Treatment of rachitic and osteomalacic syndromes due to renal tubular acidosis should focus on the primary process rather than on the bone disease. Administration of alkali in the form of sodium bicarbonate or similar materials (e.g., polycitrate K) may be all that is required to correct the metabolic disorder, including the rachitic or osteomalacic syndrome. At times (particularly in type IV), sterol treatment may be necessary.

RENAL OSTEODYSTROPHY

Metabolic Aberrations. Chronic renal failure causes an extraordinary number of metabolic abnormalities that affect almost all of the body’s homeostatic mechanisms (see Plate 3-20). There are now five stages of chronic kidney disease (CKD) defined by criteria established by the National Kidney Foundation (NKF) (see Plate 3-20). Although abnormalities in bone turnover, volume, and mineralization occur in all stages of CKD and are now defined in broader terms, chronic kidney disease–mineral and bone disorder (CKD-MBD), the histomorphometric abnormalities called renal osteodystrophy become manifest by quantitative bone histomorphometry in stage 4-5 CKD. In the patient with azotemia and chronic renal failure, aberrations in water distribution, electrolyte and acid-base balances, protein synthesis, nutrition, and hormonal activities produce extensive changes in bodily structure and functions. The manifestations of chronic renal failure on the body’s connective tissues predominate in bone as a multifaceted syndrome. Known in the past as renal rickets, renal hyperparathyroidism, and renal osteomalacia, this syndrome is now generally called renal osteodystrophy (see Plates 3-20 to 3-23).

Manifestations of renal osteodystrophy in both children and adults include a number of chronic disorders of epiphyseal cartilage and bone. Among them are rickets and osteomalacia, osteitis fibrosa cystica (secondary hyperparathyroidism), osteosclerosis, and metastatic calcification. In a child with chronic disease, slipped capital femoral epiphysis may be an additional complication. Osteoporosis, osteomyelitis, and (if corticosteroids are administered) osteonecrosis are also seen frequently in both children and adults.

The pathogenetic mechanisms for the bone changes in renal osteodystrophy are complex (see Plates 3-20 to 3-23). The underlying defect is kidney damage, which includes not only a failure of glomerular filtration that results in azotemia and hyperphosphatemia but almost always a reduced renal, and thus a reduced tubular, mass. Even when vitamin D intake is normal or increased, the high phosphate concentration and the reduction in tubular function greatly reduce the synthesis of 1,25(OH)₂D and lead to elevated PTH and FGF-23. Increased serum phosphate levels and the severely lowered concentration of 1,25(OH)₂D lead to a markedly reduced absorption of calcium from the gastrointestinal tract and profound hypocalcemia. Despite acidosis, which promotes the solubilization of calcium salts, the hypocalcemia is so severe that it not only causes all the bony and soft tissue manifestations of rickets or osteomalacia but also induces a secondary hyperparathyroidism. The excessive secretion of PTH leads to osteitis fibrosa cystica (marked osteoclastic resorption of bone and brown tumors). The resorption of bone partially restores serum calcium levels to normal. The elevated FGF-23 may contribute to the impairment of bone mineralization as well and be one of the leading mechanisms for the very low bone turnover now labeled adynamic renal bone disease.