In the early stage of lower extremity deep vein thrombosis, the venous blood return is obstructed due to the thrombus formation blocking the vein. At the later stage, the thrombus in the vein is mechanized and recanalized, but the vein is narrowed, dilated and tortuous, and the venous valve is destroyed, as well as the venous valve of the traffic branch is destroyed, and the blood of the deep vein flows backward to the superficial vein, so that the veins of the lower limbs stasis, the venous pressure is increased, the tissue is hypoxic, and finally the syndrome of deep vein thrombosis of the lower limbs – stasis syndrome appears. The main clinical manifestations are: chronic ulcers of the lower extremities occur on the basis of dystrophic lesions: skin pigmentation, fibrosis sclerosis, often located in the lower 1/3 of the calf, medial or lateral, and do not heal over time. The edges of the ulcer are irregular and smooth with white neoplastic epidermis, the ulcer base is usually pink with granular tissue, surface or with yellow secretions, and the skin around the ulcer has skin damage i.e. edema, hyperpigmentation, sclerosis, dermatitis, skin fibrosis, varicose veins, etc. This kind of ulcers can not heal for a long time or still recur after healing, which seriously affects the normal life and work of patients, and some ulcers may even become cancerous, or need amputation treatment. Varicose veins of lower limbs In the late stage of deep vein thrombosis of lower limbs, due to venous insufficiency, venous reflux obstruction and long-term stagnation, varicose veins may appear, from venous anger to varicose veins and skin microvascular anger, presenting extensive varicose veins of lower limbs. The varicose veins of the lower leg branches and small veins are predominant, and small saphenous vein and saphenous vein valve insufficiency are common. Stasis dermatitis Due to long-term blood stasis in the lower extremities, eczema-like dermatitis of the lower legs may occur, also known as stasis dermatitis. Itchy skin, vesicles, oozing, and chronic ulcers in the lower legs due to low local resistance, which can easily cause infection. Skin pigmentation Skin pigmentation is usually located in the lower and middle 2/3 of the lower leg, but rarely in the upper leg. Initially, the pigmentation is patchy, gradually expanding and aggravating, showing widespread light brown and brownish color, and the skin and subcutaneous tissue are fibrous and hard, like “leather” (stasis), and dystrophic lesions of the lower limbs appear. Insufficiency of deep vein valves in the lower extremities The valves of deep veins and traffic branch veins are destroyed, and blood flows backwards from deep veins into superficial veins. The superficial femoral veins are most affected by the gravity of the blood column and have a higher chance of valve involvement; the deep femoral veins are less affected by the gravity of the blood column and have a lower chance of damage due to their anatomical relationship. Lower extremity lymphedema can occur due to obstruction of the deep veins of the lower extremities, obstruction of blood return, local tissue stasis and increased tension, which affects lymphatic return. In the late stage of lower limb deep vein thrombosis, the skin is stagnant and hypoxic, the resistance of the affected limb is weakened, and secondary infections, such as dermatitis and acute cellulitis, are likely to occur repeatedly, resulting in the obstruction or destruction of lymphatic vessels, the obstruction of lymphatic flow at the distal end, and the abnormal increase of lymphatic fluid in the tissue interstitial space, resulting in lymphedema. Secondary infections of the lower extremities The lower extremities are in a state of chronic stasis, local resistance is reduced, and minor skin injuries can easily lead to secondary cellulitis, dermatitis, and other diseases. The most common is chronic inflammation of the lower leg with redness and burning painful hardness (chronic stasis inflammation), without fever. Clinically, it is common to see that lower extremity deep vein thrombosis syndrome, often secondary to recurrent episodes of dermatitis, with high fever of 39°C to 41°C, thick, hard and tough affected areas, and the formation of elephantiasis. Lower extremity deep vein thrombosis complicated by elephantiasis (chronic stasis of severe disease) is quite difficult to treat.