed across the compressed segment to hold it open, usually as a day procedure under local anaesthesia with sedation. Where an acute clot is present, it is cleared first. A period of blood-thinning medication and ultrasound surveillance follows.