Pathophysiology and recent advances in the management of renal osteodystrophy

The fall and rise of parathyroidectomy in U.S. hemodialysis patients, 1992 to 2002

Although the therapeutic approach to managing hyperparathyroidism has changed dramatically, it is unknown whether parathyroidectomy rates continue to decline in the United States. Parathyroidectomy rates were studied in successive annual national cohorts, prevalent on hemodialysis on January 1 of 1992 to 2002, with Medicare as primary payer. Parathyroidectomy was defined as International Classification of Diseases, Ninth Revision, Clinical Modification code 068. The annual incidence of parathyroidectomy was 11.6 per 1000 patient-years in 1992. The incidence declined progressively after 1994, reaching a low of 6.8 per 1000 patient-years in 1998. Rates increased progressively after 1998, reaching 11.8 per 1000 patient-years in 2002. Using proportional hazards modeling, with adjustment for comorbidity and 1992 as the reference group, the lowest adjusted hazards ratio, 0.32 (P < 0.0001), was seen in 1998, followed by hazards ratios of 0.39 (P < 0.0001) in 1999, 0.41 (P < 0.0001) in 2000, 0.52 (P < 0.0001) in 2001, and 0.53 (P < 0.0001) in 2002. Other antecedents of parathyroidectomy in multivariate models included ESRD network, younger age, female gender, white race, absence of diabetes, longer duration of previous hemodialysis, use of intravenous vitamin D, previous renal transplantation, several comorbid conditions, and parathyroid hormone measurement in the preceding year. With a case-control method, parathyroidectomy was associated with higher mortality rates immediately after surgery, followed, subsequently, by lower long-term rates. Parathyroidectomy rates in U.S. hemodialysis patients increased between 1998 and 2002, a period in which the therapeutic armamentarium for preventing severe hyperparathyroidism expanded considerably.

A clinical assessment of the relationship between bone scintigraphy and serum biochemical markers in hemodialysis patients

BACKGROUND

Renal osteodystrophy is a metabolic bone disease and a common complication of end-stage chronic renal failure and maintenance dialysis treatment. In this study, we examined the correlation between quantifying bone scintigraphy and serum biochemical markers in hemodialysis patients.

METHODS

Bone scintigraphy with technetium-99m-hydroxy-methylene-diphosphonate (99mTc-HMDP) was performed on 28 patients on maintenance hemodialysis. Bone scintigraphy was performed using a standard protocol and was quantified by setting regions of interest (ROIs) over selected regions. The bone-to-soft-tissue ratio (B/ST ratio) at each region was calculated in all patients. The B/ST ratios were then compared with serum biochemical markers.

RESULTS

The B/ST ratio for the skull correlated well with serum bone-specific alkaline phosphatase (BAP) (r = 0.735, p < 0.001), serum deoxypyridinoline (DPD) (r = 0.806, p < 0.001) and intact parathyroid hormone (intact PTH) (r = 0.701, p < 0.001). The B/ST ratio for the lumbar spine correlated with intact PTH (r = 0.387, p < 0.05) but not with serum BAP or serum DPD. The B/ST ratio for the femoral neck correlated with serum DPD (r = 0.431, p < 0.05) and intact PTH (r = 0.449, p < 0.05) but not with serum BAP.

CONCLUSIONS

Our data suggest that quantitative bone scintigraphy is a sensitive and useful method for evaluating bone metabolism in hemodialysis patients. The B/ST ratio for the skull may reflect changes of bone metabolism in hemodialysis patients.

The mineralization index--a new approach to the histomorphometric appraisal of osteomalacia

The histomorphometric diagnosis of osteomalacia depends on the conjunction of two or three independent criteria but for several reasons, both clinical and pathophysiologic, it would be useful to have a single index of severity. Accordingly, using an extensive library of normal values in 143 healthy women, we constructed the mineralization index (MI), defined as [osteoid thickness (O.Th) (microm) + osteoid volume/bone volume (OV/BV) (%)] x 1.15 - osteoid mineralization rate (%/day) - [bone formation rate/bone surface (BFR/BS) (microm3/microm2/year) x 0.15]. MI was normally distributed with mean 8.0, SD 3.3, and range 0-15 (arbitrary units); it was unaffected by race, menopausal status, age or bone turnover, and was slightly lower in osteoporotic patients with nontraumatic vertebral fracture than in healthy white postmenopausal women (6.83 vs. 7.95). In hypovitaminosis D osteopathy (HVO) stage I, MI was normal in 18/26 cases (70%; HVOia), demonstrating more rigorously than before that osteoid accumulation is initially due entirely to secondary hyperparathyroidism and increased bone turnover. In the remaining 30% (HVOib), MI was increased, indicating the onset of impaired mineralization while bone formation was still increased and before the appearance of osteomalacia. In secondary hyperparathyroidism due to renal bone disease, 10/20 cases (50%) had normal MI and higher BFR than in HVOia (93 vs. 32), and there was a significant inverse correlation between MI and BFR. In patients with osteomalacia according to current criteria, MI ranged from 29.2 to 166.5; an MI of 30 had high sensitivity and specificity for the diagnosis of osteomalacia. Including all patients with HVO, there was a significant (P < 0.001) inverse correlation between MI and calcium x phosphate product, but the unexplained variance of >70% suggests that vitamin D deficiency impairs mineralization by multiple mechanisms. We conclude that the MI clarifies the early effects of vitamin D deficiency on bone and the relationship between different components of renal bone disease simplifies the histologic diagnosis of osteomalacia and may contribute to its management, and explicates the mechanisms of mineralization.

Kidney-bone, bone-kidney, and cell-cell communications in renal osteodystrophy

The relationship between bone and the kidney in renal osteodystrophy is a complex interplay of kidney to bone connections, bone to kidney connections, and cell to cell connections. In addition, such interactions have a profound effect on the vasculature. In this review, we discuss the role of the bone morphogenetic proteins (BMPs) in the skeleton, kidney, and vasculature. In addition, we propose that deficiencies of these BMPs seen in chronic kidney disease (CKD) result in decreased bone remodeling and a compensatory secondary hyperparathyroidism (high turnover state). Treatment of the hyperparathyroidism blocks this compensatory arm and thus decreased bone remodeling occurs (low turnover). We review animal models of CKD in which treatment with BMP-7 resulted in normalization of both high and low turnover states. Finally, we discuss vascular calcification as it relates to bone metabolism. We discuss the roles of BMP-7 and 2 other bone regulatory proteins, osteoprotegerin (OPG) and alpha2-HS glycoprotein (AHSG, human fetuin), in the human vasculature and their implications for vascular calcification.

Comparison of the effects of calcitriol and maxacalcitol on secondary hyperparathyroidism in patients on chronic haemodialysis: a randomized prospective multicentre trial

BACKGROUND

To identify differences between the effects of calcitriol and the calcitriol analogue, maxacalcitol, on parathyroid hormone (PTH) and bone metabolisms, we conducted a randomized prospective multicentre study on patients on chronic haemodialysis.

METHODS

We randomly assigned 91 patients with secondary hyperparathyroidism [intact PTH (iPTH) > or =150 pg/ml] to have either calcitriol (47 patients) or maxacalcitol (44 patients) therapy, for 12 months after a 1 month control period. Serum electrolytes, bone alkaline phosphatase (bAP), iPTH, total PTH and PTH(1-84) (whole PTH) levels were measured periodically. The first end point was a serum iPTH of <150 pg/ml, the second was the iPTH levels obtained.

RESULTS

Treatment was discontinued for various reasons in nine patients in each group, but no serious side effects were observed in either group. The numbers of cases reaching the first end point were not significantly different between the two groups. Serum calcium concentration was significantly higher in the maxacalcitol than the calcitriol group during early treatment, but not at the end of treatment. Throughout the treatment period there were no significant differences between the two groups in serum iPTH, inorganic phosphate, the product of the serum calcium and inorganic phosphorus concentrations, bAP, or the ratio of whole PTH to total PTH minus whole PTH. Nor were the changes in these parameters significantly different between the two groups comparing the patients with moderate to severe hyperparathyroidism (basal iPTH > or =500 pg/ml).

CONCLUSION

Calcitriol and maxacalcitol are equally effective on PTH and bone metabolism.

Long-term fracture risk following renal transplantation: a population-based study

Abnormal bone metabolism is a recognized complication of end-stage renal disease, but fracture risk following renal transplantation has not been well quantified. We followed the 86 Olmsted County, Minnesota, residents who underwent initial renal transplantation in 1965-1995 for 911 person-years (median, 10.6 years per subject) in a retrospective cohort study. Fractures, and possible risk factors, were assessed through review of each subject's complete community medical records. Altogether, 117 fractures were observed during follow-up extending to 33 years. The cumulative incidence of any fracture at 15 years was 60% versus 20% expected ( P<0.001). There was a significantly increased risk of fractures generally [standardized incidence ratio (SIR), 4.8; 95% CI, 3.6-6.4] and vertebral (SIR, 23.1; 95% CI, 12.3-39.6) and foot fractures (SIR, 8.4; 95% CI, 5.1-12.9) especially. Age at first transplantation, renal failure due to diabetes, pancreas transplantation, peripheral neuropathy, peripheral vascular disease and blindness were all associated with overall fracture risk. In a multivariate analysis, however, only age and diabetic nephropathy were independent predictors of fracture risk generally, while higher activity status was protective. Diabetes was the only independent predictor of lower limb fractures, whereas age and osteoporosis history predicted vertebral fractures. Cumulative corticosteroid dosage was not associated with increased fracture risk in this analysis. Despite the fact that our patients had few risk factors for preexisting bone disease attendant to postmenopausal osteoporosis, prior corticosteroid use or renal osteodystrophy, these data indicate that renal transplantation is associated with a significant increase in fracture risk among unselected patients in the community. Diabetic patients, particularly, experience excess lower limb fractures. Patients and their care providers should be aware of this elevated fracture risk, which continues long-term.

Indicações de paratireoidectomia no hiperparatireoidismo secundário à insuficiência renal crônica

O hiperparatireoidismo é uma manifestação comum na insuficiência renal crônica (IRC), com alta morbi-mortalidade e difícil manejo clínico. As indicações clássicas da paratireoidectomia são: hipercalcemia persistente, principalmente após transplante renal, prurido intratável, fraturas patológicas, dor óssea refratária ao tratamento medicamentoso e calcificação metastática. Infelizmente, esta última não responde à paratireoidectomia e a calcificação dos vasos está relacionada ao aumento da mortalidade. Assim, novos critérios para indicação mais precoce de paratireoidectomia são necessários. Níveis séricos de PTH maiores que 10 vezes o limite da normalidade, apesar da adequada reposição de cálcio e calcitriol, produto cálcio x fósforo maior que 70(mg/dl)2, tumor marrom quando é urgente a regressão da massa, artrite e/ou periartrite incapacitantes e ruptura de tendões estão entre outras indicações a serem consideradas. Alguns cuidados são necessários para excluir doenças ósseas concomitantes, como amiloidose e intoxicação por alumínio. Esta revisão visa a orientar os endocrinologistas sobre as indicações e melhor momento de realizar paratireoidectomia no hiperparatireoidismo da IRC.

Postmenopausal osteoporosis in the dialysis patient

PURPOSE OF REVIEW

Osteoporosis is the most prevalent bone disorder in the general population, particularly in the middle and older age groups. Although more than half of the prevalent dialysis population is within these age groups, little concern has been given to the possible role of estrogen deficiency in the pathogenesis of bone disease in end-stage renal disease. The purpose of this review is to summarize the recent published evidence that supports a potential role of the postmenopausal state in the pathogenesis of bone disease in end-stage renal disease and their implications for treatment.

RECENT FINDINGS

Recent studies have shown that although the risk factors for fracture in end-stage renal disease are similar to the general population, the incidence is three to fourfold higher. The high prevalence of older population, the frequently observed premature amenorrhea and early menopause in dialysis patients may play a role. Similarly, the proportion of end-stage renal disease women receiving hormone replacement therapy is at least three times lower than the general population. Recent evidence on the risk of hormone replacement therapy should caution about its use in end-stage renal disease patients. New evidence suggests that selective estrogen receptor modulators may increase bone mass without significant secondary effects. Other alternatives, such as the use of bisphosphonates, should be considered with caution due to the risk of excessive suppression of bone turnover, worsening or favoring the development of adynamic bone disease.

SUMMARY

Osteoporosis should be recognized as an important entity that may modify the current conception of renal osteodystrophy in postmenopausal patients with end-stage renal disease. Further clinical studies are needed in order to propose strategies that may reduce the impact of postmenopausal osteoporosis in the dialysis population.

Renal bone disease: a new conceptual framework for the interpretation of bone histomorphometry

My purpose in this article is to restore the histologic appraisal of renal bone disease to the mainstream of bone and mineral metabolism from which it has been separated for many years. Historically, both the two major components were found in varying degrees in most patients, although one or other of them often predominated. For more than 15 years bone biopsy has been used almost exclusively to classify individual patients into hyperparathyroid, osteomalacic, mixed and adynamic categories according to rigid non-overlapping criteria, and remarkably few histologic data have been reported. All metabolic bone diseases result from disordered bone remodeling, the physiologic mechanism for replacing bone that has become too old to carry out its mechanical or metabolic functions. Bone remodeling is not directly concerned with the regulation of plasma calcium, which reflects the level of equilibration at quiescent bone surfaces between systemic and bone extracellular fluid set by parathyroid hormone. The separation of remodeling from homeostasis explains the concurrence of increased turnover and decreased plasma calcium in chronic renal failure; it is the homeostatic system, rather than the remodeling system, which is resistant to parathyroid hormone. The effect of mild hyperparathyroidism is a nonspecific increase in bone turnover, of which the best index is the bone formation rate measured by double tetracycline labeling expressed per unit of bone surface. Increased turnover is always accompanied by increased reversible mineral deficit. In prolonged hyperparathyroidism there is also accelerated irreversible bone loss manifested mainly as thinning of cortical bone, detectable in chronic renal failure before any symptoms, due to increased resorption depth on the endocortical surface. In severe hyperparathyroidism resorbed bone is replaced, not by a lesser quantity of normal bone, but by a mixture of vascular fibrous tissue and woven bone, referred to as osteitis fibrosa. In osteomalacia there is increased accumulation of osteoid, due not to increased turnover, but to prolongation of mineralization lag time, which in conjunction with increased thickness, surface and volume of osteoid is diagnostic. Converting histomorphometric data into category assignment discards most of the useful information, which can be retained by two-dimensional representation of severity. For the hyperparathyroid dimension, bone formation rate measured by double tetracycline labeling expressed per unit of bone surface is the most useful although not ideal. For the osteomalacic dimension a mineralization index was constructed that is unaffected by age or race. In patients with osteitis fibrosa, bone formation rate per unit of bone surface and mineralization index were inversely correlated. For the third dimension a structure/formation index was constructed which increases with age in healthy women and shows weak inverse correlation with bone formation rate. The structure/formation index is lower than normal in patients with osteitis fibrosa, and should be useful in the study of osteopenia in chronic renal failure. Bone formation rate is low in osteomalacia, but some patients have subnormal rates through quite a different mechanism. The frequency of this finding has been overestimated for several reasons: failure to exclude atypical osteomalacia (increased surface and volume but not thickness of osteoid), use of inappropriate reference values, and failure to measure the bone formation rate on endocortical and intracortical surfaces. In healthy women bone formation rate can be zero on the cancellous surface alone. Low bone formation rate is sometimes due to diabetes but most often is the expected response to subnormal parathyroid hormone secretion accompanying an excess of calcium, a situation recognized only recently because of improvement in parathyroid hormone assay methodology. Low cancellous bone formation rate should not increase fracture risk because turnover is much lower in the peripheral than in the central skeleton, and all reports of increased fracture risk are flawed or open to different interpretation. Low bone formation rate is associated with reduced skeletal buffering of calcium and increased soft tissue calcification. This is not a new disease needing its own treatment, however, but represents the final stage of skeletal adaptation to a surfeit of calcium. The concept of adynamic bone disease has been harmful by directing attention away from the most important consequence of over-treatment of hyperparathyroidism.