Rapid Deterioration of the Renal Function Caused by the Coexistence of Intratubular Amyloidosis and Myeloma Cast Nephropathy

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Multiple myeloma presents with various kidney injuries, including cast nephropathy, light chain deposition disease, and amyloidosis. Cast nephropathy is the most common form and mostly consists of monoclonal immunoglobulin light chains with Tamm-Horsfall protein. Immunoglobulin light chain (AL) amyloidosis may affect all compartments of the kidney, but it is rare in the tubuli. We herein present a rare case with rapid progression of renal failure caused by the co-occurrence of intratubular amyloidosis and cast nephropathy due to multiple myeloma. Our case suggests unique amyloidogenic light chain cast, which can form amyloid fibrils under specific tubular fluid conditions, and illustrates the complicated light chain pathophysiology.