Diagnosis: Mucous cyst

Mucous cysts are pseudocysts (cystic structures lacking a true lining) that usually appear on the middle or index finger of the dominant hand; toe involvement is uncommon. Although most mucous cysts arise in the proximal nail fold or near the distal interphalangeal (DIP) joint, our patient’s cyst occurred in the PIP joint. Cystic structures adjacent to a joint or a tendon are considered a type of ganglion. Subungual mucous cysts may be mistaken for fibrokeratomas or osteophytes. 

Usually about 1 cm in diameter, mucous cysts manifest as solitary, round, dome-shaped, translucent, fluctuant nodules. Smaller mucous cysts are considered papules. Some mucous cysts are hard, especially if the overlying skin is thick; the surface may be warty. Mucous cysts contain mucin, a viscous, gelatinous, clear, yellow- or strawberry-colored liquid. Light applied to the surface of a mucous cyst will penetrate and illuminate the mucin. A fibrous capsule and myxomatous stroma interspersed with fibroblasts surround the cyst. The epidermis manifests with compact hyperkeratosis and a collarette of hyperplastic epidermis. 

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Mucous cysts in the area of the DIP joint have many other names, the most common of which are mucinous cysts, mucoid cysts, synovial cysts, myxoid cysts, nail cysts, digital mucoid cysts, digital myxoid cysts, and digital mucinous pseudocysts. 

Differential diagnoses include rheumatoid nodule (Bouchard node at the PIP and Heberden node at the DIP), epidermoid inclusion cyst, myxoid malignant fibrous histiocytoma, myxoid variant of liposarcoma, knuckle pad, xanthoma, giant cell tumor of the tendon sheath, gout, wart, Gottron papule, granuloma annulare, and fibrokeratoma (Koenen tumor). All are rare; none transilluminates. 

The cause of mucous cysts is yet to be determined. Some evidence suggests they arise from mucoid, or myxomatous, degeneration of connective tissue often linked with osteoarthritis and resulting in communication with the adjacent interphalangeal joint. Other possibilities include synovial herniation, increased fibroblast hyaluronic acid production, and extensor retinacular metaplasia.

While most mucous cysts are asymptomatic, there can be associated complications. Uncommonly, pain associated with a ganglion may be a result of underlying arthritis, bone tumor, ligament injury, or fracture. Impingement on adjacent nerve fibers can cause a throbbing pain. A mucous cyst in the proximal nail fold can lead to nail deformities, in particular formation of a groove through the length of the nail due to disruption of the matrix (the so-called gutter or groove sign). This can lead to transverse ridging and thinning of the nail overlying the cyst.

Some mucous cysts protrude into the underlying bones. With time, mucous cysts can weaken the overlying skin, allowing liquid to drain out but also allowing bacteria to enter the joint, sometimes resulting in a serious infection.

Asymptomatic mucous cysts may not require treatment. When treatment is desired, the approaches include aspiration, electrocautery, chemical cautery, steroid injection, and various types of surgical excision. If surgical excision is undertaken, methylene blue injected along with the local anesthetic will highlight the entire cyst and pedicle for complete removal. Imaging can also help define the nature and extent of a mucous cyst prior to surgery. Plain x-rays cannot define a cyst but will reveal accompanying nonspecific soft-tissue density and osteoarthritic alterations. Ultrasound can demonstrate a rounded or lobulated mass as a markedly hypoechoic image with smooth, well-defined walls abutting the involved synovial compartment and a tapering structure that is the neck of the cyst. CT scan will reveal a well-defined mass with increased density relative to the surrounding normal soft tissue. MRI demonstrates areas of increased water density, hallmark of the mucous cyst.  

If the mucous cyst is painful or inflamed, a nonsteroidal anti-inflammatory drug, such as aspirin, ibuprofen, naproxen, or ketoprofen, can help. One folk remedy involves hitting the cyst with a book to rupture it. This can work, but some cysts have become infected or more painful due to the trauma. The medical mythology involves using a bible, leading to ganglions’ sometimes being referred to as “Gideon’s disease.” Another home remedy calls for sticking a needle in a mucous cyst, but this can lead to joint infection and is not recommended. 

This patient opted to leave her cyst alone.