A ganglion cyst is a fluid-filled sac that is usually attached to either a tendon sheath or a joint lining. Ganglion cysts usually appear on the back of the wrist. They may also be on the underside of the wrist, the hand, the fingers, or the feet. Ganglion cysts are always noncancerous.

Ganglion Cyst
Nucleus factsheet image
© 2011 Nucleus Medical Media, Inc.

The cause is unknown.