The lymphatic system is part of the body’s defense mechanism. The lymphatic channels are like streams and the lymph nodes are like ponds with a filtration system. The lymph fluid flows through these channels and is filtered in the lymph nodes. This system is part of the body’s immune system to fight infection and disease, including cancer. Cancer cells from the breast can break away and travel to the lymph nodes. Part of breast cancer staging is to see if cancer has traveled to the axillary lymph nodes. The surgical sampling of the lymph nodes in breast cancer, which involves removing lymph nodes to be sent to the pathologist, is with sentinel lymph node biopsy and axillary dissection.

When lymph nodes are removed the lymphatic channels are disrupted. These lymphatic channels are sealed close during surgery with heat (electrocautery), vibration (ultrasound dissection) or mechanical closure (suture or clips). During an axillary dissection and sentinel lymph node biopsy there is a space where the lymph nodes were removed. If the space is large and many lymph nodes were removed, a drain may be placed to help close that space.

If there is too much pressure on the lymphatic channels after lymph node surgery, these sealed lymphatic channels can break open and cause a lymph fluid collection (called a seroma). The analogy of a dam bursting if there is too much upstream pressure can be used to explain how this may occur. If there is a large open space with low pressure, fluid will freely collect there. A drain will create suction to pull the fluid out of the space and close the space, and help prevent lymphatic fluid accumulation.

Your surgeon will give you instructions on when you may resume exercising with your affected arm. Vigorous upper arm activity may predispose you to developing an axillary seroma. Patients who have had chemotherapy or radiation are also at higher risk for developing a seroma because they do not heal as well.

An axillary seroma may feel like a soft lump under the arm on the side of axillary surgery. If it is larger, it may show up as a bulge in the armpit. Your surgeon can diagnose a seroma by examining you and if needed by using ultrasound. Axillary seromas can be treated with aspiration (removing the fluid with a needle) or by placing a drain into the seroma. Small seromas may resolve on their own.