What is May-Thurner Syndrome?
May-Thurner Syndrome (MTS), also known as iliac vein compression syndrome, is an anatomical condition in which the right common iliac artery crosses over and compresses the left common iliac vein against the lumbar spine. This compression narrows the vein, impeding the return of blood from the left leg and pelvis back to the heart.
This anatomical arrangement is actually present in a significant portion of the general population, but not everyone develops symptoms. Over time, however, the persistent mechanical pressure can damage the inner lining of the vein wall, trigger scar tissue formation (intraluminal spur), and dramatically increase the risk of deep vein thrombosis (DVT) — particularly in the left leg. MTS is estimated to account for a substantial proportion of left-sided DVT cases, many of which go unrecognized because the underlying compression is not routinely identified.
May-Thurner Syndrome may also contribute to Pelvic Congestion Syndrome by impairing venous drainage from the pelvic region. At Vascular Surgical Associates, our experienced vascular specialists are skilled at identifying MTS through advanced imaging and providing targeted, minimally invasive interventions to decompress the vein, restore flow, and prevent recurrent thrombosis.
Signs & Symptoms
Many individuals with May-Thurner Syndrome have no symptoms until a DVT develops. When symptoms are present, they typically affect the left leg and may develop gradually or appear suddenly with the onset of clot formation.
Left Leg Swelling
Persistent or progressive swelling of the left leg, calf, or thigh caused by impaired venous drainage through the compressed iliac vein.
Left Leg Pain or Heaviness
A dull ache, pressure, or sense of heaviness in the left leg that often worsens with prolonged standing and improves with elevation.
Deep Vein Thrombosis
Sudden onset of significant leg swelling, warmth, and pain may signal an acute DVT — a blood clot in the deep veins of the leg — which requires urgent evaluation.
Varicose Veins of the Left Leg
Visible varicose veins on the left leg or pelvic region that develop as collateral vessels attempt to bypass the obstructed iliac vein.
Chronic Pelvic Pain
In women, MTS can contribute to impaired pelvic venous drainage, producing symptoms that overlap with Pelvic Congestion Syndrome, including chronic pelvic heaviness and pain.
Skin Changes
Long-standing venous obstruction may cause skin discoloration, thickening, or ulceration in the lower left leg — signs of post-thrombotic syndrome and chronic venous insufficiency.
Risk Factors
Female Sex
May-Thurner Syndrome is diagnosed more frequently in women, particularly those of childbearing age, though men can also be affected by iliac vein compression.
Scoliosis or Lumbar Spine Changes
Abnormal spinal curvature or lumbar vertebral changes can alter the anatomical relationship between the iliac artery and vein, worsening compression.
Dehydration
Reduced blood volume increases clotting tendency, raising DVT risk in patients who already have underlying iliac vein compression from MTS.
Prolonged Immobility
Extended bed rest, long flights, or sedentary periods reduce venous flow velocity and significantly increase the risk of clot formation at the compressed vein segment.
Pregnancy
The gravid uterus increases pelvic pressure and blood volume, which can exacerbate iliac vein compression and further elevate DVT risk in affected women.
Hormonal Contraceptives
Oral contraceptives and hormone therapy increase blood clotting factors, compounding the thrombotic risk already present due to venous compression in MTS.
Diagnosis at VSA
At Vascular Surgical Associates, we use our ICAVL-accredited vascular laboratory to provide precise, reliable diagnostic imaging for May-Thurner Syndrome. Because MTS is frequently missed on standard imaging, identifying it requires both clinical suspicion and the specialized expertise our vascular team brings to every evaluation.
A duplex ultrasound is typically the first step, allowing our technologists to assess blood flow velocity and detect signs of obstruction or DVT in the iliac and femoral veins. However, ultrasound has limitations in visualizing the deep pelvic vessels, so CT venography (CTV) or MR venography (MRV) is often used to directly image the iliac veins and confirm the degree of arterial compression. These cross-sectional studies provide detailed anatomical mapping essential to planning any intervention.
When intervention is being considered, intravascular ultrasound (IVUS) — performed at the time of a catheter-based procedure — is the gold-standard method for precisely measuring the extent of compression and guiding stent placement. IVUS reveals compression that is often underestimated on cross-sectional imaging alone, ensuring that treatment is both accurate and complete. Every study is reviewed by our experienced vascular specialists to develop a personalized, evidence-based treatment plan.
Treatment Options
Treatment for May-Thurner Syndrome is tailored to whether an acute DVT is present, the severity of chronic symptoms, and your overall vascular health. Our vascular surgeons develop individualized plans using the latest interventional techniques and evidence-based guidelines.
Venous Stenting
For patients with significant iliac vein compression and symptoms, endovascular stenting is the definitive treatment. A self-expanding metallic stent is placed inside the compressed left iliac vein through a catheter introduced via a small puncture in the groin or back of the knee. The stent holds the vein open against the arterial compression, restoring normal blood flow. Venous stenting for MTS has excellent long-term patency rates and provides durable relief from leg swelling, pain, and recurrent DVT risk. The procedure is performed under sedation on an outpatient or short-stay basis.
Catheter-Directed Thrombolysis
For patients who present with an acute DVT in the setting of MTS, catheter-directed thrombolysis (CDT) is used to dissolve the clot before stenting. A catheter is positioned directly within the clot, delivering a clot-dissolving medication over hours to days to rapidly restore venous flow. Once the thrombus is cleared, the underlying compression is addressed with stent placement to prevent recurrence. This combined approach dramatically reduces the risk of post-thrombotic syndrome — the chronic leg swelling and skin changes that often follow untreated DVT.
Anticoagulation Therapy
Anticoagulant medications are a critical component of DVT management in MTS and are used both acutely and after intervention to maintain stent patency. Blood thinners prevent further clot formation and reduce the risk of pulmonary embolism. The duration of anticoagulation depends on whether stenting was performed, the extent of prior clotting, and individual patient factors. Our vascular team coordinates closely with hematology when underlying clotting disorders are identified.
Surveillance & Monitoring
For patients with mild compression who have not developed DVT, periodic surveillance with duplex ultrasound and clinical evaluation is appropriate. Compression stockings and lifestyle modifications — including hydration, regular movement, and avoiding prolonged immobility — reduce thrombotic risk while the clinical picture is monitored. If symptoms progress or a DVT occurs, the team is prepared to move quickly to definitive intervention.
Frequently Asked Questions
The anatomical variant underlying MTS — the right iliac artery crossing over the left iliac vein — is actually quite common, estimated to be present in up to 20–25% of the general population. However, only a subset of these individuals develop symptomatic compression, DVT, or other complications. MTS is considered underdiagnosed because the left-sided DVT it causes is often attributed to other factors without imaging specifically targeted at the iliac veins.
The classic anatomical compression in MTS involves the left iliac vein, so the left leg is predominantly affected. However, atypical variants of iliac vein compression can occur on the right side or in other vascular configurations, producing similar symptoms. A thorough vascular evaluation with cross-sectional imaging is essential to identify the specific anatomy in each patient.
Long-term outcomes after iliac vein stenting for MTS are generally very good. Studies report primary patency rates — meaning the stent remains open without additional intervention — of 80–90% at one to two years. Most patients experience significant improvement in leg swelling, pain, and quality of life. Anticoagulation after stenting and routine duplex surveillance help identify and address any early stent narrowing before symptoms recur.
MTS and PCS frequently co-exist, as impaired outflow from the left iliac vein due to arterial compression can increase backpressure in the pelvic venous system, contributing to pelvic varicose veins and chronic pelvic pain. In women evaluated for PCS, MTS should always be excluded as a contributing cause, and treatment may need to address both conditions simultaneously for optimal results.
