Case Report of 59-Year-Old Woman with Bilateral Upper Limb Musculoskeletal Amyloid, Initially Diagnosed as Rheumatoid Arthritis.

BACKGROUND Amyloid light-chain (AL) amyloidosis is usually due to deposition of immunoglobulin lambda light chains from plasma cells in patients with multiple myeloma. AL amyloid may involve the salivary glands, gastrointestinal tract, peripheral nerves, and skin. However, musculoskeletal amyloid and amyloid arthropathy are rare. This report is of a woman with bilateral upper limb musculoskeletal amyloid and amyloid arthropathy associated with multiple myeloma, initially diagnosed and managed as a case of rheumatoid arthritis. CASE REPORT A 59-year-old woman who was initially diagnosed with rheumatoid arthritis presented with bilateral polyarthritis in the upper limbs. Despite treatment with corticosteroids, methotrexate, and hydroxychloroquine, her symptoms did not improve. After 4 months, she revisited our hospital with the appearance of swollen soft tissue in the upper right arm and numbness of the right hand. She had an arthroscopic synovectomy of the right shoulder joint, and the mass in the right elbow area was removed. These specimens were positive by Congo red stain and confirmed the deposition of light chain protein as amyloid. She was diagnosed with multiple myeloma according to International Myeloma Working Group criteria, including bone marrow plasma cells more the 10%, lytic lesions in bone, and anemia. CONCLUSIONS This report highlights the importance of imaging, biopsy, and laboratory investigations in patients with arthropathy and musculoskeletal disease. In this case, the patient was seronegative for rheumatoid arthritis, and the presentation with very thick and nodular synovium supported an alternative diagnosis. The identification of musculoskeletal amyloid and amyloid arthropathy confirmed underlying multiple myeloma.