Bluish, Tender Nodule on the Finger

Discussion of Answer

Digital mucous cysts (synovial cysts or myxoid pseudocysts) are found most commonly on the terminal digits of the fingers.^[1,2] Rarely, they develop on the toes. They typically present as noninflamed, solitary, skin-colored or bluish nodules ranging in size from 5-8 mm. Most are not painful, but tenderness may develop if the cyst is traumatized. The nodules are not true cysts but are formed by focal accumulations of mucin in the dermis without a defined lining or cyst wall. Therefore, some clinicians refer to these lesions as pseudocysts. They are filled with a clear, viscous, sticky mucinous fluid. When the cyst occurs at the proximal nail fold, a depressed groove in the nail plate may form along the entire nail surface. Women are more commonly affected than men and osteoarthritis is sometimes present in the adjacent joint, but it is unclear why these associations exist.

Researchers do not agree on the pathogenesis of digital mucous cysts. One theory is that mucin accumulates -- either spontaneously or due to trauma -- in areas near but not connected with the joint space. Another theory is that there is a connection from the joint space to the cyst with extrusion of mucin from the joint to the nodule. Proponents of this theory may refer to the entity as a synovial cyst.