What Is May-Thurner Syndrome?

May-Thurner Syndrome (MTS), also called iliac vein compression syndrome, is a vascular condition in which the left iliac vein is compressed by the right iliac artery. This pressure reduces blood flow from the left leg back to the heart, potentially causing symptoms and increasing the risk of deep vein thrombosis (DVT)—a type of blood clot that forms in the deep veins.

The iliac veins are large blood vessels in the pelvis responsible for returning blood from the lower limbs to the heart. In individuals with MTS, the right iliac artery crosses over and compresses the left iliac vein against the lumbar spine. Over time, this compression can damage the vein, narrow its walls, and disrupt circulation.

MTS is considered underdiagnosed because it often produces few or no symptoms until a complication, such as a blood clot, occurs. It affects approximately 2–3% of the population, though some studies suggest that as many as 20–25% of people may have some form of iliac vein compression without knowing it.

Symptoms and Warning Signs

May-Thurner Syndrome can be asymptomatic in the early stages. In many cases, the first sign is a blood clot in the leg. However, there are early symptoms to watch for, particularly if they occur more often in the left leg:

  • Persistent swelling in one leg (usually the left)
  • Heaviness, fullness, or pressure sensation in the leg
  • Aching or throbbing pain, especially after long periods of standing or sitting
  • Enlarged, bulging varicose veins in the upper thigh or groin
  • Skin discoloration, especially reddish or purplish spots
  • Warmth in the leg or tenderness when touched
  • Chronic pelvic or lower back pain
  • Skin ulcers or open sores in severe cases

If a blood clot (DVT) forms as a complication of MTS, additional symptoms may include:

  • Sudden increase in swelling
  • Leg pain that mimics a muscle cramp (Charley horse)
  • Redness or purple skin discoloration
  • Leg that feels hot or warm to the touch
  • Tightness or tension in the skin due to swelling

These symptoms require immediate medical evaluation, as untreated DVT can lead to pulmonary embolism—a potentially fatal condition where the clot travels to the lungs.

What Causes May-Thurner Syndrome?

The primary cause of MTS is anatomical compression of the left iliac vein by the right iliac artery. This positioning can either be congenital (present at birth) or acquired over time.

The right iliac artery pulsates with every heartbeat, and this constant pressure on the vein can damage the endothelium (inner lining) of the vein, leading to:

  • Inflammation of the vessel wall
  • Development of venous spurs (fibrous bands)
  • Narrowing or obstruction of the vein
  • Blood pooling in the lower extremities
  • Higher risk of clot formation

Risk Factors for MTS

Several factors may increase a person’s likelihood of developing MTS or experiencing complications:

  • Assigned female at birth (AFAB) – more common in women than men
  • Age between 20 and 50
  • Pregnancy or recent childbirth
  • Use of oral contraceptives or hormone replacement therapy
  • Sedentary lifestyle or prolonged periods of immobility
  • Dehydration or low fluid intake
  • Recent surgery (especially pelvic or orthopedic)
  • Family history of blood clots
  • Smoking or chronic vascular conditions

These risk factors can amplify the effects of vein compression and further increase the risk of DVT.

Stages of May-Thurner Syndrome

May-Thurner Syndrome progresses gradually through three main stages:

  • Stage 1 – Asymptomatic Compression: The left iliac vein is compressed, but there are no structural changes or symptoms.
  • Stage 2 – Venous Spur Formation: Scar-like tissue begins to form within the vein, causing partial obstruction and possible circulation issues.
  • Stage 3 – DVT Formation: A blood clot forms due to restricted blood flow, causing swelling, pain, and potentially serious complications.

Early diagnosis can prevent progression to stage 3 and reduce long-term health impacts.

Diagnostic Process

If May-Thurner Syndrome is suspected, your healthcare provider will begin with a thorough medical history, symptom review, and physical examination. Signs like unilateral leg swelling, varicose veins, and skin changes may prompt further testing.

Diagnostic Tests:

  1. Venous Duplex Ultrasound
    • A non-invasive imaging test that uses sound waves to view blood flow and detect clots or narrowed areas.
  2. CT Venography (MDCT)
    • Involves injecting a contrast dye into the veins to highlight blood vessels on X-rays and detect compression or obstruction.
  3. MR Venography (MRV)
    • Uses magnetic fields and contrast dye to produce 3D images of veins.
  4. D-Dimer Blood Test
    • Measures levels of D-dimer, a protein fragment produced when clots dissolve. Elevated levels suggest active clotting.
  5. Intravascular Ultrasound (IVUS)
    • A catheter-based procedure to view the inside of blood vessels, often used during treatment.

Treatment Options

The goal of treatment is to improve blood flow, relieve symptoms, and prevent complications. Treatment depends on the stage and presence of DVT.

Without Blood Clots:

  • Angioplasty and Stenting
    • A catheter is inserted into the vein. A balloon inflates to open the vein, followed by placement of a stent to keep it open.
  • Compression Therapy
    • Graduated compression stockings help support veins, reduce swelling, and promote blood return.
  • Lifestyle Modifications
    • Staying active, avoiding prolonged sitting or standing, elevating legs, and maintaining hydration.

With Blood Clots (DVT):

  • Anticoagulant Medications
    • Blood thinners like warfarin, heparin, or DOACs prevent new clot formation and allow the body to dissolve existing clots.
  • Thrombolytic Therapy
    • Powerful clot-busting drugs are delivered via catheter directly into the clot to dissolve it quickly.
  • Inferior Vena Cava (IVC) Filter
    • A mesh filter placed in the IVC vein to trap clots and prevent them from traveling to the lungs.
  • Surgical Options
    • Rare, but bypass surgery or endovenectomy may be necessary if stents fail or are not feasible.

Preventing Complications

Even though May-Thurner Syndrome cannot always be prevented, you can lower your risk of developing DVT or worsening symptoms by following these guidelines:

  • Move frequently during travel or long work shifts
  • Drink plenty of fluids to stay hydrated
  • Avoid tight clothing that restricts circulation
  • Maintain a healthy body weight
  • Avoid smoking
  • Manage chronic illnesses like diabetes or hypertension
  • Use prescribed compression garments
  • Follow medication instructions if taking blood thinners

Associated Conditions

May-Thurner Syndrome may increase the risk of developing other venous conditions:

  • Deep Vein Thrombosis (DVT): Most common complication due to impaired blood flow.
  • Pulmonary Embolism (PE): A serious, potentially fatal condition when a clot travels to the lungs.
  • Chronic Venous Insufficiency (CVI): Weak or damaged veins lead to long-term pooling of blood.
  • Post-Thrombotic Syndrome (PTS): Chronic pain, swelling, and skin changes after a DVT.

Life with May-Thurner Syndrome

Most people with MTS lead full lives, especially with early intervention and ongoing care. After receiving a diagnosis, you may be advised to:

  • Take blood thinners as prescribed
  • Wear compression socks daily
  • Attend regular follow-up appointments
  • Adjust physical activity levels to improve circulation
  • Avoid long periods of inactivity

Elevating your legs after long periods of standing and drinking water throughout the day can also help. If you’ve had DVT, you may need long-term anticoagulation therapy to reduce the risk of recurrence.