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The term edema refers to the accumulation of an excessive amount of fluid in cells, tissues, or organs and is clinically characterized by swelling. A variety of conditions can produce edema including chronic venous insufficiency, infection, and trauma. Lymphedema is a progressive condition which may be defined as the swelling of subcutaneous tissue as a result of obstruction of lymphatic vessels or lymph nodes resulting in the accumulation of large amounts of lymph in the affected region. Any disease process that interferes or disrupts the normal mechanical process by which the lymphatic system collects, filters, and returns lymph fluid back into the circulatory system can result in excessive accumulation of lymph fluid in the area affected by the blockage. The capacity of the lymphatic system to drain excess fluid away from body tissue is reduced or interrupted. Since the arterial capillaries continue to deliver fluid to the tissue, the volume of fluid increases significantly until the venous capillaries are no longer able to compensate for the defective lymph vessels. At this point, when drainage has been reduced by 80% or more, symptoms of lymphedema become apparent.

Although lymphedema can develop in any area where the normal flow of lymph fluid has been interrupted, it is most commonly seen in the extremities and almost always takes place at the most superficial level of the vessels under the skin (subcutaneous). Other areas where lymphedema can develop include the head/neck region, chest, body cavities, pelvic region, and genitals. When lymph fluid accumulates due to blockage, it can reduce the amount of nutrients that reach the cells and can interfere with wound healing which results in increased risk of infection.

There are varying degrees of severity of lymphedema, from mild swelling to extreme swelling which can cause life-threatening complications. Initially, the patient may notice that one limb is slightly larger than the other. Approximately 50% of patients with mild lymphedema report feelings of heaviness or fullness in the affected limb. As the swelling progresses, clothing it may become difficult to fit clothing over the affected limb. Eventually, the size and weight of the edematous limb may cause fatigue, embarrassment, and have a significant impact on the quality of life. Maximal girth of the limb is usually reached within the first year of onset of symptoms. The earlier the diagnosis of lymphedema is established and treatment initiated, the lower the risk of developing a large distorted limb and multiple skin changes.

Lymphedema may develop immediately after surgery. It is usually mild and resolves within a couple of weeks. If lymphedema develops approximately 6-8 weeks following surgery or radiation, it also typically resolves within a few weeks with treatment. The most common form of lymphedema, however, develops very slowly over time and may become manifest many months or years following treatment. It can be mild or severe but in any case, must be treated as soon as possible and as aggressively as possible to prevent further complications.

Lymphedema differs from edema in that edema is a secondary symptom of an underlying medical condition while lymphedema itself is a medical condition. Edema is a result of too much fluid being released from the capillaries into the tissue which overwhelms the capacity of the lymphatic system and results in swelling and fluid accumulation. In lymphedema, on the other hand, it is the lymphatic system itself which is compromised. The capillaries continue to function normally and continue to release their fluid in the interstitial spaces but that fluid is not being drained away from the site. The progression of lymphedema occurs when the balance between the increasing lymph load and reduced drainage or outflow capacity becomes too great for any compensatory mechanism to be effective. This is why lymphedema may be latent for many years before the symptoms become evident.