This condition occurs when the left iliac vein is compressed between the right iliac artery and the spine, resulting in increased venous pressure due to the compression of the outflow tract of the left extremity and increasing the risk of thrombosis.

This occlusion prevents the outflow of blood from the left leg and the left side of the pelvis through the left iliac vein into the inferior vena cava. Many patients develop a marked collateral circulation to work around blood flow blockages. This results in a change of flow in the left internal iliac vein, thus pooling blood into the pelvis and generating strong venous plexuses that surround certain organs such as the uterus, ovaries, rectum, and bladder.