Answer: D

Digital mucous cysts (DMCs) are relatively common benign cysts of the digits, occasionally involving the nail matrix and nail fold. First described as “synovial lesions of the skin” by Markin and Jones in 1880 and by Hyde in 1883,1-3 these cysts are known by various names, including mucoid cysts, mucous cyst of the finger, mucinous pseudocyst, myxomatous cutaneous cyst, and periarticular fibroma, among others.1-3

DMCs are one of the most common growths of the ungual region, second only to verruca vulgaris.1 The condition most frequently affects adults aged 40 to 70 years.1,3 Women are twice as likely as men to suffer from DMCs; there is no known proclivity for any race.1,3 Fingers are more common sites for DMCs than toes, but toes are more likely to be refractory to treatment.1-3 DMCs most commonly develop on an individual’s dominant hand, with the middle finger being most common site, then the index finger.1,3

The etiology of DMCs remains unknown. Patients with degenerative joint diseases such as osteoarthritis are at increased risk for DMCs.1 It is thought that some DMCs may arise from arthritic spurs, but direct causation between osteophytes and DMCs has not been well established.1-3 Trauma, especially in younger patients, or repetitive stress due to occupational activities also have been theorized to be inciting factors.1

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Histologic categories have been devised based on proposed pathogenesis.1,2,4 The myxomatous, or superficial, subtype is often located near the nail fold and is caused by overactivity of fibroblasts leading to excessive mucin and hyaluronic acid production. The ganglion, or deep, subtype forms from a herniated tendon or joint lining, allowing contents to leak. This type typically is seen in patients with degenerative joint disease.1,2,4  These 2 subtypes often cannot be distinguished clinically.1

DMCs, which are pseudocysts because they are lined by connective tissue rather than epithelial cells, appear histologically as a localized myxoid or collagenous mass, with intermixed fibroblasts, clefts filled with mucin, and, occasionally, an inflammatory infiltrate.1,3 On hematoxylin and eosin stain, the mucinous material appears basophilic. Alcian blue and colloidal iron stains are positive for acid mucopolysaccharides.1,4

Clinically, DMCs present between the distal interphalangeal (DIP) joint and the cuticle on the dorsal or lateral face of the digit or adjacent to the proximal nail fold. The lesions measure <1 cm in diameter, are oval or circular, and appear as translucent, shiny, smooth, singular nodules. They can fluctuate in volume, unlike an ungual fibroma.1,5 Typically, these cysts are asymptomatic, but patients may find the mass to be bothersome, leaking jelly-like discharge if compressed, impairing range of motion of the DIP joint, and becoming inflamed or mildly painful.1

The differential diagnosis for DMC includes rheumatoid nodule, epidermoid cyst, acral mucinous fibrokeratoma, fibrous histiocytoma, and Heberden nodes. Careful diagnosis is necessary to rule out other conditions and pursue the appropriate treatment.

 The diagnosis of DMCs usually is straightforward, especially with expression of characteristic clear, gelatin-like contents. However, several clinical and laboratory procedures can confirm the diagnosis of DMCs if the presentation is unusual. DMCs will transilluminate under a penlight, fine-needle aspiration will reveal clear gelatinous material, and ultrasound will reveal an anechoic mass with a defined border. Injection of methylene blue can track the pedicle of a cyst to help confirm the ganglion subtype and plan any potential operation. Additionally, electron microscopy, magnetic resonance imaging, and histopathologic analysis can confirm the diagnosis, if necessary.1

If asymptomatic, DMCs may resolve without intervention, but they have a high recurrence rate. Conservative treatment options include firm compression, repetitive needling, steroid injection, cryotherapy, carbon dioxide laser, or removal with infrared coagulation. Surgery also is an option and it has the highest cure rate.1-3,5

Surgical procedures may remove the cyst, joint capsule, skin lying above, and any arthritic spurs lying below. Closure is achieved via primary closure, skin flap, or skin graft. Complications of surgery include nail deformation, tendon injury, pain, and bleeding. The more aggressive the surgery, the less likely there will be a recurrence but the more likely there will be complications, such as digit and nail deformation. Despite the high recurrence rate, prognosis is good.1 Patients should be counseled about all the available treatment options, their efficacy, and their complications because the gold standard for treatment has yet to be defined.3 The patient in this case was given a diagnosis of DMC based on history and physical examination and assured that the lesion was benign. He chose to have the lesion removed in the office that day. The procedure resulted in no complications, and the patient did not experience recurrence of the DMC.

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