Hypercalciuria in a child with primary Fanconi syndrome and hearing loss

A hitherto undescribed association of sensorineural hearing loss and Fanconi syndrome (FS) is reported in a 10 year old black male. The patient presented with growth failure developing at the age of 6 and rachitic changes were detected the following year. No known cause for FS was identified, and renal biopsy was within normal limits. Distal tubular acidification and the threshold for proximal tubular bicarbonate reabsorption were normal, as was urine concentrating capacity. He was found to have significant hypercalciuria (urine calcium excretion 10 mg/kg/day) despite dietary calcium restriction, and urine calcium excretion increased further following an oral calcium load. Dietary sodium restriction to 16 mEq/kg/day resulted in a fall in urine calcium loss, which remained elevated at 6 mg/kg/day. Serum parathyroid hormone and 1,25 dihydroxy vitamin D3 (1,25(OH)2D3) concentrations were in the normal range. Treatment with neutral phosphate dietary supplementation resulted in partial healing of rickets and normal growth rate. Hypercalciuria resolved during phosphate administration (urine calcium excretion 3 mg/kg/day) without a fall in urine sodium excretion. It is concluded that in this patient with FS, hyperphosphaturia resulted in phosphate depletion and secondary hypercalciuria. A similar mechanism of hypercalciuria may be operative in a variety of renal tubular disorders affecting children and adults.