May Thurner syndrome symptoms come from one thing: blood struggling to leave the left leg and pelvis because the left iliac vein is pinched by the artery that crosses it. Some people notice a slowly swelling, heavy left leg. Others have no warning at all until a blood clot forms. This page explains what the symptoms feel like, which ones are urgent, and how doctors check whether the vein is really the cause. For background on the condition itself, see our guide to May-Thurner syndrome.
Many people have no symptoms
Before looking at symptoms, it helps to know that a narrowed left iliac vein is common in people who feel completely well. The body often grows small side veins (collaterals) that carry blood around the narrowing. So a scan report mentioning compression does not mean you will develop symptoms. Doctors only call it a syndrome when the narrowing is causing trouble.
Early signs of May-Thurner syndrome
Early signs tend to be mild, one-sided and easy to put down to a long day. Look out for:
- A left leg or ankle that swells more than the right, especially by the evening, and goes down overnight
- A heavy, tired or tight feeling in the left leg after standing or sitting for a long time
- Sock marks or shoe tightness on one side only
- New or worsening varicose veins in the left leg, or varicose veins that keep coming back after treatment
None of these is specific to May-Thurner. But a pattern that sticks to the left leg is worth mentioning to a doctor.
Leg symptoms as the narrowing matters more
When blood backs up for longer, the symptoms become more obvious.
- Persistent swelling. The whole left leg, from thigh to ankle, may look fuller than the right.
- Aching or throbbing, often worse at the end of the day and eased by putting the leg up.
- Bursting pain when walking. Some people get a tight, bursting pain in the thigh or calf after walking a short distance, which eases slowly with rest and raising the leg. Doctors call this venous claudication.
- Skin and vein changes around the lower leg, described below.
May-Thurner syndrome and the feet
Fluid that cannot drain well tends to collect at the lowest point, so the ankle and top of the foot often swell first. Over months or years, high pressure in the leg veins can affect the skin:
- Brownish or darker patches around the inner ankle
- Dry, itchy, eczema-like skin
- Skin that feels hard or tight
- Rarely, a slow-healing sore (venous ulcer) near the ankle
These skin changes are signs of long-standing vein pressure from any cause, not only May-Thurner. They deserve a check by a doctor.
When the first sign is a clot (DVT)
For many people, May-Thurner is only discovered after a deep vein thrombosis (DVT) in the left leg. Slow flow through the narrowed vein makes a clot more likely, especially when other risks are added, such as pregnancy or the weeks after giving birth, the contraceptive pill or hormone treatment, surgery, illness or long journeys.
Clots linked to May-Thurner often start high up in the pelvis, so the swelling can affect the whole leg, not just the calf. Our sister site explains deep vein thrombosis in more detail.
DVT warning signs: see a doctor the same day
- Swelling in one leg that comes on over hours or a few days
- Throbbing or cramping pain in the calf or thigh
- Warm skin over the painful area, or skin that looks red or darker than usual
- Hard, tender veins
Signs of a clot in the lungs: call 995
A piece of clot can travel to the lungs (a pulmonary embolism). This is an emergency. In Singapore, call 995 if you have sudden breathlessness, chest pain or upper back pain that is worse when you breathe in, cough up blood, or feel faint, especially with a painful, swollen leg.
After a DVT, some people are left with long-term swelling, aching and skin changes. This is called post-thrombotic syndrome, and an untreated narrowing can make it worse.
Pelvic signs in women
When the left iliac vein is narrowed, blood can take a detour through veins in the pelvis. These veins may widen and cause symptoms similar to the symptoms of pelvic congestion syndrome:
- A dull, dragging ache low in the pelvis, worse after standing
- Pain during or after sex
- Varicose veins around the vulva, buttocks or upper inner thigh
- Visible veins across the lower tummy or pubic area, which can be a sign that blood is finding a way around a blocked vein
Pelvic pain has many causes, so these signs suggest the veins are worth checking rather than prove anything on their own. Our page on whether pelvic congestion is serious covers the clot question in more depth.
What else could cause these symptoms?
Swelling and pain in one leg is not always May-Thurner. Doctors will also think about:
- A DVT from another cause
- A burst Baker’s cyst behind the knee
- Skin infection (cellulitis)
- Lymphoedema, where lymph fluid does not drain well
- A sprain, muscle tear or other injury
- Simple varicose veins from faulty valves in the leg
Swelling in both legs points more towards heart, kidney or liver problems, some medicines, or long periods of sitting.
May-Thurner syndrome diagnosis: how symptoms are confirmed
Because compression is common in people without symptoms, the key question is whether the narrowing matches your symptoms. Tests usually go in this order:
- Duplex ultrasound. Quick and painless. It is good at finding clots in the leg, but the iliac vein sits deep in the pelvis and can be hard to see clearly. See our guide to pelvic and iliac vein ultrasound.
- CT or MR venography. These scans show where the artery crosses the vein and look for other causes of swelling.
- Catheter venogram with intravascular ultrasound (IVUS). A tiny ultrasound probe inside the vein measures the narrowing most accurately. It is usually done when treatment is being planned.
If the narrowing is causing symptoms or a clot, the main treatment is a stent to hold the vein open. See iliac vein stenting for how that works. A Singapore vascular surgeon’s patient page on deep vein thrombosis also explains how clots are assessed and treated.
When to see a doctor
See a doctor if you have ongoing swelling, heaviness or aching in your left leg, skin changes around the ankle, varicose veins that keep coming back on one side, or a dull pelvic ache with vulval or upper thigh varicose veins. You can use our pelvic symptom check to organise your thoughts before an appointment.
Get help the same day for new swelling and pain in one leg. Call for emergency help if you have sudden breathlessness, chest pain, coughing up blood or fainting. In Singapore, call 995 in an emergency.
Frequently asked questions
What are the early signs of May-Thurner syndrome?
Early signs are often mild and easy to dismiss. They include a left leg or ankle that swells more than the right by the end of the day, a heavy or tight feeling in the left leg, sock marks on one side only, or new varicose veins on the left. For some people there are no early signs, and the first problem is a clot.
Can May-Thurner syndrome affect the feet?
Yes, indirectly. Because blood drains poorly from the leg, swelling often collects around the ankle and top of the foot. Over time the skin around the inner ankle can darken, become itchy or dry, harden, or rarely break down into an ulcer.
What are the symptoms of iliac vein compression?
They are the same as May-Thurner symptoms: swelling, heaviness and aching in the leg on the affected side, usually the left, sometimes pain in the thigh when walking, varicose veins, and in women, pelvic congestion. Many people with compression seen on a scan have none of these.
Can May-Thurner syndrome affect the right leg?
It usually affects the left leg, because that is where the artery crosses the vein. Less commonly, other vein compressions cause similar problems on the right side or in both legs.
How is May-Thurner syndrome diagnosed?
Usually with a duplex ultrasound first, then CT or MR venography to show where the artery crosses the vein. A catheter venogram with intravascular ultrasound (IVUS) measures the narrowing most accurately and is usually done when treatment is being planned.
Sources and further reading
- Mangla A, Hamad H. May-Thurner Syndrome. StatPearls (NCBI Bookshelf)
- Society for Vascular Surgery — A guide for patients: May Thurner syndrome (PDF)
- Knuttinen MG et al. May-Thurner: diagnosis and endovascular management. Cardiovasc Diagn Ther 2017
- NHS — Deep vein thrombosis (DVT)
- NHS — Pulmonary embolism
This article is general information, not personal medical advice. Please speak to a doctor about your own symptoms. How we create our content.
