Ganglion cyst. Artwork of a ganglion cyst on the back of a wrist. This cyst (fluid filled sac) is named for its similarity to ganglion nerve clusters. Its cause is unknown, but it may result from overuse of a specific joint, causing the degeneration of the surrounding fibrous tissue and the development of a fluid-filled cystic structure. The cysts are usually asymptomatic, but may be removed surgically for cosmetic reasons.