Pathophysiology and recent advances in the management of renal osteodystrophy.

Bone disease is observed in 75-100% of patients with chronic renal failure as the glomerular filtration rate (GFR) falls below 60 ml/minute. Hyperparathyroid (high turnover) bone disease is found most frequently followed by mixed osteodystrophy, low-turnover bone disease, and osteomalacia. With advancing renal impairment, "skeletal resistance" to parathyroid hormone (PTH) occurs. To maintain bone turnover, intact PTH (iPTH) targets from two to four times the upper normal range have been suggested, but whole PTH(1-84) assays indicate that amino-terminally truncated fragments, which accumulate in end-stage renal disease (ESRD), account for up to one-half of the measured iPTH. PTH levels and bone-specific alkaline phosphatase (BSAP) provide some information on bone involvement but bone biopsy and histomorphometry remains the gold standard. Calcitriol and calcium salts can be used to suppress PTH and improve osteomalacia but there is growing concern that these agents predispose to the development of vascular calcification, cardiovascular morbidity, low-turnover bone disease and fracture. Newer therapeutic options include less calcemic vitamin D analogues, calcimimetics and bisphosphonates for hyperparathyroidism, and sevelamer for phosphate control. Calcitriol and hormone-replacement therapy (HRT) have been shown to maintain bone mineral density (BMD) in certain patients with end-stage renal disease (ESRD). After renal transplantation, renal osteodystrophy generally improves but BMD often worsens. Bisphosphonate therapy may be appropriate for some patients at risk of fracture. When renal bone disease is assessed using a combination of biochemical markers, histology and bone densitometry, early intervention and the careful use of an increasing number of effective therapies can reduce the morbidity associated with this common problem.