Lambda light chain induced nephropathy: a rare cause of the Fanconi syndrome and severe osteomalacia

The Fanconi syndrome is a generalized disorder of proximal renal tubular transport characterized by wasting of phosphate, amino acids, glucose, bicarbonate, and uric acid. The association of the acquired Fanconi syndrome with lambda light-chain proteinuria is rare. We report the third case in the English language literature. A 65-year-old man presented with severe pelvic pain. Investigations showed an elevated serum creatinine level, and a 24-hour urine collection contained 2.56 g protein. The Fanconi syndrome was diagnosed, with findings of phosphaturia, glycosuria, and aminoaciduria. Bence Jones protein (lambda sub-type) was present in the urine at a concentration of 0.58 g/L. Monocytic cells in the bone marrow and proximal tubular cells in the kidney contained cytoplasmic crystalline inclusions. Undecalcified bone sections confirmed the clinical diagnosis of osteomalacia. The patient was treated with phosphate, calcium, and ergocalciferol and experienced significant symptomatic improvement. The Fanconi syndrome caused by light-chain deposition in proximal tubular cells is well described in the literature. However, it is rare for the light chains to be of the lambda subtype. This may reflect differences in the physicochemical properties of kappa and lambda light chains.