Myxoid pseudocysts of the digits

The many synonyms for mixed pseudocyst of the digits reflect the controversial nature of this lesion:

Figure 5.27

Large periungual myxoid pseudocyst.

Figure 5.27

Large periungual myxoid pseudocyst.

• dorsal finger cyst

• synovial cyst

• recurring myxomatous cyst

• cutaneous myxoid cyst

• dorsal distal interphalangeal joint ganglion

• digital mucinous pseudocyst

• focal myxomatous degeneration

Whereas some authors regard it as a synovial cyst, most now believe it to be a periarticular degenerative lesion.

Myxoid cysts occur more often in women. They are typically found in the proximal nail fold of the fingers and rarely on toes (Figures 5.27-5.29). Usually asymptomatic, these lesions vary from soft to firm, cystic to fluctuant, and may be dimpled, dome-shaped or smooth-surfaced. Transillumination confirms their cystic nature. They are always located to one side of the midline and rarely exceed 10-15 mm in diameter. The skin over the lesion is thinned and may be verrucous or even ulcerated. Rarely, a paronychial fistula may develop under the proximal nail fold, less commonly under the nail plate. Longitudinal grooving of the nail results from pressure on the matrix. Occasionally a series of irregular transverse grooves are seen, suggesting alternating intermittent decompression and refilling of the cyst. Degenerative, 'wear and tear' osteoarthritis, frequently with Heberden's nodes, is present in most cases.

Figure 5.28

Nail plate gutter due to myxoid pseudocyst.

Figure 5.28

Nail plate gutter due to myxoid pseudocyst.

Figure 5.29

Subungual myxoid pseudocyst with nail plate disruption.

Figure 5.29

Subungual myxoid pseudocyst with nail plate disruption.

Histopathological investigation reveals the pseudocystic character. Cavities without synovial lining are located in an ill-defined fibrous capsule. The structure is essentially myxomatous with interspersed fibroblasts. Areas of myxomatous degeneration may merge to form a multilocular pseudocyst. In the cavities, a jelly-like substance is found which stains positively for hyaluronic acid. In some cases a mesothelial-like lining is found in the stalk connecting the pseudocyst with the distal interphalangeal joint. It has been suggested that the lesion arises from the joint capsule or tendon sheath synovia, as do ganglia in other areas.

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