Answer:
(From Grabb and Smith CD-ROM:)
“Historically, lymphedema has been defined as a deficiency in the lymphatic system. The etiology of the lymph flow abnormality has led to a traditional classification scheme that categorizes lymphedema as primary or secondary. In primary lymphedema, the disease is believed to be genetically determined, and expression can occur at birth (Milroy’s Disease), puberty (lymphedema praecox), or mid-life (lymphedema tarda). In secondary lymphedema, the disease develops as a result of a known inciting event such as infection or surgical ablation.
"However, why do the vast majority of patients following regional node dissection not develop lymphedema? We believe a congenital predisposition exists and a spectrum of dysfunction is likely. Patients who are classified as having primary (congenital) lymphedema may in fact have a more severe form of lymphatic dysfunction, while patients with secondary (acquired) lymphedema develop swelling only after some serious predisposing event. The capacity for collateral flow and regeneration of damaged lymphatics may vary considerably among patients and may explain whether or not lymphedema develops.
"The uncertainties regarding the etiology of primary lymphedema derive from its many unusual, unexplained features: women are afflicted at least three times more frequently than men and often develop edema around the time of menarche; the left leg is affected more often than the right, and upper extremity involvement is rare.
"The most common cause of secondary lymphedema worldwide is direct infestation of lymph nodes by the parasite Wuchereria bancrofti. In western countries, however, damage or removal of regional lymph nodes by surgery, radiation, tumor invasion or the result of infection or inflammation are the most common causes of secondary lymphedema. The overall incidence of lymphedema following breast cancer surgery has been reported to be 8%, with the greatest prevalence among those who undergo extensive axillary surgery followed by axillary radiation. The swelling is usually delayed for approximately a year as a result of the ongoing fibrotic process and generally begins in the upper arm.