Cytoplasmic crystals in multiple myeloma-associated Fanconi's syndrome. A morphological study including immunoelectron microscopy.

We report a case of Fanconi's syndrome associated with multiple myeloma, which displayed some unusual features. Although serum immunoelectrophoresis showed no spike, urine electrophoresis revealed monoclonal kappa light chain. The myeloma cells in multiple organs including bone marrow, lymph nodes, spleen, and kidneys were distended with characteristic intracytoplasmic crystals. In the kidneys, identical intracytoplasmic crystals were found in some proximal tubules, distal tubules, collecting ducts, glomerular cells (mostly parietal epithelial and endocapillary cells), and renal interstitial cells. Only monoclonal kappa light-chain protein was demonstrated in these crystals by immunofluorescence and immunoperoxidase techniques, a finding confirmed for the first time (to our knowledge) by immunoelectron microscopic study of the renal biopsy specimens.