A ganglion is a small sac (cyst) filled with clear, jellylike fluid that often appears as a bump on the hands and wrists, but it can also develop on feet, ankles, knees, or shoulders. Ganglions are not cancerous.

The cause of ganglions is not known, but it may be a reaction to an injury that causes the tendon sheath (which covers the tendon) or joint capsule (which protects the joint) to form extra fluid and balloon out.

A ganglion grows out of a joint, pushing up out of the joint tissue like a balloon. Most people with ganglions notice that the bumps appear suddenly. They may be as small as a small green pea or larger, are usually not painful, and may be movable. They may grow as activity increases, because more fluid collects in the sac. They may also shrink and may break open on their own.

Ganglions are not serious and may go away on their own. If a ganglion is not bothersome, treatment is usually not needed. If a ganglion is painful or unsightly or limits activity, it can be drained (aspirated) and possibly injected with a corticosteroid, although ganglions often come back after being drained. A ganglion can also be surgically removed.

Smashing a ganglion with a heavy object is not recommended because it usually does not work and may cause injury.