What exactly is primary lymphedema?

Hereditary: Lymphedema usuallly occurs secondary to trauma to the lymphatic system from surgical removal of lymph nodes. Primary lymphedema occurs due to inherited conditions that affect the development of lymph nodes or the valves within the lymphatics. These diseases are very rare.

Not an identifiable: Cause. As dr. Rosen said, most lymphedema is secondary, meaning it is due to something like surgery and/or radiation. Primary lymphedema is divided into three categories: congenital lymphedema, lymphedema praecox, and lymphedema tarda. Congenital means you are born with it and a small group of these are hereditary. Praecox develops before age 35 (most have this) and tarda develops after 35.