Pain and swelling that affects only one leg most often occurs because of trauma or disease to that leg, rather than a systemic disease. A number of conditions, some serious, can cause one leg to swell and hurt. Leg pain and swelling merit medical attention, especially if it's accompanied by other symptoms such as fever, redness or chest pain.
Deep Vein Thrombosis
Formation of blood clots in the lower leg, called deep vein thrombosis, or DVT, occurs commonly after injury to a vein or if blood flow slows, allowing clots to form. People who have blood that clots more than usual because of genetic tendencies or because they’re taking medications that foster clotting, like birth control pills, also may develop DVT. Rarely, people who sit for long times without moving their legs often can develop DVT, although this normally occurs only in people with other risk factors. DVT can cause leg swelling over the blood clot, pain and tenderness to the touch and redness.
A clot can break loose and travel through the blood vessels, lodging eventually in the pulmonary system. Called a pulmonary embolism, a clot in the pulmonary system causes chest pain or shortness of breath and can lead to collapse or sudden death. Medication to thin the blood prevents clots in susceptible people or treats those with DVT.
Cellulitis, a common bacterial infection that causes pain, redness and swelling, most often occurs on the lower leg, along with fever and warmth. The skin may develop small red spots or blisters. Injury from something as minor as an insect bite at the site allows bacteria to enter and causes cellulitis. People with weakened immune systems have an increased risk of developing cellulitis. Antibiotic treatment cures the infection.
Accumulation of lymphatic fluid in an injured leg, known as lymphedema, can occur after trauma, surgery, infection, radiation or cancers in which lymph nodes in the leg or pelvis were removed. Swelling leads to pain, a heavy feeling in the leg and a decrease in function as the leg becomes difficult to bend or hard to stand on. Manual drainage of the area, which encourages the lymph to travel out of the damaged area, and compression bandaging treat lymphedema.
Chronic Deep Vein Insufficiency
Previous DVT in a leg can cause scarring that results in permanent damage to the valves in the leg that maintain normal blood flow, a condition called chronic deep vein insufficiency. Edema accumulates in the lower leg, causing swelling, and the skin becomes red, itchy and scaly. Pain occurs when standing or walking on the leg. The fragile skin over the injury breaks and tears easily, resulting in ulcers and sores. Compression bandages help reduce the swelling in the leg.