A digital mucous cyst, also known as a myxoid cyst, is a benign, fluid-filled sac that typically develops on the fingers or toes, most often near the nail bed or the distal joint. The cyst itself is a localized collection of a thick, jelly-like substance called mucin, which is rich in hyaluronic acid. While this common growth is generally not harmful, it frequently causes cosmetic concern and can lead to discomfort or nail deformity. Their presence often prompts people to seek treatment to address the appearance or prevent complications.
Physical Characteristics and Location
Digital mucous cysts present as a solitary, dome-shaped papule or nodule on the dorsal surface of a digit, specifically near the distal interphalangeal (DIP) joint. They are typically found adjacent to the nail matrix, the area where the nail grows from, or the proximal nail fold. The size of these cysts usually ranges from 5 to 10 millimeters.
The surface of the cyst is often smooth and shiny, appearing translucent or sometimes slightly bluish or pink. Light can shine through the cyst, a feature known as transillumination, confirming its fluid-filled nature. While the cyst may feel firm, it is generally painless unless it presses directly on the underlying nail matrix. Pressure on the nail matrix can cause a characteristic longitudinal groove or indentation along the length of the nail plate.
Underlying Causes of Formation
These growths are classified as pseudocysts because, unlike true cysts, they lack an epithelial lining or capsule surrounding the fluid collection. The contents are a thick, clear, gelatinous fluid derived from mucopolysaccharides. The formation of the majority of digital mucous cysts is strongly linked to underlying degenerative changes in the joint, specifically osteoarthritis of the DIP joint.
In these cases, the cyst is considered a type of ganglion cyst that arises when synovial fluid leaks out from the joint capsule. This leakage occurs through a weakened area, creating a stalk that connects the cyst to the joint space. The fluid accumulates under the skin because the stalk acts like a one-way valve, preventing its return to the joint.
The underlying osteoarthritis often causes the formation of bone spurs, or osteophytes, around the joint. These osteophytes irritate the surrounding soft tissues and increase pressure, contributing to cyst formation. A less common type, known as focal mucinosis, forms independently of the joint and is caused by an abnormal overproduction of mucin by cells in the skin’s connective tissue.
Diagnosis and Initial Management
The diagnosis of a digital mucous cyst is primarily clinical, relying on visual inspection and physical examination of the affected digit. The characteristic location, dome-shape, and translucent appearance usually allow a healthcare provider to identify the growth. Transillumination, where a light source is shone through the cyst, is a simple technique that confirms the presence of fluid within the lesion.
X-rays of the finger are often utilized to check for signs of underlying osteoarthritis. Radiographs can reveal evidence of joint degeneration, such as joint space narrowing or the presence of bone spurs. If the cyst is small and asymptomatic, the initial management strategy is observation, also known as a “wait-and-see” approach.
If the cyst is bothersome, non-surgical treatments may be attempted first. These include aspiration, which involves draining the thick, gelatinous fluid with a sterile needle, sometimes followed by the injection of a corticosteroid. Aspiration alone is associated with a high recurrence rate, sometimes reaching 50 percent, because the underlying connection to the joint is not addressed. Repeated needling or cryosurgery (freezing the cyst) are other non-invasive options, though corticosteroid injections carry a risk of thinning the skin.
Definitive Treatment Procedures
For cysts that are symptomatic, recurrent after conservative treatment, or causing a significant nail deformity, definitive treatment involves surgical excision. The goal of surgery is to remove the cyst and eliminate the source of fluid leakage to prevent recurrence. This procedure involves excising the cyst, removing the stalk connecting it to the joint, and addressing the underlying joint pathology.
A hand surgeon will scrape the joint and remove any underlying bone spurs (osteophytes) that may be perpetuating the cyst formation. Removing these bony outgrowths and debriding the dorsal joint capsule is important for achieving a low recurrence rate. In some cases, the cyst itself is not removed, but addressing the osteophyte and degenerative joint tissue allows the cyst to resolve over time.
Surgical removal offers a high success rate for long-term resolution. Potential complications include the risk of infection, scarring, or temporary stiffness in the distal interphalangeal joint. Careful surgical technique is required to avoid damage to the nearby nail matrix, which could result in permanent nail deformity. If the cyst had thinned the overlying skin, the surgeon may need to use a local skin flap to cover the defect.