# May-Thurner Syndrome and Iliac Vein Stenting

## Anatomy and Pathophysiology

May-Thurner syndrome (MTS), also known as iliac vein compression syndrome or Cockett syndrome, occurs when the left common iliac vein is compressed between the right common iliac artery anteriorly and the L5 vertebral body posteriorly. The pulsatile arterial compression causes intimal hyperplasia (venous spurs) and progressive luminal narrowing over time. While right-sided and bilateral variants exist, they are less common. Anatomic compression is present in 20-30% of the general population on imaging, but only a minority become symptomatic. The condition predominantly affects young to middle-aged women, with a female-to-male ratio of 2-3:1.

## Clinical Presentation

Thrombotic MTS presents as acute left iliofemoral DVT, often in young patients without typical VTE risk factors. The classic presentation is a young woman with left leg swelling and DVT, and MTS is found as the underlying cause in 18-49% of left-sided DVTs. Non-thrombotic MTS (also termed non-thrombotic iliac vein lesion or NIVL) presents with chronic left leg swelling, heaviness, varicose veins, and venous stasis changes. The diagnosis of non-thrombotic MTS is more controversial because imaging compression alone does not confirm clinical significance, and symptoms overlap with other causes of chronic venous insufficiency.

## Diagnostic Workup

### Non-Invasive

Duplex ultrasound may show iliofemoral DVT, but direct visualization of iliac vein compression is difficult due to overlying bowel gas. CT venography or MR venography demonstrates the anatomic compression of the left common iliac vein and allows measurement of cross-sectional area reduction. Greater than 50% compression is generally considered significant, though there is no universally accepted threshold, and many asymptomatic patients exceed this level.

### Invasive (Gold Standard)

Catheter-based venography with manometry involves transfemoral venous access and identifies collateral veins, the presence of which (transpelvic or ascending lumbar collaterals) indicates hemodynamic significance. Intravascular ultrasound (IVUS) has become the key diagnostic tool and the de facto standard for confirming MTS before stenting. IVUS provides cross-sectional area measurements, with greater than 50% area reduction generally considered significant. It identifies intimal spurs not visible on venography and is superior to venography for characterizing the degree and extent of compression.

## Treatment

### Thrombotic MTS

Treatment follows a sequential approach: first, thrombus removal through catheter-directed thrombolysis or pharmacomechanical thrombectomy to clear the acute DVT; second, stenting of the underlying compression after thrombus clearance, with IVUS confirmation; and third, anticoagulation typically for 6-12 months, with some advocating indefinite therapy if no reversible provoking factor is identified.

### Non-Thrombotic MTS

Conservative management is tried first, including compression stockings, exercise, and leg elevation. Stenting is reserved for patients with persistent symptoms despite conservative measures and IVUS-confirmed significant compression. The controversy centers on the fact that many patients have anatomic compression without symptoms, and stenting asymptomatic compression is not indicated. Patient selection is critical, with symptoms needing to correlate with imaging findings.

## Iliac Vein Stenting

### Stent Selection Controversy

| Stent | Type | Design | Max Diameter | Radial Force | Foreshortening | Key Advantage |
|-------|------|--------|--------------|-------------|----------------|---------------|
| Wallstent | Generic | Braided elgiloy | 24 mm | Moderate | Significant | Flexibility, availability |
| Venovo | Dedicated venous | Nitinol open-cell | 20 mm | High | Minimal | Precise deployment |
| VICI | Dedicated venous | Nitinol | 20 mm | High | Minimal | Large diameter options |
| Zilver Vena | Dedicated venous | Nitinol laser-cut | 20 mm | High | Minimal | Proven durability |
| ABRE | Dedicated venous | Nitinol | 20 mm | High | Minimal | Crush resistance |

The Wallstent (bare metal, flexible, braided) was historically the most commonly used stent, offering good flexibility, conformability, and availability in large diameters up to 24 mm. Its disadvantages include foreshortening during deployment, limited radial force, and susceptibility to deformation under chronic compression. Dedicated venous stents designed specifically for venous compression are now available: the Venovo (Bard/BD) features nitinol open-cell design with high radial force and precise deployment; the VICI (Boston Scientific) is designed for large venous diameters; the Zilver Vena (Cook) is available in large sizes; and the ABRE (Medtronic) offers high radial force. These purpose-built stents offer higher radial force to resist extrinsic compression, less foreshortening, and better conformability to venous anatomy. Growing evidence supports dedicated venous stents over the Wallstent.

### Stent Sizing

Sizing is based on the diameter of the uncompressed vein measured on IVUS or at an uncompressed segment. Typical diameter is 14-16 mm for the common iliac vein. Length should cover the entire compressed segment with adequate landing in healthy vein, extending into the IVC by 1-2 cm if needed. The stent should not cross the inguinal ligament into the common femoral vein if avoidable, as this location carries risk of stent fracture and compression.

### Technical Considerations

Access is through the ipsilateral popliteal or femoral vein. The compressed segment is predilated before stent deployment, and the stent is deployed under IVUS guidance to confirm full expansion and coverage. Post-dilation to nominal diameter is performed, with extension into the IVC considered if the compression extends to the iliac vein confluence. Completion IVUS and venography confirm stent apposition and flow.

### Post-Stenting Antithrombotic Therapy

There is no consensus on the optimal regimen. Approaches vary by center and include anticoagulation for 3-6 months (or longer if there was a prior DVT), antiplatelet therapy with aspirin or clopidogrel added by some centers, and antiplatelet therapy alone for non-thrombotic cases.

## Outcomes

For thrombotic MTS, 12-month primary patency is 75-85% with secondary patency of 90-95%. Non-thrombotic MTS has better outcomes, with 12-month primary patency of 90-95%. Symptom improvement occurs in 80-90% of appropriately selected patients. Stent-related complications include in-stent restenosis (5-10%), stent migration (rare), and contralateral iliac vein compression by the stent (rare).

<image>Anatomic illustration of May-Thurner syndrome. An axial cross-section at the L5 vertebral body level showing the left common iliac vein compressed between the overlying right common iliac artery and the vertebral body posteriorly. The normal round cross-section of the vein is shown flattened with intimal spurs (venous webs) developing at the compression point. A companion coronal illustration shows the relationship of the iliac vessels at the aortic bifurcation with the compressed left common iliac vein highlighted. Labels identify the right common iliac artery, left common iliac vein, L5 vertebral body, and intimal spurs.</image>

<image>IVUS images demonstrating May-Thurner compression. Three panels: (1) Normal left common iliac vein cross-section proximal to the compression showing a round, fully expanded lumen; (2) IVUS at the compression point showing a flattened, elliptical lumen with >50% area reduction and visible intimal spur; (3) Post-stenting IVUS showing a fully expanded venous stent with restoration of the round lumen. Each panel includes cross-sectional area measurements annotated on the image. A companion angiographic image shows transpelvic collateral veins indicating hemodynamically significant compression.</image>

<image>Comparison of venous stent designs for iliac vein compression. Four panels showing side-by-side illustrations: (1) Wallstent (braided, flexible, with notable foreshortening); (2) Venovo (nitinol, open-cell, high radial force); (3) VICI (nitinol, designed for large venous diameters); (4) Zilver Vena (nitinol, laser-cut). Each panel lists key specifications (available diameters, radial force, foreshortening percentage) and a brief note on advantages. A comparison chart below summarizes patency data where available.</image>

## Clinical Pearls

IVUS is the diagnostic gold standard for MTS because venography and CT alone may underestimate the degree of compression. Not every patient with anatomic compression needs a stent; symptoms must correlate with imaging findings. In acute left-sided iliofemoral DVT, always evaluate for MTS after thrombus clearance, because untreated compression leads to rethrombosis. Dedicated venous stents (Venovo, VICI, Zilver Vena) are replacing the Wallstent due to superior radial force and purpose-built design. Avoid crossing the inguinal ligament with the stent, as this increases the risk of stent-related complications. Non-thrombotic MTS has excellent stent patency (greater than 90% at 1 year) when patients are carefully selected. Post-stenting antithrombotic regimens are not standardized, and institutional protocols vary.

## References

- May R, Thurner J. The cause of the predominantly sinistral occurrence of thrombosis of the pelvic veins. Angiology 1957
- Defined by the AVF/SIR/SVS Guidelines on Iliac Vein Stenting. J Vasc Surg Venous Lymphat Disord 2019
- Defined by the VIDIO trial: Gagne PJ et al. Dedicated venous stent vs. Wallstent for iliofemoral venous disease. J Vasc Surg Venous Lymphat Disord 2022
- Defined by Defined by the CIRSE Standards of Practice on Iliac Vein Stenting. Cardiovasc Intervent Radiol 2023