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In medicine, May–Thurner syndrome (MTS), also known as the iliac vein compression syndrome, is a rare condition in which compression of the common venous outflow tract of the left lower extremity may cause discomfort, swelling, pain or blood clots, called deep venous thrombosis (DVT), in the iliofemoral vein.
The specific problem is compression of the left common iliac vein by the overlying right common iliac artery. This leads to pooling or stasis of blood, predisposing the individual to the formation of blood clots. Uncommon variations of MTS have been described, such as the right common iliac vein getting compressed by the right common iliac artery.
In the 21st century the May–Thurner syndrome definition has been expanded to a broader disease profile known as nonthrombotic iliac vein lesions (NIVL) which can involve both the right and left iliac veins as well as multiple other named venous segments.  This syndrome frequently manifests as pain when the limb is dependent (hanging down the edge of a bed/chair) and/or significant swelling of the whole limb.
Epidemiology and diagnosis
May-Thurner syndrome (MTS) is thought to represent between two to five percent of lower-extremity venous disorders.[medical citation needed] May–Thurner syndrome is often unrecognized; however, current estimates are that this condition is three times more common in women than in men. The classical syndrome typically presents in the second to fourth decades of life. In the 21st century in a broader disease profile, the syndrome acts as a permissive lesion and becomes symptomatic when something else happens such as, following trauma, a change in functional status such as swelling following orthopaedic joint replacement.
It is important to consider May–Thurner syndrome in patients who have no other obvious reason for hypercoagulability and who present with left lower extremity thrombosis. To rule out other causes for hypercoagulable state, it may be appropriate to check the antithrombin, protein C, protein S, factor V Leiden, and prothrombin G20210A.
Venography will demonstrate the classical syndrome when causing deep venous thrombosis.
May–Thurner syndrome in the broader disease profile known as nonthrombotic iliac vein lesions (NIVLs) exist in the symptomatic ambulatory patient and these lesions are usually not seen by venography. Morphologically, intravascular ultrasound (IVUS) has emerged as the best current tool in the broader sense. Functional testing such as duplex ultrasound, venous and interstitial pressure measurement and plethysmography may sometimes be beneficial.
In contrast to the right common iliac vein, which ascends almost vertically to the inferior vena cava, the left common iliac vein takes a more transverse course. Along this course, it lies under the right common iliac artery, which may compress it against the lumbar spine. Iliac vein compression is a frequent anatomic variant, as 50% luminal compression of the left iliac vein occurs in a quarter of healthy individuals. Compression becomes clinically significant only if causes appreciable hemodynamic changes in venous flow or venous pressure, or if it leads to acute or chronic DVT.
In addition to compression, the vein develops intraluminal fibrous spurs from the effects of the chronic pulsatile compressive force from the artery. The narrowed turbulent channel predisposes the patient to thrombosis. The compromised blood flow often causes collateral blood vessels to form. These are most often horizontal transpelvis collaterals, connecting both internal iliac veins, thus creating outflow through the right common iliac vein. Sometimes vertical collaterals are formed, most often paralumbar, which can cause neurological symptoms, like tingling and numbness.
Management of the underlying defect is proportional to the severity of the clinical presentation. Leg swelling and pain is best evaluated by vascular specialists (vascular surgeons, interventional cardiologists, interventional radiologists) who both diagnose and treat arterial and venous diseases to ensure that the cause of the extremity pain is evaluated. The diagnosis needs to be confirmed with some sort of imaging that may include magnetic resonance venography, venogram and usually confirmed with intravascular ultrasound because the flattened vein may not be noticed on conventional venography. In order to prevent prolongued swelling or pain from the consequences of the backed up blood from the compressed iliac vein, flow needs to be improved out of the leg. Uncomplicated cases may be managed with compression stockings.
Severe May–Thurner syndrome may require thrombolysis if there is a recent onset of thrombosis, followed by angioplasty and stenting of the iliac vein:1006-1007 after confirming the diagnosis with a venogram or an intravascular ultrasound. A stent may be used to support the area from further compression following angioplasty. As the name implies, there classically is not a thrombotic component in these cases, but thrombosis may occur at any time.
If the patient has extensive thrombosis, it may be appropriate to consider pharmacologic and/or mechanical (also known as pharmacomechanical) thrombectomy. This is currently being studied to determine if this will decrease the incidence of post-thrombotic syndrome.
- Chronic cerebrospinal venous insufficiency – Venous compression in the neck.
- Thoracic outlet syndrome (TOC) – Compression of the brachial plexus or subclavian vessels.
- Paget–Schroetter disease – Upper extremity deep vein thrombosis in the axillary or subclavian veins, related to TOS.
- Budd–Chiari syndrome – Venous compression or obstruction in the liver.
- Nutcracker syndrome - Compression of the left renal vein between aorta and upper mesenteric artery.
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