Multiple myeloma presented as acute interstitial nephritis and rheumatoid arthritis-like polyarthritis.

Acute interstitial nephritis and rheumatoid arthritis (RA) or RA-like polyarthritis are among the very rare paraneoplastic manifestations of multiple myeloma (MM). A 47-year-old man with acute renal failure due to interstitial nephritis was admitted to our university hospital and successfully treated with corticosteroid. He later developed a symmetric distal polyarthritis with morning stiffness mimicking RA. On follow-up, the patient had a rise in serum creatinine, hypercalcemia, anemia, and a monoclonal spike (Bence Jones protein) on the urine protein electrophoresis. Bone marrow biopsy demonstrated a diffuse neoplastic plasma cell infiltration. Diagnosis of MM was made and the patient received chemotherapy. After four-course chemotherapy, the patient's articular manifestations resolved, urine monoclonal spike disappeared, and serum creatinine returned to a near normal level. We hypothesize that in this case, immunologic hypersensitivity reactions to the light-chain molecules or other tumoral antigens deposited within the kidney or joint spaces, in the context of MM cytokine milieu may have resulted in this unusual presentation. Ultimately, clinicians and pathologists should consider MM in the differential diagnosis of the acute interstitial nephritis and RA-like polyarthritis.