What is May-Thurner Syndrome People can have narrowing of a vein for several reasons. One of them is an anatomic variation called May-Thurner syndrome.

In normal anatomy, the artery leading to the right leg (called the right common iliac artery) rests on top of the vein coming from the left leg (the left common iliac vein).

In some people, the artery puts increased pressure on the vein, causing the vein to be narrowed (Figure 2). People are born with this variant and have it throughout their life.

The narrowing can range from mild to severe. Some studies estimate that 20% of people have some degree of narrowing. Severe narrowing is less common.

Veins can also become chronically narrowed as a result of scarring from previous DVTs or from external compression (eg, from a cancer).

Significant narrowing can lead to blood flow disturbance and can increase the chance of developing a DVT.