Communicating synovial pseudocyst in small joint of the hand assessed by ultrasound

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Illustration 1.

The patient, a 53-year-old woman with osteoarthritis of the hands, presented with a painless fluctuant lesion on the fourth proximal interphalangeal joint. A very small mucous cyst was observed in the second distal interphalangeal joint (arrow in A). A longitudinal ultrasound of the fourth proximal interphalangeal joint at the dorsal aspect revealed a communicating synovial pseudocyst measuring 1.26 × 0.85 cm. The cyst was connected to the joint through a small tubular structure (arrows B and C). After gentle pressure on the cyst with the probe, the fluid was displaced from the cyst to the joint (asterisks in B and C), resulting in an increase in the size of the joint effusion (C). Aspiration of the entire cyst was performed and a corticosteroid injection was administered under ultrasound guidance. The synovial fluid was clear and very viscous. This passage of fluid from the cyst to the joint as seen on ultrasound might be attributed to a check-valve mechanism, similar to what occurs in a Baker's cyst at the popliteal fossa (Canoso JJ, Goldsmith MR, Gerzof SG, Wohlgethan JR. Foucher's sign of the Baker's cyst. Ann Rheum Dis 1987;46:228–32). Rheumatic nodular lesions identified on ultrasound, such as rheumatoid nodules or tophi, are typically noncystic lesions with no joint connection. Ganglion cysts, both isolated and paucisymptomatic, are usually anechoic or hypoechoic noncompressible masses with posterior acoustic enhancement on ultrasound. Atypical ganglion cysts, however, might be difficult to distinguish from communicating pseudocysts on ultrasound. A clue in the differential diagnosis is the additional subjacent joint damage, commonly noted in the communicating synovial pseudocyst.

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