Lower limb edema is a common clinical condition. With the gradual and widespread clinical application of ultrasound technology, especially the development of ultrasound high-frequency technology, the application of ultrasound for lower limb edema has gradually increased, so we have made this topic to review the causes, pathogenesis, and ultrasound characteristics of lower limb edema. Lower extremity edema is the accumulation of excessive fluid in the tissue spaces of the lower extremities, resulting in tissue swelling. It is classified as cardiogenic edema, nephrogenic edema, hepatogenic edema, dystrophic edema, mucinous edema, drug-related edema, premenstrual tension syndrome edema, idiopathic edema, lymphoedema, venous edema, etc. Cardiogenic edema: It is mainly a manifestation of right heart failure, and the pathogenesis is mainly due to the decrease of effective circulating blood volume, decrease of renal blood flow, secondary aldosterone increase causing sodium and water retention and venous stasis, decrease of capillary filtration pressure, and decrease of tissue fluid reabsorption. The edema is characterized by a symmetrical, depressed edema that first appears in the hypochondriac region of the body. It is often combined with other manifestations of right heart failure. 2, nephrogenic edema: mainly seen in various types of nephritis and nephropathy, the pathogenesis is mainly caused by a variety of factors that reduce renal excretion of water and sodium, resulting in sodium and water retention, increased extracellular fluid, and increased capillary hydrostatic pressure, causing edema. Edema is characterized by eyelid and facial edema when waking up in the morning in the early stage of the disease, and later develops into generalized edema. There are often urinary changes, hypertension, and renal impairment. 3. Hepatogenic edema: It is mainly seen in the decompensated stage of cirrhosis, and the main pathogenesis is portal hypertension, hypoproteinemia, impaired hepatic lymphatic fluid reflux, and secondary aldosterone increase. There are often manifestations of both hepatic hypofunction and portal hypertension. 4, malnutrition edema: mainly due to chronic wasting disease long-term nutritional deficiency, protein-losing gastrointestinal disease, severe burns and other causes of hypoproteinemia or vitamin deficiency. Characteristically, edema is often preceded by wasting, weight loss and other manifestations. Edema often starts from the feet. 5. Idiopathic edema: It occurs mostly in women, mainly in the hypophysical area, and the cause is unknown, probably due to endocrine disorders and abnormal response to upright body position. 6.Mucinous edema: mostly caused by high protein content in tissue fluid, mostly seen in hypothyroidism, obvious in face and lower limbs. 7, drug edema: mostly seen in the treatment of glucocorticoids, androgens, estrogens, insulin, etc. 8, lymphedema: divided into two categories: primary and secondary. The former is caused by normal lymphatic vessels development disorder, which is less common in clinical practice. The latter is often caused by recurrent dengue, filariasis infection or regional lymph node dissection, etc. and is more common. Although the etiology of lymphedema is different, the pathological changes are generally similar: at the beginning of the disease, the lymphatic reflux is obstructed, resulting in increased pressure in the lymphatic vessels, distortion and expansion of the lymphatic vessels, and gradual loss of valve function, resulting in the backflow of lymphatic fluid, which eventually affects the absorption of intertissue fluid and macromolecular substances by capillary lymphatic vessels, resulting in the accumulation of fluid and protein in the tissue interstitial space and thickening of subcutaneous tissue, when the skin is still smooth and soft, with indentations by finger pressure. If the lesion persists, under the long-term stimulation of high protein edema fluid, the skin and subcutaneous tissues produce a large amount of fibrous connective tissue, and the walls of lymphatic vessels gradually thicken, fibrosis and even sclerosis, making it more difficult for tissue fluid to enter the lymphatic vessels, and the interstitial edema of tissues becomes more serious. At the same time, local sweat and sebaceous gland dysfunction also easily leads to infection, which further promotes local tissue fibrosis and aggravates lymphatic duct obstruction, and so on in a vicious circle, making the lesions of lymphatic ducts aggravate. In time, the skin becomes thickened and rough, hard like elephant skin, so it is also called elephant skin swelling. 9. Venous edema: Mostly due to deep vein thrombosis or venous valve insufficiency, which leads to the increase of deep vein pressure and poor reflux, small molecules of blood penetrate into the tissue interstices around the vessels, forming low protein edema.