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 approximately 3.7% of cases. Myeloma-associated amyloid arthropathy (MAA), a musculoskeletal manifestation of AL amyloidosis, often mimics rheumatoid arthritis (RA), making accurate diagnosis challenging. Musculoskeletal ultrasonography (MSKUS) has emerged as a valuable tool in the differential diagnosis of arthralgia; however, the specific sonographic features of AL amyloidosis-related arthritis remain poorly characterized. We present a case of AL amyloidosis-associated arthritis initially misdiagnosed and treated as seronegative RA. A comprehensive diagnostic workup-including MSKUS, 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 poor response to RA therapies, involvement of non-classical joints, or systemic manifestations such as proteinuria or peripheral neuropathy. Although not diagnostic on its own, MSKUS may reveal atypical synovial changes suggestive of amyloid deposition and assist in guiding targeted biopsies, which remain essential for 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 recognized.