Residency · Residency · Vascular Surgery
May-Thurner Syndrome and Iliac Vein Obstruction
Overview
May-Thurner syndrome (MTS) refers to the compression of the left common iliac vein by the overlying right common iliac artery as it presses the vein against the lumbar spine. This anatomical anomaly was first described by May and Thurner in 1957, based on an autopsy study that found iliac vein spurs in 22% of cadavers. The compression leads to venous stasis and intimal hypertrophy, resulting in the formation of intraluminal spurs or webs that predispose patients to left iliofemoral deep vein thrombosis (DVT). MTS accounts for approximately 2-5% of lower extremity venous thrombotic events and is more commonly seen in young women aged 20 to 40 years. With the increasing use of intravascular ultrasound (IVUS), recognition of this syndrome has become more frequent.
Anatomy and Pathophysiology
Anatomic Compression
At the L5/S1 vertebral level, the right common iliac artery crosses anteriorly over the left common iliac vein. The vein becomes compressed between the artery in front and the vertebral body behind. This chronic pulsatile compression causes endothelial injury and intimal fibrosis within the vein. Over time, fibrotic adhesions known as intraluminal "spurs" or "webs" develop inside the vein lumen. Although rare, a right-sided variant exists where the right iliac vein is compressed by the right iliac artery.
Cockett Classification of Iliac Vein Spurs
The Cockett classification categorizes iliac vein spurs into three types: Type I involves a central spur and is the most common; Type II is characterized by a lateral spur; and Type III represents a combination of both central and lateral spurs.
Spectrum of Disease
The clinical spectrum ranges from asymptomatic compression, which is often an incidental finding on imaging and occurs in over 50% of the population, to symptomatic non-thrombotic iliac vein lesions (NIVL) presenting as chronic left leg swelling without DVT. More severe manifestations include acute iliofemoral DVT precipitated by MTS and chronic post-thrombotic syndrome resulting from MTS-related DVT.
<image>Cross-sectional anatomical diagram at the L5 level showing the right common iliac artery compressing the left common iliac vein against the lumbar vertebral body, with development of intimal spurs</image>
Clinical Presentation
Patients with MTS typically present with left lower extremity swelling, heaviness, and pain that worsen with standing or activity. Acute left iliofemoral DVT often occurs in young, otherwise healthy individuals. Chronic venous insufficiency (CVI) symptoms such as edema, skin changes, and ulceration may also be present. MTS may become apparent during pregnancy or in patients using oral contraceptives, both of which are additional risk factors for DVT. In any young patient presenting with isolated left leg swelling without obvious provocation, suspicion for MTS should be raised.
Diagnosis
Duplex Ultrasound
Duplex ultrasound is the first-line diagnostic study and may reveal iliofemoral DVT or post-thrombotic changes. However, its ability to directly visualize iliac vein compression is limited due to bowel gas and the depth of the vessel. Typical findings include dampened phasic flow in the left common femoral vein and signs of post-thrombotic changes. The absence of respiratory phasicity in venous flow suggests proximal obstruction.
CT Venography
CT venography can identify iliac vein compression and quantify the degree of stenosis. It delineates the anatomical relationship between the iliac artery and vein, detects thrombus, collateral formation, and assesses compression severity. A limitation is that cross-sectional imaging may overestimate compression, as more than 50% compression is commonly seen in asymptomatic individuals.
MR Venography
MR venography serves as an alternative to CT, avoiding contrast nephropathy and radiation exposure. It uses time-of-flight or contrast-enhanced sequences to identify compression and collateral pathways effectively. This modality is particularly useful during pregnancy.
Conventional Venography
Conventional venography demonstrates the compression defect, collateral formation, and pressure gradients. It can identify intraluminal webs and spurs and is often performed during intervention. Multiple views, including anteroposterior and oblique, are used to characterize the compression.
Intravascular Ultrasound (IVUS)
IVUS is considered the gold standard for diagnosing and quantifying iliac vein obstruction. It provides cross-sectional measurements of the luminal area and directly visualizes spurs, webs, external compression, and post-thrombotic changes. A reduction of more than 50% in luminal area on IVUS is deemed significant. IVUS also guides stent sizing and deployment and is superior to venography for identifying non-thrombotic iliac vein lesions.
<image>IVUS images showing a normal iliac vein lumen compared to a significantly compressed left common iliac vein with intimal spur, alongside corresponding venographic appearance with extrinsic compression defect</image>
Treatment
Acute Iliofemoral DVT with May-Thurner
In cases of acute iliofemoral DVT associated with MTS, anticoagulation should be initiated immediately. Catheter-directed thrombolysis (CDT) or pharmacomechanical thrombectomy is used to clear the acute thrombus. Following thrombus clearance, venography and IVUS are performed to reveal the underlying compression. Iliac vein stenting of the stenotic lesion is then carried out either during the same session or in a staged manner.
Non-Thrombotic Iliac Vein Lesion (Symptomatic)
For symptomatic non-thrombotic iliac vein lesions, venography combined with IVUS is used to confirm hemodynamically significant compression. Iliac vein stenting is indicated if there is more than 50% area reduction on IVUS accompanied by corresponding symptoms. Long-term anticoagulation is not required in the absence of prior DVT; antiplatelet therapy alone is sufficient.
Iliac Vein Stenting
Technical Considerations
Dedicated venous stents are preferred for iliac vein stenting. Historically, the Wallstent was used, but newer devices such as the VICI, Venovo, and Abre stents are designed specifically for the venous system. These stents accommodate larger diameters, offer crush resistance, and maintain flexibility. Typical stent diameters range from 14 to 16 mm for the common iliac vein and 12 to 14 mm for the external iliac vein. The stent must cover the entire compressed or diseased segment and may need to extend into the inferior vena cava (IVC) if the common iliac vein ostium is involved. Balloon angioplasty is performed before stent deployment (predilatation), followed by post-dilation to the nominal stent diameter. IVUS is used to confirm adequate stent expansion and wall apposition.
Landing Zones
Proximally, the stent should extend to the IVC confluence if the common iliac vein ostium is compressed. Distally, it should extend into the external iliac vein to cover the full extent of the disease. Extension below the inguinal ligament should be avoided if possible, as this area is associated with a higher risk of stent occlusion due to hip flexion. Extending the stent into the IVC generally does not compromise contralateral iliac vein flow.
Post-Stenting Management
After stenting for DVT, anticoagulation is typically continued for 3 to 6 months. For non-thrombotic lesions, antiplatelet therapy alone may suffice. Surveillance with duplex ultrasound is recommended at 1, 3, 6, and 12 months post-procedure, and then annually, to assess stent patency, flow velocity, and detect in-stent stenosis.
Outcomes of Iliac Vein Stenting
Primary patency rates at one year range from 85% to 95% for non-thrombotic lesions and 70% to 85% for post-thrombotic lesions. More than 80% of patients experience improvement in symptoms such as swelling, pain, and ulcer healing. Venous ulcer healing rates with stenting are between 60% and 80%. The reintervention rate for in-stent restenosis or disease progression is approximately 10% to 20%.
Post-Thrombotic Syndrome and Venous Stenting
Post-thrombotic syndrome (PTS) following iliofemoral DVT is often driven by persistent iliac vein obstruction with or without reflux. Stenting the obstructive component improves symptoms even when distal reflux is present. The VIDIO trial, a non-randomized study, demonstrated that iliac vein stenting combined with medical therapy improved quality of life compared to medical therapy alone in patients with PTS. Patient selection involves symptomatic PTS with documented iliac obstruction on IVUS showing more than 50% area reduction. When concurrent superficial venous reflux is present, superficial venous ablation may be performed in combination.
<image>Venogram sequence showing catheter-directed thrombolysis for acute left iliofemoral DVT followed by unmasking of May-Thurner compression and subsequent iliac vein stent deployment with restored patency</image>
Special Considerations
Bilateral Iliac Vein Disease
Although right-sided MTS variants are uncommon, they do exist. Bilateral stenting may occasionally be necessary for bilateral post-thrombotic disease. When performing bilateral stenting, care must be taken to ensure that the contralateral vein is not compressed by the stent.
Pregnancy
MTS may present during pregnancy due to the combined effects of a hypercoagulable state and uterine compression. Anticoagulation with low molecular weight heparin (LMWH) is recommended, and stenting is generally deferred until the postpartum period if possible. In select cases, placement of a suprarenal inferior vena cava filter may be required.
Recurrent DVT
In patients with recurrent left-sided DVT, evaluation for underlying MTS is essential. Stenting addresses the anatomic substrate and reduces the risk of recurrence.
Clinical Pearls
May-Thurner syndrome should be considered in any young patient, especially females, presenting with unprovoked left iliofemoral DVT, as it accounts for up to 50% of such cases. Intravascular ultrasound is vastly superior to venography for diagnosing and quantifying iliac vein compression and should always be used during venous interventions. Asymptomatic compression detected on CT or MRI is extremely common, occurring in over half the population, so incidental findings should not prompt stenting without clinical correlation. Dedicated venous stents, rather than arterial stents, must be used because they are specifically designed to meet the biomechanical demands of the venous system, including crush resistance and larger diameters. After thrombolysis for DVT, it is crucial to look for underlying anatomic lesions, as clearing the thrombus without addressing the cause leads to rethrombosis. Finally, non-thrombotic iliac vein lesions can cause significant symptoms without ever causing DVT, so chronic left leg swelling in a young patient should never be dismissed without thorough investigation.
References
- Raju S, Neglen P. High prevalence of nonthrombotic iliac vein lesions in chronic venous disease: a permissive role in pathogenicity. J Vasc Surg. 2006;44(1):136-144.
- Meissner MH, et al. Early thrombus removal strategies for acute deep venous thrombosis: clinical practice guidelines of the SVS and the AVF. J Vasc Surg. 2012;55(5):1449-1462.
- Neglén P, et al. Stenting of the venous outflow in chronic venous disease: long-term stent-related outcome, clinical, and hemodynamic result. J Vasc Surg. 2007;46(5):979-990.
- Murphy EH, et al. Iliac vein compression syndrome: outcome of endovascular treatment. J Vasc Surg Venous Lymphat Disord. 2020;8(6):934-943.
- Birn J, Vedantham S. May-Thurner syndrome and other obstructive iliac vein lesions: meaning, myth, and mystery. Vasc Med. 2015;20(1):74-83.
