[Multiple myeloma diagnosed due to development of amyloid arthritis].

Here, we present the case of a 72-year-old male who presented with swelling, stiffness, and dysesthesia in the bilateral fingers, wrists, and ankles. Although rheumatoid arthritis was initially suspected, laboratory tests were negative for rheumatoid factor (RF) and anti-cyclic citrullinated peptide (CCP) antibody. Based on the findings of immune globulin G (IgG)-λ M proteins and 26% plasma cells in the bone marrow, multiple myeloma was diagnosed. Joint sonography revealed thickening of the tendon synovial sheaths around the bilateral wrist joints, palmar flexor tendon sheaths, and extensor digitorum tendon sheaths, and magnetic resonance imaging (MRI) revealed soft tissue masses around the bilateral hip joints. Carpal tunnel syndrome associated with amyloid arthritis was suspected. Amyloid deposits were observed in synovectomy specimens, and the patient was then diagnosed with amyloid arthritis. He had concurrent pulmonary fibrosis, and treatment with lenalidomide/dexamethasone (Ld therapy) was initiated. The symptoms in the bilateral fingers, wrists, and ankles improved with the treatment course, and joint sonography revealed that thickening of tendon sheath and soft tissue masses disappeared after seven courses of Ld therapy. However, MRI still revealed soft tissue masses around the bilateral hip joints. In patients with joint symptoms that do not fulfill the diagnostic criteria for rheumatoid arthritis, differentiation with amyloid arthritis is necessary.