Yellowish, thick, creamy synovial fluid in a joint effusion



Illustration 1.

The patient, a 67-year-old woman with type 2 diabetes mellitus and a 14-year history of rheumatoid arthritis (RA), was evaluated for painless swelling of her right wrist. Physical examination did not reveal any sign of active arthritis or the presence of rheumatoid nodules but did show a 4 × 3–cm–wide subcutaneous oval mass on the volar surface of the wrist; the common flexor sheath of the fingers was mildly swollen. The patient reported that the cyst had been present for 2 years. Laboratory findings, including the lipid profile, were within the normal range. Rheumatoid factor and anti–cyclic citrullinated peptide antibodies were positive. RA was well controlled by anti–tumor necrosis factor α therapy (the Disease Activity Score in 28 joints was 2.1). Radiography revealed a radiotransparent soft tissue swelling of the volar surface of the wrist and erosions of the styloid process. Ultrasonography showed a unilocular cyst filled with a thick, echogenic material in narrow communication with the wrist joint cavity. Aspiration of the cyst yielded 7 ml of highly viscous, yellowish, thick, creamy fluid (left). Compensated polarized light microscopy evaluation did not show white blood cells but revealed the presence of a large number of plate-like cholesterol crystals (viewed at 40× magnification) (right). The majority of such crystals were large, flat, rectangular clusters with or without notched corners, while a few were isolated, long, rod-shaped crystals. They showed strong birefringence with positive or negative elongation. A drop of ether completely dissolved the crystals. At the last followup, 2 months after the aspiration, we did not observe reappearance of the cyst. Cholesterol crystals, although unusual, are a possible and well-recognized finding in the synovial effusions of patients with RA and other rheumatic diseases, such as gout and osteoarthritis (1). It is not clear what induces deposition of cholesterol crystals, but local factors, including defective drainage, increased permeability, intraarticular bleeding, and local destruction, seem pathogenetically relevant. Systemic factors may also play a role, but usually patients do not present with high serum cholesterol and lipoprotein levels (2).