Lymphatic malformations are sponge-like collections of abnormal channels and spaces that contain clear fluid. The lymphatic system normally collects excess fluid from the tissues and transports it through a series of small vessels back into the venous system. With a lymphatic malformation, however, transfer of this fluid through these vessels is slowed. The excess fluid accumulates and dilates the vessels, resulting in a swelling of the affected area and sometimes in more extensive enlargement of soft tissues and bones.

These lesions are most commonly seen in the neck (illustration 1) and axilla (armpit) (illustration 2), but they can involve any area of the body. In certain areas the dilated lymphatic vessels tend to be small (illustration 3), while in other areas, they tend to be large (illustration 4). As with other vascular malformations, lesions may be superficial or deep, and localized or diffuse. They steadily increase in size, although some enlarge more rapidly than others. Conditions such as infection or trauma can result in sudden but temporary enlargement.

Although the exact cause of lymphatic malformations is unknown, they are thought to be caused by errors in the formation and development of lymphatics during fetal development. The cause is not related to any known drug or medication that may have been taken during pregnancy or to any environmental exposure that may have occurred during that time.