May Thurner Syndrome: left iliac vein compressed by the overlying artery
The slow blood flow can also lead to blood clots forming in the arms or more commonly the legs, called a Deep Vein Thrombosis (DVT). This can cause sudden swelling of the limb. There is also a danger of the blood clot breaking off and travelling to the lungs, known as a Pulmonary Embolus (PE). This is a medical emergency and can be life threatening.
In the longer term the DVT scars and narrows the vein resulting in worse flow of blood, more swelling, skin changes and eventually ulceration. This is known as 'Deep Venous Insufficiency' and 'Post Thrombotic Syndrome'.
A common place for the deep vein narrowing is left common iliac vein, which is in the pelvis at the top of the left leg. The vein is compressed between the overlying right iliac artery and the bones of the pelvis behind them. This is also called 'May-Thurner Syndrome'.
The vein narrowing of May-Thurner Syndrome is quite a common finding and doesn't cause problems in most people. For some patients it may cause mild swelling of the leg for many years, but more commonly it suddenly causes a DVT with swelling and pain of the leg. The DVT can be diagnosed with ultrasound, CT and MRI.