Both deep vein thrombosis (DVT) and thrombophlebitis are vascular conditions in which the veins become inflamed and blood clots form inside. These conditions almost always develop in the veins in the legs, but occasionally occur in the arms or, more rarely, other parts of the body. Thrombophlebitis occurs in the veins close to the skin (the superficial veins), while DVT occurs in the deep veins (those farther below the skin surface).
The most significant danger for people with DVT is a pulmonary embolism, an infrequent but potentially fatal complication. A pulmonary embolism occurs when part of a blood clot breaks off, travels through the veins to the heart, and from there into the pulmonary arteries, which supply the lungs with blood. The clot can cut off the lung's blood supply, causing shortness of breath, chest pain, and other symptoms.
DVT can also cause post-thrombotic syndrome, a condition in which the veins are permanently damaged. Patients with post-thrombotic syndrome develop varicose veins, pain, swelling, and, in some cases, skin ulcers.
Thrombophlebitis is generally caused by irritation to the lining of the vein from, for example, prolonged intravenous injection of medications or infection. There is a very small risk of the blood clot traveling from the superficial veins into the deeper veins, but serious complications due to thrombophlebitis are extremely rare.