A case of AL amyloidosis–associated arthritis initially misdiagnosed as rheumatoid arthritis

Excessive production of immunoglobulin light chains can result in amyloid light-chain (AL) amyloidosis, which manifests as arthritis in ~3.7% of cases. Myeloma-associated amyloid arthropathy, a musculoskeletal manifestation of AL amyloidosis, often mimics rheumatoid arthritis, making an accurate diagnosis challenging. Musculoskeletal ultrasonography has emerged as a valuable tool in the differential diagnosis of arthralgia; however, the specific sonographic features of AL amyloidosis-related arthritis remain poorly characterised. We present a case of AL amyloidosis-associated arthritis initially misdiagnosed and treated as seronegative rheumatoid arthritis. A comprehensive diagnostic workup - including musculoskeletal ultrasonography, magnetic resonance imaging, ultrasound-guided biopsy of the left shoulder joint, and systemic evaluations such as serum and urine protein electrophoresis and bone marrow examination - led to a definitive diagnosis of AL amyloidosis secondary to multiple myeloma. This case highlights the need to consider AL amyloidosis in the differential diagnosis of atypical arthritis, particularly in seronegative patients with a poor response to rheumatoid arthritis therapies, the involvement of nonclassical joints, or systemic manifestations such as proteinuria or peripheral neuropathy. Although not diagnostic on its own, musculoskeletal ultrasonography may reveal atypical synovial changes suggestive of amyloid deposition and assist in guiding targeted biopsies, which remain essential for a definitive diagnosis. Notably, this case demonstrated positive power Doppler signals, in contrast to previous reports, suggesting a broader spectrum of ultrasonographic findings in amyloid arthropathy than previously recognised.