May-Thurner Syndrome

If a scan says May-Thurner and your leg feels fine, do not start treating it. If one leg swells and stays swollen, or bursts with pain when you walk, book a vascular opinion this week.

Go to A&E instead, right now, if you are short of breath, have chest pain that is worse when you breathe in, or are coughing up blood.

The Verdict

Half of healthy adults have this on a scan. Only a swollen or painful leg makes it a condition.

Think of a garden hose running under a paving slab. The slab presses the hose flat, and for most gardens the water still gets through fine, because a hose carries far more than you ever ask of it. The narrowing only starts to matter when you turn the tap up, which is exactly what walking does to a leg vein. The ache is not the squashing itself. It is pressure backing up behind the flattened section when the leg asks for more drainage than that section can deliver.

An artery crossing over the main vein in your left leg presses it against your spine, and in healthy 13 to 20 year olds the average narrowing is 69.4%, with more than half above 70%, against 1.7% of healthy 35 to 65 year olds.
The common mistake is treating the scan instead of the leg: 47.4% of 1,698 people with no symptoms had narrowing of 50% or more, and not one of them had a problem.
What to watch for is one leg that swells and stays swollen, or a bursting ache when you walk that eases when you put the leg up.

Best for

Anyone told a scan shows iliac vein compression or May-Thurner. Anyone with persistent one-sided, usually left, leg swelling or a bursting ache on walking.

Skip if

Your leg swelled suddenly over hours or days, you are short of breath, or you are already under a vascular team. Those need a doctor now, not a webpage.

Want the full evidence? Keep scrolling

What Works

Read the tiers carefully. The highest-value act in this condition is a decision, not a treatment, and almost nothing a physical therapist owns reaches the top of the list.

Cinematic anatomical rendering of the iliac venous confluence

Tier 1 · Classify before you treat Strong

Decide which of three things you are looking at: an asymptomatic anatomical variant, a non-thrombotic lesion causing real symptoms, or a clot. Four asymptomatic cohorts totalling 3,216 subjects establish that the imaging finding on its own is not a disease.

Evidence: STRONG. Kibbe 2004, Zhu 2022, Li 2024, Wang 2025. Timeline: immediate.

Tier 1 · Urgent vascular management for acute clot Strong

Anticoagulation, and thrombus removal with stenting in selected patients. This is outside physical therapy scope entirely. Recognise and refer.

Evidence: STRONG. Deep venous stenting as an adjunct to early thrombus removal gave 12-month primary patency of 74 to 95% with post-thrombotic syndrome in 14.6% across 27 studies and 542 patients (Taha 2019).

Tier 2 · Iliac vein stenting for genuinely symptomatic obstruction Moderate

Reserved for people whose legs are actually causing problems. Stenting lowered the walking pressure rise by 26.2 mmHg (95% CI -41.2 to -11.3) and improved quality of life by 25.3 points, though in only 12 patients.

Evidence: MODERATE. 12-month patency 94.6% in non-thrombotic lesions versus 84.1% post-thrombotic (OR 0.29, 95% CI 0.14-0.63, I²=0%, Rodrigues 2021). No randomised comparison against conservative care exists, and one systematic review calls the evidence quality "currently weak" (Wang S 2020).

Tier 2 · Keep moving, do not go to bed Moderate

In treated acute DVT, walking with compression is part of the treatment rather than a risk to be managed.

Evidence: MODERATE. New pulmonary embolism 7.0% (95% CI 3.9-11.4) in iliofemoral DVT among 631 patients walking with compression and low-molecular-weight heparin, significantly below previously reported bed-rest rates (p<0.01), with one fatal PE at 0.2% (Partsch 1997). Non-randomised, 1997, never contradicted since.

Tier 3 — clinical reasoning only

Compression for symptoms Emerging

Reasonable for aching and heaviness. It has not been shown to prevent post-thrombotic syndrome, and no pressure has ever been tested in this specific condition.

Calf-pump activity, walking, elevation Expert opinion

Physiologically sound and never tested here. The authors of the source recommendation label it expert opinion themselves (Machin 2021).

Exercise Prescription

No trial has ever tested an exercise, a set, a repetition or an amount of walking in this condition. A dedicated rehabilitation literature sweep returned five papers and zero trials. Every number below is sensible practice, not a proven dose, and each is tagged accordingly. If you are under a vascular team or an anticoagulation clinic, their instructions override all of it.

Walking · 10-20 minutes · 2-3× daily consensus

The calf muscle is the pump that pushes blood back up the leg, and it only works when it is used. Stop and elevate if the leg becomes tight or bursting, and report that symptom.

Ankle pumps · 2 × 20 · every hour of sitting consensus

Point the toes away, then pull them back up. Should be effortless.

Calf raises · 2 × 15 · daily consensus

Rise onto the toes holding a worktop, lower slowly. Muscle effort is fine; leg tightness is not.

Leg elevation · 15-20 minutes · 2-3× daily consensus

Leg above heart height on pillows. The single most reliably useful thing on this list.

What Doesn't Work

Treating the scan. Intervening on asymptomatic compression. Roughly half of asymptomatic adults and the majority of asymptomatic teenagers have it, and Wang 2025 questions the need for routine intervention in that group outright.
Compression stockings to prevent post-thrombotic syndrome. Not demonstrated in randomised trials, and NICE NG158 advises against them for this purpose. They stay reasonable for symptom relief, which is a different claim.
Bed rest for acute DVT. Contradicted by the only large cohort that measured it, and it costs conditioning exactly when the calf pump is needed.
Ruling it out with a normal duplex scan. Duplex does not reliably see the iliac segment, and even venography reads normal in 34% of lesions confirmed on intravascular ultrasound. The symptomatic person with a clean scan is the one who gets missed for years.
A strengthening programme for the obstruction itself. No trial has tested one. Strength work is not forbidden, it just is not treatment for a squashed vein.

Red Flags

Refer, do not rehabilitate

Dark cinematic rendering of the deep venous system of the pelvis and thigh

Breathlessness, chest pain worse on breathing in, coughing blood, fainting. Pulmonary embolism, present at diagnosis in 9.9% of women and 1.6% of men in the symptomatic literature. A&E immediately.
One leg swelling suddenly over hours to days, painful, warm, tight. Deep vein thrombosis. Same-day medical assessment and imaging.
Collapse or shock with a swollen left leg, typically a woman around 60. Spontaneous iliac vein rupture: 76 pooled cases, 96.1% left-sided, 84.2% female, and 22.4% did not survive. Emergency, and frequently missed.
New or worsening leg swelling in pregnancy. Pregnancy can turn silent anatomy into symptomatic compression with no clot at all. Obstetric and vascular review.
Bursting leg pain on walking that is relieved by raising the leg. Venous claudication. Vascular referral, and do not load through it.
A stented leg where swelling or pain returns. In-stent restenosis or stent compression. Back to the implanting service.
Progressive skin changes or ulceration in one limb. Established outflow disease. Vascular referral.

Refer to: A&E for suspected embolism or rupture. Same-day GP or the ambulatory DVT pathway for a suspected clot. Vascular surgery for chronic one-sided swelling, venous claudication, ulceration or a stent problem. Paediatric vascular services for anyone under 18.

Return to Training

These apply after a thrombotic episode or a stent. An asymptomatic scan finding requires none of this, because there is nothing to return from.

Medical clearance obtained, and the prescriber's position on anticoagulation and contact or impact activity documented
Limb circumference back within 1 cm of the unaffected side by each morning
Can walk 20 minutes on the flat with no bursting leg pain
No new swelling in the 24 hours after a training session
Symptom-free through two consecutive weeks of progressive loading
If stented: no return of swelling or pain, and surveillance up to date with the implanting service

On anticoagulation the limiting factor is the drug, not the vein. Bleeding risk in contact sport, fall-prone activity and heavy overhead work is the prescriber's call. No set, repetition, load or frequency landmark applies, because none has ever been tested in this population.

Conviction

Moderate overall

Endpoint-stratified, because the claims are not equally supported:

Compression is a common anatomical variant, most severe in the young High
Symptomatic disease presents as left iliofemoral clot or chronic one-sided outflow symptoms High
Stenting achieves high technical patency Moderate-High
Severe compression raises absolute clot risk Moderate
Walking is safe in treated acute clot Moderate
Stenting beats good conservative care for how patients actually feel Low
Compression prevents post-thrombotic syndrome Low and negative
Any specific exercise prescription helps No evidence

What would change my mind about the prevalence claim

A prospective cohort of at least 2,000 asymptomatic adults with baseline venous imaging, followed five years or more with protocol-driven ultrasound surveillance, reporting absolute clot and chronic venous disease incidence by compression decile. The best current natural-history study followed 500 people for one year and recorded six events in total.

What would change my mind about stenting

A randomised trial of stenting versus structured conservative management in at least 300 patients with symptomatic non-thrombotic lesions, powered on a patient-reported outcome at 12 months rather than on whether the stent stayed open. Every treatment study retrieved here is single-arm or a comparative cohort.

The Full Picture — Anatomy, Diagnosis & Evidence

What's Actually Going On

Cinematic cross-section of the right common iliac artery crossing the left common iliac vein

The right common iliac artery crosses over the left common iliac vein and presses it against the fifth lumbar vertebra. The vein is soft, the artery pulses against it several billion times over a lifetime, and the vein wall answers by laying down fibrous tissue: the webs and spurs May and Thurner described.

Three different things can follow, and treating them as one condition is the commonest error here:

Anatomical compression alone. No symptoms. The majority state. A variant, not a disease.
Non-thrombotic lesion. The narrowing restricts outflow enough to raise pressure during activity, giving swelling, heaviness and bursting pain on walking, with no clot. Raju 2006 found these in 53% of 938 limbs with iliac obstruction and argued they are permissive rather than causative.
Thrombotic May-Thurner. Stasis above the narrowing plus vessel-wall injury produces a left iliofemoral clot, classically in a young person with no other risk factor.

The detail that reframes the whole condition is that compression is worse in the young and eases with age, which is the opposite of a wear-and-tear disease. Being underweight was associated with narrowing (OR 4.69, 95% CI 2.70-8.14) and obesity looked protective (OR 0.38, 95% CI 0.23-0.64) in 1,698 asymptomatic patients. The likely reason is mechanical rather than metabolic, since fat behind the abdomen cushions the vein against the bone. That is a hypothesis the study did not test, and it is not a reason to gain weight.

How to Identify It

Dark clinical rendering of a lower limb under vascular assessment

There is no bedside test, and that absence is the finding. Nothing across 58 papers reports a sensitivity or specificity for any physical examination manoeuvre in this condition. Figures do exist for general clot prediction rules, and they were deliberately not borrowed here, because those were built in people suspected of a clot rather than people with chronic outflow obstruction. A borrowed number in a diagnostic slot is an invented number.

Intravascular ultrasound, the reference standard Sn: >90% | Sp: data unavailable
Venography, which underestimates the problem Sn: 66% | Sp: data unavailable and reads normal in 34% of confirmed lesions
Any physical examination test Sn: data unavailable | Sp: data unavailable

What clinical assessment does contribute is pattern recognition, and the pattern is specific: unilateral, left-sided swelling that does not fully settle overnight, plus venous claudication. That last one is the highest-yield question available. Bursting leg pain on walking relieved by elevation is venous; pain relieved by rest and hanging the leg down is arterial. The pressure event behind it has been measured directly: common femoral vein pressure rose 34.8 ± 23.1 mmHg during walking in obstructed limbs, against 3.9 ± 5.8 mmHg in the same patients' unaffected limbs, roughly a nine-fold difference. Tellingly, the effect was absent at the foot, so a measurement taken lower down would have missed it entirely.

Cinematic comparison rendering of swollen versus normal lower limbs

The first question is always whether the swelling is one-sided. Bilateral swelling is systemic until proven otherwise: heart, kidney, liver, or a drug. One-sided and left points here, with lymphoedema, lipoedema, cellulitis, arterial disease and reflux-driven venous insufficiency as the working differentials.

The Debate

Rare condition, or ordinary anatomy?

Framed as rare since 1957. Yet compression sits in about 47% of asymptomatic adults, and 55.7% of healthy 13 to 20 year olds exceed 70% narrowing against 1.7% of healthy 35 to 65 year olds.

Resolution: the syndrome may well be uncommon, the anatomy is not. Case reports multiplied because imaging improved. Population prevalence of the actual syndrome has never been established, and a 2020 review exists to say exactly that.

A ten-fold risk, or a small one?

People with 50% or more compression had a 12-month clot rate of 6.8% against 0.7%, an adjusted relative risk of 10.162.

Resolution: both readings are true and the gap between them is everything. That multiplier rests on six events in 500 people, with a confidence interval running from 1.149 to 89.865. A large multiplier on a small base rate, in a variant half the population carries, does not turn the variant into a disease.

Does tighter compression mean more danger?

Severe compression was associated with lower risk of a clot travelling to the lungs (adjusted OR 0.18, 95% CI 0.06-0.54).

Resolution: mechanically coherent, since a tighter narrowing filters better. It is one single-centre retrospective study. It must not be used to reassure anyone, because the same anatomy increases the leg-clot burden it is diverting away from the lungs.

Stockings after a clot

Long-standing standard practice, but the effectiveness of elastic compression stockings for preventing post-thrombotic syndrome "has not been conclusively demonstrated in randomized trials", and NICE now advises against them for that purpose.

Resolution: wear them for how the leg feels, not as insurance against future damage.

Honest Limitations

The symptomatic literature has no denominator

The main review of who gets this condition explicitly excluded asymptomatic people with anatomical compression, and it synthesises case reports and series. Everything it says about age, sex and presentation comes from people already selected for being publishable.

Use it to recognise a presentation. Never use it to estimate how likely a given person is to have the disease.

Technical success is measured well, patient benefit badly

Stent patency is reported consistently and looks good. The field's own standard severity score discriminated improvement from non-improvement poorly across three years in 433 patients, and in-stent narrowing reached 74% by three months in 578 limbs.

A stent that stays open in a leg that still hurts counts as a success in this literature. Track what the person reports and the tape measure instead.

There is no rehabilitation evidence to translate

Not thin. Absent. The one exercise recommendation available for post-thrombotic legs is labelled expert opinion by its own authors, and no trial has tested a programme in iliac vein obstruction at all.

Everything prescribed here is reasoned from plumbing physiology and general practice. It should be labelled that way to the patient rather than sold as proven.

The Nuance

Conservative management success rate: not established. No study retrieved followed a conservatively managed symptomatic group to a defined outcome. That absence is the central gap in this condition.

Stenting: 12-month primary patency 94.6% in non-thrombotic lesions and 84.1% in post-thrombotic limbs. Patency of 73.4 to 98% at 12 months across nine studies and 953 patients. Ulcer healing 71.4 to 100% across twelve studies and 2,292 patients, though those authors call their own evidence base weak.

The honest position: stenting works well as a plumbing operation. Patency is high, complications are uncommon, and in symptomatic people it lowers the pressure causing the symptoms. What nobody has done is randomise symptomatic patients to stenting against good conservative care and follow what they actually report, so the comparative question is genuinely open despite how confident the field sounds. Against that, the case for not intervening on an asymptomatic scan finding is about as settled as anything here.

And a stent is a commitment rather than a repair. In-stent narrowing reaches 74% by three months, stent compression was present in 80% of limbs on day one, and 16.4% needed a further procedure at a median of 11 months.

Sources

Kibbe MR, et al. (2004). Iliac vein compression in an asymptomatic patient population. J Vasc Surg. PMID 15111841. N=50. 24% above 50% compression, 66% above 25%.
Wang K, et al. (2025). Frequency of left common iliac vein compression in asymptomatic adolescents and young adults. J Vasc Surg Venous Lymphat Disord. PMID 40545194. N=122. Mean stenosis 69.4%; 55.7% above 70% versus 1.7% aged 35-65.
Li HL, et al. (2024). Prevalence and predictors of radiological left common iliac vein compression in asymptomatic patients. J Vasc Surg Venous Lymphat Disord. PMID 37572778. N=1,698. Mean 46.2%; 47.4% at or above 50%.
Wu MK, et al. (2016). Incidence and risk factors of deep venous thrombosis in asymptomatic iliac vein compression: a prospective cohort study. Chin Med J. PMID 27625083. N=500 over 12 months; adjusted RR 10.162 (95% CI 1.149-89.865) on six events.
Zhu Q, et al. (2022). Prevalence of left iliac vein compression in an asymptomatic population and patients with left iliofemoral deep vein thrombosis. Phlebology. PMID 35786081. N=896 plus 93.
Raju S, Neglen P (2006). High prevalence of nonthrombotic iliac vein lesions in chronic venous disease. J Vasc Surg. PMID 16828437. N=4,026; venography 66% sensitive versus IVUS above 90%.
Shi Y, et al. (2023). Impact of common iliac vein compression on the incidence of pulmonary embolism in patients with acute deep vein thrombosis. Eur J Vasc Endovasc Surg. PMID 36931552. N=226; severe compression adjusted OR 0.18.
Partsch H, et al. (1997). The effect of mobilisation of patients during treatment of thromboembolic disorders with low-molecular-weight heparin. Int Angiol. PMID 9405014. N=631; new PE 7.0% in iliofemoral DVT while walking.
Kurstjens RLM, et al. (2018). The effect of stenting on venous hypertension, using a treadmill stress test with invasive pressure measurements. Eur J Vasc Endovasc Surg. PMID 29866528. N=12; walking pressure rise 34.8 mmHg versus 3.9 mmHg.
Kaltenmeier CT, et al. (2018). Systematic review of May-Thurner syndrome with emphasis on gender differences. J Vasc Surg Venous Lymphat Disord. PMID 29290600. 174 articles, N=1,569.
Rodrigues LDS, et al. (2021). Association between deep vein thrombosis and stent patency in symptomatic iliac vein compression syndrome. J Vasc Surg Venous Lymphat Disord. PMID 32827731. Meta-analysis, N=1,050.
Li M, et al. (2025). Iliac vein stenting outcomes in non-thrombotic and thrombotic diseases. Biomol Biomed. PMID 41090385. Meta-analysis, 27 studies, N=4,782.
Wang S, et al. (2020). Iliac vein stenting is a safe and effective treatment for iliac vein compression syndrome. Phlebology. PMID 32715909. 12 studies, N=2,292; authors state evidence quality is weak.
Taha MA, et al. (2019). A systematic review on the use of deep venous stenting for acute venous thrombosis of the lower limb. Phlebology. PMID 29788818. 27 studies, N=542.
Jayaraj A, et al. (2022). In-stent restenosis and stent compression following stenting for chronic iliofemoral venous obstruction. J Vasc Surg Venous Lymphat Disord. PMID 34174500. N=578 limbs.
Bai H, et al. (2023). Venous Clinical Severity Score has a suboptimal ability to detect improvement after iliac vein stenting. J Vasc Surg Venous Lymphat Disord. PMID 36906105. N=433.
Kooiman AL, et al. (2023). Spontaneous iliac vein ruptures: a systematic review. Vasc Endovascular Surg. PMID 36913198. 76 pooled cases.
Pikovsky O, Rabinovich A (2018). Prevention and treatment of the post-thrombotic syndrome. Thromb Res. PMID 28736157.
Machin M, et al. (2021). Surgical and non-surgical approaches in the management of lower limb post-thrombotic syndrome. Expert Rev Cardiovasc Ther. PMID 33455484.
Harbin MM, Lutsey PL (2020). May-Thurner syndrome: history of understanding and need for defining population prevalence. J Thromb Haemost. PMID 31821707.

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