May-Thurner Syndrome, also known as iliac vein compression syndrome, occurs when the right iliac artery squeezes the left iliac vein against the lumbar spine inside the abdomen. Because of this compression, blood flow is slowed and pressure is increased in the left pelvis and leg. The majority of patients are asymptomatic and do not require treatment. Some patients may show symptoms such as large Deep Venous Thrombosis (DVT), which is a type of blood clot that can be very serious.