Varicose Veins are extremely common with Klippel-Trenaunay Syndrome. The natural flow of blood returning to the heart through the veins is prevented from flowing in the wrong direction by valves within the veins. Varicose veins are caused when the decreased efficiency of blood circulating through the vessels causes greater than normal pressure or weight from the blood. The excess pressure from gravity acting on the blood within the veins causes the walls of the vein to weaken, the valves may become damaged or defective and collapse. When this occurs, a varicose vein has developed. The picture below demonstrates this principle.