A ganglion cyst is a swollen, closed sac under the skin. The sac is attached to the sheath of a tendon or may be attached to a joint. The cyst contains fluid similar to joint fluid. It can vary in size from a small pea to a golf ball.
Ganglion cysts are the most common type of mass that occurs in the hand or wrist.
They may also occur in the foot. The cause of ganglion cysts is not known. You may feel discomfort or pain. Sometimes the area of the cyst becomes swollen or disfigured. Your health care provider may stick a needle into the cyst to take a sample of the fluid inside it.