Lymphedema is the swelling of tissues (edema), usually the feet and legs, due to lymphatic obstruction.

Lymphatic fluid seeps out of the blood circulation into the tissues. It returns to the heart through separate channels called lymphatics, carrying waste products. On its way to the heart, it passes through lymph nodes, where infecting germs (including some cancers) are attacked by the body's defense mechanisms.

If lymphatic channels are obstructed or inadequate, fluid backs up and causes edema.  Tissue fluid can also return to the circulation through tissues, without using the lymphatics, but gravity hinders this flow. So lymphedema is usually confined to the feet and legs.

There are several types of congenital abnormalities associated with other birth defects of the lymphatics, which cause this condition. One in 10,000 people have this type of lymphedema.

Lymphatics can be damaged or obstructed by many different agents. Repeated bouts of blood poisoning can scar the vessels. Surgery to remove cancerous lymph nodes or radiation therapy can damage them. Cancer itself, as it invades the lymph system, as well as several other infectious and inflammatory conditions, can result in blockage of lymph flow. The most common worldwide cause of lymphedema is a group of worms known as filaria.  Filaria can be found in most of the developing regions of the world. They enter humans through insect bites, mostly mosquitos, and take up residence in lymphatic channels, irritating them enough to scar them and impair their ability to carry lymph. Long-standing lymphatic filariasis can cause massive swelling of the legs, earning the name elephantiasis.  

When lymphedema is left untreated, the outcome is often disabling, painful, and continually progressive. With lymphatic drainage impaired, edema fluid and protein gradually collect in the soft tissue eventually causing the production of fibrotic tissue and providing a natural medium for bacterial infection

Complex decongestive physical therapy has been the accepted treatment approach for lymphedema in Europe and Australia for many years. Though relatively new in the United States, it is quickly replacing the traditional use of pumps and garments alone.