Normal veins have healthy (competent) valves. The venous walls are strong enough to withstand the lateral pressure of blood exerted on them. Blood flows through competent valves in one direction, toward the heart. In varicose veins, also known as varicosities, dilation of veins from long periods of pressure prevents complete closure of the valves. Unhealthy or damaged (incompetent) valves do not close completely. The incompetent valves result in a backflow and pooling of blood in the veins. This pooling causes varicosites that contribute to enlarged, twisted superficial veins, called varicose veins. They commonly appear blue, bulging, and twisted. If left untreated, varicos veins can cause aching and feelings of fatigue as well as skin changes. Because the blood pools, the risk of thrombosis is increased as well. Treatment consists of sclerotherapy and such surgical intervention as EVLA of the greater saphenous (large) veins in the legs and dmicrophlebectomies of the lesser saphenous (small) veins.