Secondary lymphoedema develops in people who previously had a normal lymphatic system that then becomes damaged.
It can have a number of different causes. Some of the most common causes are explained below.
Surgery for cancer
Treatment for cancer can involve surgery to remove sections of the lymphatic system.
The surgeon will try to limit damage to your lymphatic system, although this is not always possible.
There's a particular risk of lymphoedema occurring after treatment for any cancer where lymph glands are removed.
Some of the more common cancers where this happens are:
Radiotherapy uses controlled doses of high energy (radiation) to destroy cancerous tissue, but it can also damage healthy tissue.
If radiotherapy is needed to destroy cancerous cells in your lymphatic system, there's a risk that the lymphatic system could become permanently damaged and unable to drain fluid properly.
An infection, such as cellulitis, can sometimes cause lymphoedema. Severe cellulitis can damage the tissue around the lymphatic system, causing it to become scarred.
Filariasis can also cause lymphoedema. Lymphatic filariasis is a parasitic disease caused by microscopic, thread-like worms.
The adult worms only live in the human lymphatic system and block lymph drainage. It's a common cause of lymphoedema worldwide, but is not generally a risk in the UK.
Medical conditions that cause tissue to become red and swollen can also permanently damage the lymphatic system.
Conditions that can cause lymphoedema include:
Diseases that affect the flow of blood through the veins can cause lymphoedema in some people.
The abnormal or damaged veins can cause fluid to overflow from the veins into the tissue spaces.
This overwhelms and eventually exhausts the parts of the lymphatic system responsible for draining this fluid.
Some venous diseases that can lead to lymphoedema include:
Obesity is another possible cause of secondary lymphoedema.
People who are obese, particularly those who are severely obese, have an increased risk of developing swollen body parts.
It's not clear exactly why this is, but it's been suggested that the extra fatty tissue affects the lymphatic channels in some way, reducing the flow of fluid through them.
Check if you are obese on the BMI calculator.
In these cases, weight loss is an important part of treatment and even just starting to lose weight can make a big difference to the swelling.
Trauma and injury
In a small number of cases, lymphoedema can be caused by an accidental injury to the lymphatic system.
For example, it can sometimes occur after a road traffic accident where there's extensive bruising or soft tissue loss.
Movement and exercise help lymph drainage because muscle activity surrounding the lymphatic vessels massages fluid into and along them.
Reduced movement can therefore lead to lymphoedema because the fluid in the lymphatic system does not get moved along.
For example, people who have limited mobility for a long period of time as a result of an illness, nerve damage or arthritis may be at risk of lymphoedema.