Hypophosphatemic osteomalacia and adult Fanconi syndrome due to light-chain nephropathy. Another form of oncogenous osteomalacia.

Two patients, one with myeloma (Patient 1) and the other with probable chronic lymphocytic leukemia (Patient 2), had reduced renal tubular phosphate reabsorption in the absence of hyperparathyroidism together with other features of the Fanconi syndrome, as consequences of the nephropathy associated with light-chain proteinuria. Both patients had hypophosphatemic osteomalacia, demonstrated for the first time in this condition by iliac bone histomorphometry after in vivo double tetracycline labeling, despite absence of bone pain or Looser zones. Neither patient was vitamin D-depleted, but plasma calcitriol level was normal in Patient 1 and low in Patient 2; only the latter patient had severe muscle weakness. Complete histologic correction of osteomalacia was achieved by treatment in accordance with the biochemical defects--oral phosphate therapy alone in Patient 1 and combined with calcitriol in Patient 2. Both patients are now symptom-free, five and three years after the initial diagnosis of bone disease and hematogenous malignancy. Thirteen previous instances of the same form of osteomalacia were reviewed; in most cases, the Fanconi syndrome developed before its probable cause became apparent. The Fanconi syndrome has also been reported in two cases of osteomalacia due to mesenchymal tumor, but not in osteomalacia associated with prostatic carcinoma. Light-chain nephropathy and consequent renal tubular dysfunction appears to be a third form of oncogenous osteomalacia.