May-Thurner syndrome

# Overview
MayThurner 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. MayThurner syndrome is therefore more common in the left leg as the artery acutely overlaps the left iliac vein.  In the 21st century the MayThurner 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 and/or significant swelling of the whole limb.

# Epidemiology and Diagnosis
It causes between two and five percent of lower-extremity venous disorders. MayThurner 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 MayThurner 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, check the patient'sAntithrombin, Protein C, Protein S, Factor V Leiden, and Prothrombin G20210A.
Venography will demonstrate the classical syndrome when causing deep venous thrombosis.
MayThurner 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.

# Mechanism
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 underlies the right common iliac artery, which may compress it against the lumbar spine.
In addition to compression the vein develops 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, numbness etc.
This compressed, narrowed outflow channel causes stasis of the blood, which is one element of Virchow's triad that precipitates deep vein thrombosis.

# Treatment
Management of the underlying defect is imperative to preventing further exacerbations and aggravation of the underlying malformation. Patients with MayThurner syndrome may require an angioplasty and stenting of the iliac vein. A braided stainless steel stent is used to support the area from further compression following angioplasty. In the 21st century, ambulatory patients with significant symptoms affecting their quality of life increasingly have non-thrombotic iliac vein lesions treated with angioplasty as well as stent placement. As the name implie,  there is not a thrombotic component in these cases.
If the patient has extensive thrombosis, it may be appropriate to consider pharmacologic and/or mechanical (also known as pharmacomechanical) thrombectomy to decrease the incidence of post-thrombotic syndrome. At the time of the thrombectomy the use of an inferior vena cava filter may be considered to prevent pulmonary embolism in these patients although filter use continues to evolve and will not correct the underlying lesion alone.