Imaging Studies
Plain radiography findings are not diagnostic for digital mucous cysts (DMCs) but will demonstrate a nonspecific soft-tissue density and adjacent bony involvement consistent with osteoarthritic changes.
Ultrasonography evaluation reveals a rounded or lobulated mass of markedly hypoechoic appearance with smooth, well-defined walls immediately adjacent to the involved synovial compartment. A tapering margin, which constitutes the "neck" of the cyst, is observed. Ultrasound is faster and better tolerated than MRI, but MRI is less operator dependent.
CT scanning usually demonstrates a well-defined water density mass with normal surrounding soft tissue.
On MRIs, homogenous low-intensity lesions are seen on T1-weighted images, with markedly increased signal and sharp borders on T2-weighted images. Other cyst features that may be observed are intracystic septa, satellite cysts, cyst pedicles, osteoarthritis of the distal interphalangeal (DIP) joint, subungual cysts, and multiple flattened cysts. MRI is an excellent modality for visualizing soft tissue cyst structures and may be particularly useful preoperatively. [9]
Other Tests
Transillumination with a penlight may assist in making the diagnosis and in differentiating digital mucous cysts from giant-cell tendon sheath tumor.
Approximately 12 hours before surgery, the DIP joint may be injected with methylene blue and local anesthetic. The coloring of the entire cyst and pedicle at the time of surgery may facilitate removal of the entire cyst and minimize the risk of recurrence.
Injection of radio-opaque contrast material into the cyst and passage of this material into the adjacent joint may yield a radiograph that reveals the entire extent of the cyst. This technique provides an image of the cyst extent prior to the surgery itself; therefore, it may be more practical than methylene blue infusion. [10]
Chemical analysis of matrix is a research test that has no role in routine diagnosis. Cellulose acetate paper electrophoresis and enzymatic digestion liquid chromatography demonstrates that digital mucous cysts contain copious quantities of glycosaminoglycans, primarily hyaluronic acid.
Procedures
Fine needle aspiration may have an ancillary role in diagnosis. On such aspiration, a large amount of clear gelatinous fluid is expressed. Rare macrophages and histiocytes are found in the thick, mucoid matrix, as are a few tight clusters of cells, some collagen fibers, and red blood cells with altered shapes. [11]
Histologic Findings
On microscopy, a pseudocyst with a fibrous capsule and myxomatous stroma with scattered fibroblasts is seen. A partial mesothelial lining, but not a true cyst wall, may be found. The overlying surface epithelium demonstrates compact hyperkeratosis with a collarette of hyperplastic epidermis. The mucinous contents stain basophilic with hematoxylin and eosin and can be seen clearly when stained for acid mucopolysaccharides with colloidal iron or Alcian blue. Note the image below.
On scanning electron microscopy, a porous network of collagenous fibers form the cyst wall and a fibrillated inner surface composed of a random arrangement of collagen fibers is observed. An intermittent thin membrane occurs on the inner surface, but no evidence of a cellular lining is apparent.
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Digital mucous cyst proximal to nail unit.
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Digital mucous cyst at proximal nail fold.
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Histopathology of digital mucous cyst.
