Spider veins are a type of varicose veins. They are small dilated blood vessels near the surface of the skin. They can develop anywhere on the body, including the legs, specifically on the upper thigh, below the knee joint, and around the ankles. While many people think that varicose veins are simply a cosmetic issue, there is in fact often an underlying medical problem that causes varicose veins. This medical condition is called venous reflux disease.