Definition and epidemiology
May-Thurner syndrome (MTS) refers to extrinsic compression of the left common iliac vein by the crossing right common iliac artery, which pins the vein against the underlying lumbar vertebral body (typically L5). Repetitive pulsatile compression leads to intimal hyperplasia, venous spurs, and fibrosis, which narrows the vein lumen and promotes stasis and thrombus formation.
The anatomic variant is common in the general population (found incidentally on imaging in a substantial minority of asymptomatic people), but symptomatic MTS resulting in venous thromboembolism or chronic venous insufficiency is much less frequent. It is classically described in women in the second to fourth decades of life, with a female predominance, though it can occur at any age and in men.
MTS is an important cause of unprovoked left-sided deep vein thrombosis (DVT) in young patients and should be considered when standard risk factors are absent.
- Left common iliac vein compressed by right common iliac artery against the lumbar spine
- Classic patient: young woman with unprovoked left leg DVT
- IVUS is the gold standard for assessing degree of compression
- Treatment combines anticoagulation with endovascular stenting after thrombus removal
- Virchow triad (stasis, endothelial injury, hypercoagulability) applies here
- Asymptomatic anatomic compression is common and does not require treatment
- Right-sided symptoms are atypical and should prompt an alternative diagnosis
- Pregnancy and OCP use are common precipitating factors for symptomatic presentation
Etiology and risk factors
The underlying anatomic abnormality is congenital or developmental: the right common iliac artery crosses anterior to the left common iliac vein at the pelvic brim, and chronic pulsatile compression against the vertebral column causes venous wall changes over time.
Risk factors for symptomatic presentation
- Female sex
- Pregnancy and the postpartum period (increased pelvic venous pressure and hypercoagulability)
- Scoliosis or lumbar hyperlordosis (alters vascular anatomy)
- Dehydration or immobility
- Oral contraceptive or hormone use
- Hypercoagulable states (inherited thrombophilia, malignancy)
- Recent surgery or trauma
Many people with the anatomic compression never develop symptoms; a second hit (hypercoagulability, stasis, or vessel wall injury) is usually needed to precipitate thrombosis.
Pathophysiology
Chronic compression of the left common iliac vein between the right common iliac artery and the lumbar spine produces three classic contributors to thrombosis (Virchow triad):
- Stasis: mechanical narrowing slows venous return from the left lower limb
- Endothelial injury: repetitive pulsatile trauma causes intimal thickening, fibrosis, and intraluminal venous spurs or webs
- Hypercoagulability: often provided by an additional factor (pregnancy, OCP use, inherited thrombophilia)
Over time, the vein wall changes become fixed (non-thrombotic iliac vein lesion), and superimposed thrombosis can occur acutely. Even without thrombosis, chronic outflow obstruction leads to venous hypertension in the left leg, producing symptoms of chronic venous insufficiency independent of clot.
Clinical features
Presentation ranges from asymptomatic (incidental finding) to acute DVT or chronic venous insufficiency.
Acute presentation (DVT)
- Left leg swelling, often sudden onset
- Left leg pain or heaviness
- Warmth and erythema of the affected limb
- Dilated superficial collateral veins over the thigh or pelvis
- Phlegmasia cerulea dolens in severe iliofemoral thrombosis (rare, limb-threatening)
Chronic presentation (without acute clot)
- Chronic left leg swelling, worse with standing
- Varicose veins, skin pigmentation, venous stasis changes
- Leg heaviness or aching, pelvic congestion symptoms in some patients
- Venous claudication with exertion
Symptoms are almost always left-sided given the anatomy; right-sided presentations are rare and suggest an alternative diagnosis.
Diagnosis
Diagnosis requires a combination of clinical suspicion (especially unprovoked left leg DVT in a young patient) and imaging that demonstrates venous compression.
Initial work-up
- Duplex ultrasound: first-line to confirm DVT; may show compression but is operator-dependent and limited for pelvic veins
- D-dimer: supports presence of acute thrombus but is nonspecific
Confirmatory imaging
- CT venography or MR venography: visualize the site and degree of iliac vein compression, collateral vessels, and extent of thrombus
- Intravascular ultrasound (IVUS): considered the gold standard for assessing degree of compression and guiding stent placement, often performed at the time of catheter-directed intervention
- Catheter venography: can show characteristic filling defects, collateral filling, and compression; often performed with pressure gradient measurement across the stenosis
| Modality | Role |
|---|---|
| Duplex ultrasound | Initial DVT screen |
| CT/MR venography | Noninvasive confirmation of compression |
| IVUS | Gold standard, used during intervention |
| Catheter venography | Direct visualization, pressure gradients |
A pressure gradient across the stenosis and a compression of more than 50% on cross-sectional imaging support a hemodynamically significant lesion.
Differential diagnosis
- Primary (idiopathic) DVT from inherited or acquired thrombophilia without anatomic compression
- Pelvic or abdominal malignancy causing extrinsic venous compression
- Retroperitoneal fibrosis
- Nutcracker syndrome (left renal vein compression, different vein and symptom pattern)
- Post-thrombotic syndrome from a prior unrelated DVT
- Lymphedema (non-pitting edema, no clot on imaging)
- Cellulitis (fever, skin changes, but no venous obstruction)
The key distinguishing feature of MTS is left-sided symptoms with imaging evidence of iliac vein compression at the point where the right common iliac artery crosses.
Management
Management depends on whether the patient has acute thrombosis, chronic venous insufficiency symptoms, or an asymptomatic incidental finding.
Acute DVT due to MTS
- Anticoagulation is first-line for all patients with confirmed DVT, per ACCP/ASH and European guidelines
- Catheter-directed thrombolysis or pharmacomechanical thrombectomy is considered for extensive iliofemoral DVT, especially in younger patients with low bleeding risk and short symptom duration, to reduce post-thrombotic syndrome
- Endovascular stenting of the compressed iliac vein segment after clot removal is standard once the underlying stenosis is treated, since anticoagulation alone does not address the mechanical lesion and recurrence risk is high
Chronic or non-thrombotic MTS with symptoms
- Venous stenting is the primary treatment for hemodynamically significant compression confirmed by IVUS or venography
- Compression stockings and conservative measures for mild symptoms
Asymptomatic incidental compression
- No treatment needed; the anatomic finding alone is not an indication for intervention
Duration of anticoagulation
- Typically at least 3-6 months after an episode of provoked DVT with a correctable anatomic lesion (stenting); duration is individualized and some patients continue longer if risk factors persist, per guideline committee discretion (ACCP/ASH vs individual case-based practice differ slightly)
Surgical bypass or venous reconstruction is reserved for cases failing endovascular approaches.
Complications and prognosis
- Post-thrombotic syndrome: chronic pain, swelling, and skin changes from venous valve damage, significant risk if untreated acute DVT
- Pulmonary embolism: can occur from iliofemoral DVT, though lower extremity compression limits but does not eliminate this risk
- Recurrent DVT: common if underlying compression is not corrected with stenting
- Stent complications: in-stent restenosis or thrombosis, requiring surveillance imaging
- Chronic venous insufficiency: venous ulcers in long-standing untreated cases
- Phlegmasia cerulea dolens: rare but limb-threatening if iliofemoral thrombosis is massive
With appropriate anticoagulation and endovascular stenting, long-term stent patency rates are generally favorable, and most patients have significant symptom improvement. Prognosis is worse if diagnosis and treatment are delayed, increasing risk of post-thrombotic syndrome.
"May-Thurner: Think LEFT, LEG, LADY" — Left common iliac vein compression, Leg (left leg DVT), and Lady (young woman) is the classic triad to recall.
Practice questions
1. A 28-year-old woman presents with sudden-onset left leg swelling and pain one week postpartum. She has no history of travel, surgery, or known clotting disorder. Duplex ultrasound confirms a left iliofemoral DVT. Which anatomic finding is most likely to be identified on further imaging?
2. A 34-year-old woman is found to have greater than 50% compression of the left common iliac vein on CT venography after presenting with an extensive left iliofemoral DVT. She undergoes catheter-directed thrombolysis with significant clot resolution. What is the most appropriate next step in management?
3. A 45-year-old man presents with chronic right leg swelling and aching for several years without a history of trauma or clotting. He has no left leg symptoms. Which statement is most accurate regarding this presentation?
Frequently asked questions
What causes May-Thurner syndrome?
May-Thurner syndrome is caused by chronic compression of the left common iliac vein between the right common iliac artery and the underlying lumbar vertebra. This repetitive pressure causes vein wall thickening and narrowing, which slows blood flow and predisposes to clot formation, especially when combined with pregnancy, oral contraceptive use, or an inherited clotting disorder.
What are the first symptoms of May-Thurner syndrome?
The first symptoms are usually sudden left leg swelling, pain, and heaviness, often mistaken for a simple strain or injury. Some patients notice visible varicose veins or skin discoloration if the condition has been present for a while before diagnosis. Symptoms are almost always confined to the left leg because of the anatomy involved.
How is May-Thurner syndrome diagnosed?
Diagnosis starts with duplex ultrasound to confirm a blood clot, followed by CT or MR venography to visualize the iliac vein compression directly. Intravascular ultrasound, usually performed during a catheter procedure, is considered the gold standard for measuring the exact degree of compression and guiding treatment decisions.
Is May-Thurner syndrome curable?
May-Thurner syndrome is not cured in the sense of reversing the anatomy, but it is highly treatable. Endovascular stenting of the compressed vein segment, combined with blood thinners for any associated clot, restores normal blood flow in most patients and allows long-term symptom control with good outcomes.
May-Thurner syndrome vs deep vein thrombosis — what is the difference?
Deep vein thrombosis (DVT) is a blood clot in a deep vein that can have many causes, while May-Thurner syndrome is a specific anatomic condition (vein compression) that increases the risk of DVT, particularly in the left leg. May-Thurner syndrome is essentially an underlying cause or risk factor, while DVT is the clinical event that can result from it.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: I87.1
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.