Thoracic Outlet Syndrome (TOS) is a condition that occur when blood vessels, arteries or veins in the space between your collarbone and your first rib (thoracic outlet) are compressed. There are three main types of this syndrome – neurogenic, venous and arterial. The neurogenic type is the most common type and is caused by compression of components of brachial plexus nerves. Arterial type of Thoracic Outlet Syndrome is rare and is caused due to compression of subclavian artery. Venous type is extremely rare type of TOS and is caused due to compression of subclavian vein.