May–Thurner syndrome

May–Thurner syndrome (MTS), also known as the iliac vein compression syndrome, is a condition in which compression of the common venous outflow tract of the left lower extremity may cause discomfort, swelling, pain or iliofemoral deep vein thrombosis.

Specifically, the problem is due to left common iliac vein compression by the overlying right common iliac artery. This leads to stasis of blood, which predisposes 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 twenty-first 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.