Metabolic bone disease secondary to renal and intestinal disorders

Abstract

Metabolic bone disease occurring in renal or intestinal disorders has been reviewed with particular reference to etiological factors. Hyperparathyroidism is seen as a recurring cycle of renal damage-hyperphosphatemia-hypocalcemia-parathyroid stimulation-mobilization of bone calcium and phosphate-renal tubular phosphate rejection. In intestinal cases, the initial stimulus is presumably hypocalcemia. Osteomalacia is seen as resulting from phosphate depletion for the following reasons:1. Experimentally, rickets results from dietary phosphate restriction in rats.2. Such rickets is not prevented by the presence of normally adequate amounts of dietary vitamin D, and may therefore be termed "resistant" in the clinical sense.3. Osteomalacia or rickets in intestinal malabsorption and renal tubular disorders is associated with hypophosphatemia due to excessive fecal or urinary loss.4. Renal tubular rickets has been healed by oral phosphate loading in some studies.5. Acidosis may induce osteomalacic changes, experimentally and clinically (for example, in uretero-sigmoidostomy). Reversal of systemic acidosis with oral bicarbonate has resulted in phosphate retention and a rising serum phosphate in one such case.6. Preliminary data from analysis of full-thickness bone biopsy in two osteomalacic patients shows a significant reduction in calcium and phosphate content.7. Despite the hyperphosphatemia of azotemic renal failure, over-all phosphate depletion may be present in this situation also due to: * Diminished dietary phosphate in low protein diets * Nausea and vomiting * Occasional diarrhea * The use of oral phosphatebinding antacids * Perpetuation of urinary phosphate losses by reduction in proportion of tubular reabsorbed phosphate (secondary hyperparathyroidism) and possibly high filtered load per nephron * Repeated losses of phosphate to bath fluid during dialysis.