Chronic Venous Disease (CVD) refers to other chronic conditions related to or caused by veins that become diseased or abnormal. Varicose veins are veins that become thickened, twisted, dilated, or elongated blood vessels that do not control blood flow as they should. They can be small spider veins, appear as threads, and in other cases, can appear as large clusters of grapes under the skin. Some patients may have varicose veins that are not visible to the naked eye, however, they are easily identified with a diagnostic ultrasound.