A mucous cyst is a small, fluid-filled sac that forms on the back of the finger near the base of the fingernail. It is a form of ganglion cyst that erupts from the capsule of the joint at the end of the finger, called the DIP joint. The cyst is attached to the joint capsule by a “stalk” that allows fluid to move into the cyst from the joint. Mucous cysts most commonly affect the index finger of the dominant hand. Cause Mucous cysts are caused by joint synovitis or arthritis at the DIP joint of the finger. It is believed that a weakness in the capsule leads to the formation of the cyst. Although cysts usually develop slowly, they can appear quite rapidly as well.
A mucous cyst appears as a visible bump under the skin. If it occurs near the nailbed, it may cause a groove to develop in the fingernail. A mucous cyst is typically not painful, but if it is repeatedly rubbed or bumped, the skin that covers the cyst may become irritated. Additionally, the arthritic or inflamed joint may cause discomfort. The skin that covers the cyst may become very thin and may rupture, resulting in drainage of a clear, jelly-like fluid. Because the cyst is connected to the joint, there is a risk of severe joint infection if the ruptured cyst becomes infected.
Diagnosis of a mucous cyst involves x-rays, which are used to assess the condition of the joint and the severity of any underlying arthritis or synovitis. In many cases, bone spurs have formed on the top of the joint near the cyst.
In many cases, mucous cysts do not need treatment. Small cysts that don’t cause any problems may be treated with aspiration and a steroid medication. In more severe cases where the skin is thin and at risk of rupture, the cyst has become very large or painful, or the nail has become significantly deformed, surgical excision and joint debridement may be recommended.