Disappearance of aluminic bone disease in a long term asymptomatic dialysis population restricting A1(OH)3 intake: emergence of an idiopathic adynamic bone disease not related to aluminum

Magnesium exposure increases hip fracture risks in patients with chronic kidney disease: a population-based nested case-control study

This population-based study demonstrates a strong link between Mg-containing antacid exposure and hip fracture risk in nondialysis CKD and dialysis patients. As an Mg-containing antacid, MgO is also commonly used as a stool softener, which can be effortlessly replaced by other laxatives in CKD patients to maintain bone health.

PURPOSE

Bone fracture is a severe complication in chronic kidney disease (CKD) patients, leading to disability and reduced survival. In CKD patients, blood magnesium (Mg) concentrations are usually above the normal range due to reduced kidney excretion of Mg. The present study examines the association between Mg-containing antacid exposure and the risk of hip fracture of CKD patients.

METHODS

In this nationwide nested case-control study, we enrolled 44,062 CKD patients with hip fracture and 44,062 CKD matched controls, among which the mean age was 77.1 years old, and 87.9% was nondialysis CKD.

RESULTS

As compared to non-users, Mg-containing antacid users were significantly more likely to experience hip fracture (adjusted odds ratio (OR) 1.36, 95% CI, 1.32 to 1.41; p < 0.001). Subgroup analysis showed that such risk exists in both nondialysis CKD patients and long-term dialysis patients. In contrast, aluminum or calcium-containing-antacid use did not reveal such association. Next, we examined the influence of Mg-containing antacid dosage on hip fracture risk, the adjusted ORs in the first quartile (Q1), Q2, Q3, and Q4 were 1.20 (95% CI, 1.15 to 1.25; p < 0.001), 1.35 (95% CI, 1.30 to 1.41; p < 0.001), 1.49 (95% CI, 1.43 to 1.56; p < 0.001), and 1.54 (95% CI, 1.47 to 1.61; p < 0.001), respectively, showing that such risk exists regardless of the antacid dosage. A receiver operating characteristic curve analysis demonstrated that the best cutoff value of the exposed Mg dose to discriminate the hip fracture is 532 mEq during the follow-up period.

CONCLUSION

This population-based study demonstrates a strong link between Mg-containing antacid exposure and the hip fracture risk in both nondialysis CKD and dialysis patients.

Chronic Kidney Disease-Mineral and Bone Disorders: Pathogenesis and Management

The key players of the chronic kidney disease-mineral and bone disorders (CKD-MBD) are calcium, phosphate, PTH, FGF23, and the vitamin D hormonal system. The progressive reduction of kidney function greatly modifies the tightly interrelated mechanisms that control these parameters. As a result, important changes occur in the bone and mineral hormonal axis, leading to changes in bone turnover with relevant consequences in clinical outcomes, such as decrease in bone mass with increased bone fragility and bone fractures and increased vascular and valvular calcification, also with great impact in the cardiovascular outcomes. So far, the knowledge of the mineral and bone disorders in CKD and the increased variety of efficacious therapies should lead to a better prevention and management of CKD-MBD.

Serum Magnesium Concentration is An Independent Predictor of Parathyroid Hormone Levels in Peritoneal Dialysis Patients

Background

Parathyroid hormone (PTH) is a cardinal factor in the pathogenesis of bone disease in the dialysis population. The spectrum of renal osteodystrophy has been reported to have changed during the past years, and adynamic bone disease has emerged as the most common bone disorder in these patients. Continuous ambulatory peritoneal dialysis (CAPD) is considered a risk factor for the development of this condition, and furthermore, the adynamic bone lesion is associated with a state of relative hypoparathyroidism (hypo-PTH). Calcium, vitamin D, and phosphorus play a key role in the control of parathyroid gland function in uremic patients. However, magnesium may also be able to modulate PTH secretion in a way similar to calcium.

Objective

The aims of this study were (1) to analyze the serum Mg concentration in a large group of CAPD patients, (2) to study the relationship between serum Mg and PTH levels, and (3) to investigate whether this relationship is independent of other factors, such as calcium, phosphorus, and calcitriol, that regulate parathyroid function.

Patients and Methods

We studied 51 stable patients, aged 23 – 77 years, under maintenance CAPD for more than 6 months (range 8 – 48 months). Calcium carbonate was used as a phosphate binder in all patients, and 9 subjects also received aluminum hydroxide. No patient had been previously treated with vitamin D. Biochemical parameters were prospectively evaluated over 6 months, and the mean values were computed.

Results

The mean serum Mg was 1.08 ± 0.19 mmol/L, and hypermagnesemia, defined as a Mg level higher than 1.01 mmol/L, was found in 30 patients (59%). Thirty-one subjects (60%) had an intact PTH (iPTH) level lower than 120 pg/mL and were diagnosed as having relative hypo-PTH. Except for the values of iPTH and alkaline phosphatase, the only difference between the two groups was the serum Mg concentration, which was significantly higher in patients with hypo-PTH (1.16 ± 0.15 mmol/L vs 0.91 ± 0.14 mmol/L; p < 0.001). Furthermore, iPTH levels were lower in patients with hypermagnesemia than in subjects with normal serum Mg (69 ± 49 pg/mL vs 190 ± 89 pg/mL, p < 0.001). There was a significant correlation between serum Mg and PTH levels ( r = –0.70, p < 0.01). After controlling for the effect of other variables by partial correlation analysis, a significant positive association between P and PTH ( r = 0.25, p < 0.05), and a negative relationship between Mg and PTH ( r = –0.57, p < 0.001) were evident. A forward stepwise multiple regression analysis showed that only P and Mg predicted PTH values (multiple r = 0.59, p < 0.001).

Conclusions

Hypermagnesemia and hypoparathyroidism are frequent in CAPD patients. There is a significant inverse relationship between serum Mg concentration and iPTH levels. Furthermore, this association is independent of the most important factors regulating parathyroid gland function (calcium, phosphorus, and calcitriol). These results suggest that hypermagnesemia may have a suppressive effect on PTH synthesis and/or secretion. Therefore, elevated serum Mg levels may play a role in the pathogenesis of adynamic bone disease.

Adynamic bone in patients with chronic kidney disease

Adynamic bone in patients with chronic kidney disease (CKD) is a clinical concern because of its potential increased risk for fracture and cardiovascular disease (CVD). Prevalence rates for adynamic bone are reportedly increased, although the variance for its prevalence and incidence is large. Differences in its prevalence are largely attributed to classification and population differences, the latter of which constitutes divergent groups of elderly patients having diabetes and other comorbidities that are prone to low bone formation. Most patients have vitamin D deficiency and the active form, 1,25-dihydroxyvitamin D, invariably decreases to very low levels during CKD progression. Fortunately, therapy with vitamin D receptor activators (VDRAs) appears to be useful in preventing bone loss, in part, by its effect to stimulate bone formation and in decreasing CVD morbidity, and should be considered as essential therapy regardless of bone turnover status. Future studies will depend on assessing cardiovascular outcomes to determine whether the risk/reward profile for complications related to VDRA and CKD is tolerable.

PHOSPHORUS METABOLISM AND MANAGEMENT IN CHRONIC KIDNEY DISEASE: Renal Osteodystrophy, Phosphate Homeostasis, and Vascular Calcification

New advances in the pathogenesis of renal osteodystrophy (ROD) change the perspective from which many of its features and treatment are viewed. Calcium, phosphate, parathyroid hormone (PTH), and vitamin D have been shown to be important determinants of survival associated with kidney diseases. Now ROD dependent and independent of these factors is linked to survival more than just skeletal frailty. This review focuses on recent discoveries that renal injury impairs skeletal anabolism decreasing the osteoblast compartment of the skeleton and consequent bone formation. This discovery and the discovery that PTH regulates the hematopoietic stem cell niche alters our view of secondary hyperparathyroidism in chronic kidney disease (CKD) from that of a disease to that of a necessary adaptation to renal injury that goes awry. Furthermore, ROD is shown to be an underappreciated factor in the level of the serum phosphorus in CKD. The discovery and the elucidation of the mechanism of hyperphosphatemia as a cardiovascular risk in CKD change the view of ROD. It is now recognized as more than a skeletal disorder, it is an important component of the mortality of CKD that can be treated.

A new era in phosphate binder therapy: what are the options?

Dietary restriction of phosphorus and current dialysis prescription are unable to maintain phosphorus levels within the recommended range (2.7-5.5 mg/dl) in patients with advanced chronic kidney disease (CKD). Therefore, phosphate binders that limit the absorption of dietary phosphorus are commonly prescribed for this patient group. The first phosphate binders were introduced more than 30 years ago and included aluminum salts; however, although effective binders, the use of these agents was subsequently restricted because of concerns over aluminum accumulation in the central nervous system, bone, and hematopoietic cells. In subsequent years, calcium salts, namely calcium carbonate and calcium acetate, became the most widely used phosphate binders; however, increasing evidence now suggests that prolonged use of these agents increases the total body calcium load, induces adynamic bone, and potentially increases the risk of cardiovascular and soft tissue calcification. Sevelamer is the first phosphate-binding agent that is non-absorbed, calcium-free, and metal-free. To date, this agent has been shown to effectively control serum phosphorus levels in patients with CKD. It may also attenuate coronary and aortic calcification and has a number of other beneficial effects on lipid metabolism and inflammation among others. Lanthanum carbonate is another new agent that is reported to provide similar phosphate control to calcium-based phosphate binders but concerns that the long-term administration of such compound may lead to tissue accumulation may limit its use.

Variant of adynamic bone disease in hemodialysis patients: fact or fiction?

BACKGROUND

Adynamic bone disease is a type of renal osteodystrophy characterized by low bone turnover and paucity of bone cells. It was proposed that a new type of this disease featuring high osteoclastic resorption without parathyroid hormone stimulus and designated adynamic bone disease variant occurs in hemodialysis patients. The present study is designed to evaluate the frequency and characteristics of both diseases in a large series of bone biopsy specimens.

METHODS

We reviewed 1,160 bone biopsy specimens from hemodialysis patients. Specimens in which adynamic bone disease was diagnosed were selected and categorized as classic or variant based on osteoclastic surface.

RESULTS

In 218 bone biopsy specimens (18.8%), adynamic bone disease was identified, whereas the variant form was identified in 35 specimens (38.8%). Biopsy specimens categorized as the variant form were from patients who were younger and had greater phosphorus and parathyroid hormone levels. Histologically, the variant form presented greater osteoid volume, fibrosis volume, osteoid surface, osteoblast surface, and eroded surface. Similarly, values for all dynamic parameters were greater in the variant group. Osteoclastic surface correlated with phosphorus level, parathyroid hormone level, and osteoblast surface. Age and osteoblast surface were identified as independent determinants of the variant form.

CONCLUSION

Adynamic bone disease variant seems to occur in younger hemodialysis patients with greater levels of parathyroid hormone, which acts on cell-covered bone surfaces. It probably is a transitional phase from low- to high-turnover status, rather than a true entity within the spectrum of renal osteodystrophy.

Intracapsular hip fractures in end-stage renal failure

Patients with end-stage renal failure (ESRF) have metabolic bone disease. This increases the risk of femoral neck fracture and increases the risk of complications associated with fracture fixation such as non-union and avascular necrosis (AVN). We report the results of treatment in a consecutive series of 15 intracapsular fractures of the hip occurring in 13 patients with ESRF over a 5-year period. Six intracapsular hip fractures (of which five were undisplaced) were treated by internal fixation. Five out of these six (mean=83.3%) required conversion to total hip arthroplasty because of non-union or AVN. In all six of these patients, internal fixation was considered adequate post operatively. Of the remaining nine intracapsular hip fractures treated by hemiarthroplasty, only one required conversion to total hip arthroplasty because of stem subsidence (mean=11%). The difference in the revision rate for the two groups i.e. primary fixation versus primary hemiarthroplasty was statistically significant (p-value=0.01). The six patients with undisplaced intracapsular fractures treated by internal fixation required a total of 14 major operations, at an average rate (including initial fracture fixation and revision surgery) of 2.3 per patient. The 9 displaced fractures treated by hemiarthroplasty required just 10 operations in total, at an average rate of 1.1 per patient. (The difference was significant; p-value=0.006.) The 1-year mortality in the whole group (13 patients with 15 fractures) was 44.4%. We suggest that patients with ESRF with an intracapsular fracture of the neck of femur should be treated by replacement arthroplasty irrespective of femoral head displacement because of the high risk of revision surgery associated with internal fixation.

Viewpoint: How Do Calcimimetics Fit Into the Management of Parathyroid Hormone, Calcium, and Phosphate Disturbances in Dialysis Patients?

As suggested by its American brand name (Sensipar), the calcimimetic cinacalcet sensitizes the parathyroid cells to the extracellular calcium signal, suppressing parathyroid hormone (PTH) release and synthesis and preventing parathyroid cell proliferation. This primary PTH suppression decreases the release of calcium and phosphate from bone without increasing intestinal absorption of calcium and phosphate. Therefore cinacalcet decreases the risk of hypercalcemia and hyperphosphatemia in contrast to 1alpha-OH vitamin D derivatives. Compared with calcium-containing oral phosphate binder (OPB), it increases the risk of hypocalcemia and may decrease the PTH-mediated phosphaturia in predialysis patients. This justifies its combined use with calcium-containing OPB in order to prevent hypocalcemia and enhance the hypophosphatemic effect of the latter, while improving PTH suppression. The National Kidney Foundation (NKF) Kidney Disease Outcomes Quality Initiative (K/DOQI) has recommended restriction of supplemental elemental calcium to 1.5 g/day, a recommendation that we believe should be revised. No pathophysiologic or randomized trial data have yet evidenced the absolute necessity for systematically using 1alpha-OH vitamin D derivatives and noncalcium-containing OPB rather than higher doses of calcium-containing OPB alone in uremic patients without vitamin D insufficiency. In patients with hyperparathyroidism as severe as in the "Treat to Goal Study," the Durham study showed that a calcium carbonate dose more than three times the K/DOQI limit could decrease PTH into the recommended range, with the advantage of a lower calcium-phosphate product compared with the combination of calcitriol and noncalcium OPB. Besides the efficient PTH suppression associated with lower calcium-phosphate product and a good gastrointestinal tolerance, long-term data suggest that cinacalcet may decrease the risk of parathyroidectomy and fracture, while high bone turnover lesions are improved. However, no long-term data on bone mineral density and cardiovascular calcification and complications are yet available. Such studies, along with those comparing cinacalcet and 1alpha-OH vitamin D-based approaches to hyperparathyroidism, are needed.

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.

Useful biochemical markers for diagnosing renal osteodystrophy in predialysis end-stage renal failure patients

BACKGROUND

Various biochemical markers have been evaluated in dialysis patients for the diagnosis of renal osteodystrophy (ROD). However, their value in predialysis patients with end-stage renal failure (ESRF) is not yet clear.

METHODS

Bone histomorphometric evaluation was performed and biochemical markers of bone turnover were determined in serum of an unselected predialysis ESRF population (N = 84).

RESULTS

Significant (P < 0.005) differences between the five groups with ROD (ie, normal bone [N = 32], adynamic bone [ABD; N = 19], hyperparathyroidism [N = 8], osteomalacia [OM; N = 10], and mixed lesion [N = 15]) were noted for intact parathyroid hormone, total (TAP) and bone alkaline phosphatase (BAP), osteocalcin (OC), and serum calcium levels. Serum creatinine and (deoxy)pyridinoline levels did not differ between groups. For the diagnosis of ABD, an OC level of 41 microg/L or less (< or =7.0 nmol/L) had a sensitivity of 83% and specificity of 67%. The positive predictive value (PPV) for the population under study was 47%. The combination of an OC level of 41 ng/L or less (< or =7.0 nmol/L) with a BAP level of 23 U/L or less increased the sensitivity, specificity, and PPV to 72%, 89%, and 77%, respectively. ABD and normal bone taken as one group could be detected best by a BAP level of 25 U/L or less and TAP level of 84 U/L or less, showing sensitivities of 72% and 88% and specificities of 76% and 60%, corresponding with PPVs of 89% and 85%, respectively. In the absence of aluminum or strontium exposure, serum calcium level was found to be a useful index for the diagnosis of OM.

CONCLUSION

OC, TAP, BAP, and serum calcium levels are useful in the diagnosis of ABD, normal bone, and OM in predialysis patients with ESRF.

Radial bone mineral density in hemodialysis patients with adynamic bone disease

Adynamic bone disease (ABD) has attracted attention as the most frequent type of renal osteodystrophy, but there are few reports about the bone mineral density (BMD) in ABD patients. This study investigated the BMD in hemodialysis patients with ABD and with relatively normal bone turnover. We measured the BMD of the distal one-third of the radius by dual-energy X-ray adsorptiometry. In the ABD group (intact PTH<65 pg/ml, intact osteocalcin<30 ng/ml), there were 19 men and 17 women with a mean age of 56.4 +/- 12.0 years. In the relatively normal bone turnover group (intact PTH: 120-250 pg/ml), there were 24 men and 16 women with a mean age of 57.1 +/- 14.7 years. Although there were no significant differences between the two groups with respect to age, gender, and duration of hemodialysis, a significant increase of the BMD and the calcium x phosphate product was observed in the ABD group (radial BMD: 0.648 +/- 0.137 g/cm2 versus 0.572 +/- 0.132 g/cm2, calcium x phosphate product: 57.53 +/- 14.92 mg2/dl2 versus 49.76 +/- 12.13 mg2/dl2). These findings suggest that an increase in radial BMD may not be a useful marker of the improvement in bone lesions in ABD patients.

Use of vitamin D analogs in chronic renal failure

Renal osteodystrophy is the term used to describe the spectrum of bone diseases associated with chronic renal failure. Deficiency of 1,25-dihydroxycholecalciferol (calcitriol) plays a major role in the development of renal osteodystrophy, in particular the evolution of secondary hyperparathyroidism. In recent decades, our understanding of the complex interactions between calcium, phosphorus, vitamin D, and parathyroid hormone (PTH) has increased, resulting in a rational approach to therapy in which vitamin D analogs have become an essential component. The initial vitamin D analogs that have been in widespread clinical use include calcitriol (1,25-[OH](2)D(3)) and alfacalcidol (1alpha-[OH]D(3)). These agents have been extensively studied to optimize their effects on secondary hyperparathyroidism. The occurrence of significant hypercalcemia and hyperphosphatemia limiting their use has led to the development of alternative vitamin D analogs that effectively reduce PTH secretion without causing these complications. Recently, 3 such analogs, 22-oxa-1,25-(OH)(2)D(3) (OCT), 1alpha-(OH)D(2) (doxercalciferol), and 19-nor-1,25-(OH)(2)D(2) (paricalcitol), have been released for clinical use. Only paricalcitol has been studied in comparative human clinical trials with calcitriol in dialysis patients. Preliminary findings suggest a clinical advantage over calcitriol, however, analysis of the larger comparative studies are forthcoming.

Percutaneous bone biopsy in the diagnosis of renal osteodystrophy

Renal failure is associated with many complex bone and mineral complications. The spectrum of diseases is wide, encompassing defects in bone turnover, remodeling, and mineralization. Disease is currently defined in terms of whether a high or low turnover lesion is present. Measurement of serum parathyroid hormone levels (PTH) remains an important aspect in the management of renal bone disease, however, is limited by its lack of sensitivity in many clinical settings. Multiple biochemical markers are also available both commercially and experimentally to assist in assessing the degree of bone formation or resorption. However, when definitive diagnosis is important, when the clinical setting is confusing or complex, or when parathyroidectomy is being considered, the use of percutaneous bone biopsy is an essential tool in the understanding of underlying bone pathology and in directing therapy intervention.

Peritoneal Dialysis Case Forum

Hypercalcemia in a Peritoneal Dialysis Patient

Are parathyroidectomies still appropriate in chronic dialysis patients?

The uremic milieu generates chronic stimulatory input to the parathyroid glands, which is mediated principally by low calcium, high phosphate and low calcitriol, and results in increased parathyroid hormone (PTH) synthesis and release and an increase in parathyroid mitotic activity with the development of monoclonal areas of nodular hyperplasia. Such glands do not fully express the machinery required to mediate the suppressive inputs to the parathyroids; the extracellular calcium receptor (CaR) and the vitamin D receptor (VDR) are both downregulated. In most of these patients ablation, by parathyroidectomy or ethanol injection, provides the only means of correcting the hyperparathyroidism; apoptosis in parathyroid cells is negligible and clinically irrelevant. In practice, surgery is often delayed by a doomed and ultimately futile attempt to effect control by medical means. Better predictors of the likely success or failure of optimal non surgical management are needed. Gland size exceeding 1 cm3 and elevated PTH despite hypercalcemia (implying loss of suppressibility by calcium), in the presence of good phosphate control and adequate calcitriol provision point strongly to eventual failure of medical treatment and the need for parathyroid ablation. Parathyroidectomy, usually subtotal, remains the standard management, with ultrasound guided injection of ethanol or calcitriol showing promise in some centers. The above scenario is unlikely to be changed greatly by the new emerging vitamin D metabolites, but calcimimetic agents may well increase the scope of non surgical management.

Adynamic bone and chronic renal failure: an overview

Renal osteodystrophy may present with a wide spectrum of bone lesions, ranging from high bone turnover to low bone turnover. Decreased serum calcium and 1,25-dihydroxy vitamin D synthesis and retention of phosphate are involved in the pathogenesis of high bone turnover. However, several factors may influence the evolution of this disorder. The use of different therapeutic approaches (such as calcium supplements, phosphate binders, vitamin D metabolites, etc.), the type of treatment (either hemodialysis or continuous ambulatory peritoneal dialysis), and also the changes in the type of patients to whom we are offering dialysis (more diabetics and older patients are currently included in dialysis programs) may have introduced changes modifying the form of presentation of the bone metabolic disorders. As a result, recent studies reported a greater prevalence of adynamic forms of renal osteodystrophy. Patients with adynamic bone (with or without aluminum) would have more difficulties in handling and buffering calcium loads; consequently, they would have a higher risk of extraosseous calcifications.

Bone turnover in short-term and long-term home parenteral nutrition for benign disease

6 mo afterward. Long-HPN was assessed only at baseline. In short-HPN, there was a hyperkinetic turnover at baseline. At follow-up, OC was similar to baseline, whereas cross-links, urinary calcium and magnesium decreased (P < 0.03), and parathyroid hormone increased (P < 0.001). The variation of urinary calcium correlated with that of cross-links (r = 0.73, P < 0.04). In long-HPN, OC was low or low-normal in almost all the patients, and cross-links were normal. Mean OC was lower than that of short-HPN both at baseline (P < 0. 003) and at follow-up (P < 0.002). The results suggest that in the early period of HPN bone metabolism improved from a hyperkinetic turnover to a positive balance. A low bone-formation rate appeared to be a characteristic feature of long-term HPN.

Determination of trace elements and calcium in bone of the human iliac crest by atomic absorption spectrometry

A rapid and reliable analytical method for the determination of trace elements in human bone by atomic absorption spectrometry is reported. Calcium was determined to estimate the homogeneity of samples. Human bone from the iliac crest was obtained at autopsy of adult subjects. Before analysis samples were decomposed by microwave digestion and acid digestion in a Parr bomb. Zinc, rubidium, strontium, calcium and iron were determined by flame atomic absorption spectrometry (FAAS) and aluminium, copper and lead by electrothermal atomic absorption spectrometry (ETAAS) at optimum measurement conditions. The results for the two digestion procedures agreed for zinc, rubidium and calcium within +/-5%, for copper within +/-7% and for strontium, iron, aluminium and lead within +/-10%. The repeatability of measurement (R.S.D.) for determination of calcium and trace elements after microwave digestion and acid digestion in a Parr bomb was tested in one representative autopsy bone sample by six parallel determinations. It was found to be better than +/-5% either for microwave digested samples or samples digested in a Parr bomb, for all elements determined by FAAS and ETAAS techniques. The accuracy of the applied digestion procedures was checked by analysis of trace elements in NIST SRM 1486 Bone Meal reference material. Good agreement of the results with certified values was obtained for both digestion procedures. The microwave procedure developed for digestion of small amounts of sample was applied in trace elements analysis of bone biopsy samples from dialysis patients.

Low bone turnover in patients with renal failure

Renal failure inevitably leads to metabolic bone disease. Low turnover disease or adynamic bone disease (ABD) is characterized by a low number of osteoblasts with normal or reduced numbers of osteoclasts. Mineralization proceeds at a normal rate, resulting in normal or decreased osteoid thickness. Recently, it became clear that the relative contribution of the various types of renal osteodystrophy (ROD) to the spectrum of the histologic picture in renal failure patients underwent profound changes during the last 25 years. At the moment, the exact physiopathological mechanisms behind ABD are not yet elucidated, and thus the reason(s) for its increasing prevalence remains poorly understood. A number of epidemiological and experimental data suggest a multifactorial pathophysiologic process, in which hypoparathyroidism and suppression of the osteoblast are the main actors. Compared to adynamic bone disease, osteomalacia has now become a much rarer disease (around 4%), at least in Western countries. On the other hand, recent studies indicate that this particular bone disease entity might still regularly occur in less developed countries. Osteomalacia originates from a direct effect on the mineralization process. With this type of renal bone disease, the effects of secondary hyperparathyroidism on bone are overridden by a number of metabolic abnormalities that finally result in a defective bone mineralization, as occurs, for instance, when the lag time between osteoid deposition and its mineralization is increased. The relationship between exogenous and endogenous vitamin D deficiency (mainly calcitriol) and the histologic finding of osteomalacia in uremic patients is well known. Recent data showed distinctly lowered 25-(OH) vitamin D3 levels in the presence of unaffected calcitriol concentrations in patients with osteomalacic lesions, as assessed radiologically by the presence of Looser's zones. Recently, we found that bone strontium levels were increased in patients with osteomalacia as compared to all other types of ROD. Strontium accumulation appeared to originate mainly from the use of strontium-contaminated dialysate, which resulted from the addition of strontium-containing acetate-based concentrates. Evidence for a causal role of the element in the development of a mineralization defect could be tested experimentally by adding strontium to drinking water in a chronic renal failure rat model.

What is the optimal regimen for vitamin D?

The past 30 years have seen substantial advances in our understanding of the pathogenesis of the mineral, hormonal and skeletal disorders that comprise renal osteodystrophy. The introduction of calcitriol and alfacalcidol as treatments for this disorder in the early 1970s represented an enormous step forward in clinical practice, but unfortunately, the subsequent refinement of these therapies still leaves us well short of the ideal: hyperphosphatemia and hypercalcemia induced by the vitamin D metabolites, and failure to control parathyroid hyperplasia, all remain problematic. Novel pulsed regimens using alfacalcidol and calcitriol, while clearly effective, have not fulfilled initial high expectations of superiority in the context of comparative studies. New vitamin D metabolites, some of which have exhibited desirable selectivity in experimental settings with reduced tendency to raise phosphate and/or calcium while maintaining good control of the parathyroid glands, are now being evaluated. Of these, 22-oxacalcitriol, paricalcitol (19 nor-1,25 dihydroxyvitamin D2) and doxercalciferol (1 alpha-hydroxyvitamin D2) have all shown high efficacy when compared with placebo, but so also did alfacalcidol and calcitriol in similar studies in the 1970s and 1980s. The results of randomized studies comparing the new vitamin D metabolites with current standard therapy (alfacalcidol or calcitriol) are either not yet available or show uncertain benefits in relation to hypercalcemia, hyperphosphatemia and hyperparathyroidism. The impact of these new metabolites on the increasing prevalence of low turnover bone disease is unknown, although experimentally there is evidence of potentially important differences at the level of the skeleton.

Relative hypoparathyroidism and adynamic bone disease

Renal bone disease results in significant morbidity in patients with end-stage renal failure. Renal osteodystrophy is a mixture of different conditions with different pathogenetic factors involved. Most recently a new form of renal bone disease, adynamic bone disease, has emerged as the most frequent finding on bone biopsy of patients on dialysis therapy. The etiology of this new entity is not fully understood, but relatively low levels of intact serum parathyroid hormone are frequently associated with this disorder and may play an important role in its pathogenesis.

Renal osteodystrophy in dialysis patients: diagnosis and treatment

This article reviews the clinical, biological, radiological, and pathological procedures and their respective indications for the practical diagnosis of the following various histological patterns of renal osteodystrophy: osteitis fibrosa due to parathyroid hormone (PTH) hypersecretion: osteomalacia or rickets due to native vitamin D deficiency and/or aluminum overload; and adynamic bone disease (ABD) due to aluminum overload and/or PTH secretion oversuppression. Our advice regarding bone biopsy is to restrict it to patients with symptoms and hypercalcemia, especially those who have been previously exposed to aluminum. In other cases, we propose relying merely on the determination of the plasma concentrations of calcium, protide, phosphate, bicarbonate, intact PTH, aluminum, 25(OH)D3, and alkaline phosphatase (total and bony if hepatic disease is associated) to choose the appropriate treatment. Because of the danger of the desferrioxamine treatment necessary to chelate and remove aluminum, the suspicion of aluminic bone disease (osteomalacia or ABD) will always be confirmed by a bone biopsy. In the case of nonaluminic osteomalacia, correction of the vitamin D deficiency by native vitamin D or 25(OH)D3, and of the calcium deficiency and acidosis by alkaline salts of calcium and if necessary sodium bicarbonate are sufficient to cure the disease. In the case of nonaluminic ABD, the stimulation of PTH secretion by the discontinuation of 1alpha hydroxylated vitamin D and the induction of a negative calcium balance during dialysis by decreasing the calcium concentration in the dialysate will allow an increase of the CaCO3 dose to correct for hyperphosphatemia without inducing hypercalcemia. For hyperparathyroidism, i.e., plasma intact PTH levels greater than two- or four-fold the upper limit of normal levels (according to the absence or presence of previous aluminum exposure), the treatment will consist in increasing the CaCO3 dose to correct for hyperphosphatemia together with a decrease of the calcium concentration in the dialysate if the dose of CaCO3 is so high that it induces hypercalcemia. When the hyperphosphatemia has been corrected and there is still a low or normal corrected plasma calcium level, 1alpha(OH)D3 in an oral bolus 2 or 3 times a week should be given at the minimal dose of 1 microg. When the PTH level stays above 400 pg while hypercalcemia occurs and hyperphosphatemia persists, surgical subtotal parathyroidectomy is recommended or the injection of calcitriol into the big nodular hyperplastic parathyroid glands under sonography control in high surgical risk patients. Special recommendations are given for children.

Serum magnesium concentration and PTH levels. Is long-term chronic hypermagnesemia a risk factor for adynamic bone disease?

The observation that some subjects with low PTH had elevated plasma magnesium (Mg) levels prompted us to analyze in 41 patients on maintenance hemodialysis for 44 +/- 36 months, their serum Mg concentrations, and the relationship between plasma Mg and PTH levels. The mean serum Mg concentration was 2.4 +/- 0.2 mg/dl. Twenty-four out of the 41 subjects (58.5%) had hypermagnesemia (serum Mg above 2.5 mg/dl). Patients were classified into 3 groups according to their PTH level: Group A, low PTH (below 120 pg/ml); group B, adequate PTH (120-250 pg/ml); and group C, high PTH (above 250 pg/ml). There were no differences among groups according to number of subjects, age, sex, time on dialysis, renal disease, serum calcium, phosphorus, bicarbonate, vitamin D or aluminum concentrations. Doses of calcium carbonate and aluminium hydroxide were also similar in all groups. Curiously, although the differences were not statistically significant, the total cumulative intake of calcium and aluminium were less in group A than in the other groups. Interestingly, patients with low PTH had a significantly higher serum Mg concentration than patients with adequate or high PTH (2.8 +/- 0.2 mg/dl vs 2.3 +/- 0.1 mg/dl and 2.2 +/- 0.1 mg/dl, respectively, p < 0.01). Moreover, regression analysis showed a negative linear correlation between serum PTH level and plasma Mg concentration (r = -0.6059, p < 0.001). Based on these findings, chronic hypermagnesemia could have a suppressive effect on PTH secretion, and it could be a risk factor for the development of adynamic bone disease in dialysis patients.

Harry M. Vars Research Award. Increase in lumbar spine bone mineral content in patients on long-term parenteral nutrition without vitamin D supplementation

BACKGROUND

We had previously shown that short-term withdrawal of vitamin D in patients with metabolic bone disease complicating home parenteral nutrition (HPN) corrected osteomalacia. We therefore conducted a prospective study of the effect of long term withdrawal of vitamin D in patients on home parenteral nutrition.

METHODS

Baseline measurements of bone mineral content, serum levels of calcium, phosphorus, parathormone, 25-OH and 1,25 (OH)2D; urinary calcium; and bone mineral density were measured. Then all parenteral vitamin D was withdrawn and the above parameters were followed for a mean of 4.5 years.

RESULTS

Lumbar spine bone mineral content (LSBMC) was 0.79 +/- 0.06 g/cm2 at the start of the study, well below the reference value, 1.16 +/- 0.13 g/cm2. Parathyroid hormone (PTH) (0.48 +/- 0.24 pmol/L) and 1,25-(OH)2D levels (22.8 +/- 7.9 pmol/L) were low and 25-hydroxyvitaniin D levels were normal (33.3 +/- 5.5 nmol/L) before removing vitandn D from the HPN solutions. After withdrawal of vitamin D for 4.5 +/- 0.2 years LSBMC increased from 0.79 +/- 0.06 to 0.93 0.07 g/cm2 (p < 0.005). Calcium phosphorus, magnesium and 25-hydroxyvitamin D did not change significantly, 1,25(OH)2D, and PTH levels became normal after withdrawal of vitamin D.

CONCLUSIONS

In selected patients with depressed PTH levels, long-term withdrawal of vitamin D during HPN increases LSBMC and levels of PTH and 1,25(OH)2D. There is no reduction of the mean level of 25-hydroxyvitamin D.

Predictive value of serum parathyroid hormone levels for bone turnover in patients on chronic maintenance dialysis

With the increasing occurrence of adynamic bone disease, it is essential to determine the level of bone turnover in chronically dialyzed patients before instituting vitamin D therapy. To assess the value of serum parathyroid hormone (PTH) levels for prediction of bone turnover, we determined sensitivity, specificity, and predictive value positive of serum PTH, alone or in combination with other variables, in 79 patients who underwent one or two bone biopsies. Serum PTH levels were determined by a radioimmunometric assay and were obtained at the time of bone biopsies. Patients were classified into (1) low or normal and (2) high bone turnover according to the value of activation frequency of bone. There were 57 biopsy specimens taken from hemodialysis patients and 39 specimens from continuous ambulatory peritoneal dialysis patients (CAPD). All patients with serum PTH levels within or below the normal range had low or normal bone turnover. Values of serum PTH above 450 pg/mL were 100% and 95.5% specific for high bone turnover in hemodialysis and CAPD patients, respectively. Values of serum PTH between 65 and 450 pg/mL had worse predictive value positive in CAPD patients (48.6% to 78.6%) than in hemodialysis patients (67.3% to 87.1%). When other characteristics of the patients were taken into consideration, only age in hemodialysis patients and serum ionized calcium in CAPD patients improved the predictive value of serum PTH. All hemodialysis patients younger than 45 years of age with serum PTH levels above 65 pg/mL (n = 15) had high bone turnover, and CAPD patients with low or normal bone turnover had higher serum ionized calcium. However, overall, bone turnover could not be predicted by serum PTH measurements in 30% of hemodialysis and 51.3% of CAPD patients. The data suggest that for patients with serum PTH levels between 65 and 450 pg/mL, bone biopsies are indicated to precisely assess bone turnover prior to initiation of vitamin D therapy.

The importance of bone biopsy in managing renal osteodystrophy

A case is presented in which bone biopsy results helped to resolve not only difficult issues in the clinical management of the patient's renal osteodystrophy but also disruptive psychosocial problems surrounding her clinical course. The outcome was a satisfactory resolution based on rational medical treatment and directed supportive care. The presentation highlights important principles in the procurement, processing, and interpretation of the bone biopsy, while also addressing the importance of accurate diagnosis in facilitating the overall long-term management by the entire renal team.

Evidence for abnormal calcium homeostasis in patients with adynamic bone disease

To investigate whether the derangements in calcium kinetics in patients with renal osteodystrophy are similar in the various histologic forms of this metabolic bone disease, 43 patients on chronic maintenance dialysis underwent calcium kinetic studies using the double isotope technique, iliac crest bone biopsies for mineralized bone histology and histomorphometry and determinations of serum indices of calcium and bone metabolism. Intestinal calcium absorption was not different among the three histologic groups. However, women exhibited lower calcium absorption in each histologic form (P < 0.01). Patients with predominant hyperparathyroid bone disease showed plasma calcium efflux, calcium accretion rate and calcium retention markedly above normal values. Patients with low turnover bone disease exhibited a normal or slightly decreased plasma calcium efflux and calcium accretion rate together with a disproportionately low calcium retention. Patients with mixed uremic osteodystrophy presented with a calcium kinetic profile intermediary to the two other forms. Good relationships existed between plasma calcium efflux, calcium accretion rate, calcium retention and histomorphometric parameters of bone turnover as well as serum levels of parathyroid hormone. However, no serum parameter could indicate with certainty the underlying bone disease. These findings demonstrate that adynamic bone disease does not merely represent an academic finding but is characterized by a very low bone capacity to buffer calcium and inability to handle an extra calcium load. This is particularly relevant for the daily care of end-stage renal failure patients presently receiving higher than ever amounts of vitamin D and calcium salts.

Interpretation and clinical significance of alkaline phosphatase isoenzyme patterns

Alkaline phosphatase (ALP, EC 3.1.3.1) is a membrane-bound metalloenzyme that consists of a group of true isoenzymes, all glycoproteins, encoded for by at least four different gene loci: tissue-nonspecific, intestinal, placental, and germ-cell ALP. Through posttranslational modifications of the tissue-nonspecific gene, for example, through differences in carbohydrate composition, bone and liver ALP are formed. Nowadays, most commercially available methods for separating or measuring ALP isoenzymes are easy to perform and sensitive and allow for reproducible and quantitative results. As more isoenzymes and isoforms have been characterized, confusion has arisen due to the many different names they were given. For the sake of simplicity and because of structural analogies, we propose an alternative nomenclature for the ALP isoenzymes and isoforms based on their structural characteristics: soluble, dimeric (Sol), anchor-bearing (Anch), and membrane-bound (Mem) liver, bone, intestinal, and placental ALP. Together with lipoprotein-bound liver ALP and immunoglobulin-bound ALP, these names largely fit the many forms of ALP one can encounter in human serum and tissues. The clinically relevant isoenzymes are sol-liver, Mem-liver, lipoprotein-bound liver, and Sol-intestinal ALP in liver diseases, and Sol-bone and Anch-bone ALP in bone diseases. Many different isoenzyme patterns can be found in malignancies and renal diseases. This test provides the clinician with valuable information for diagnostic purposes as well as for follow-up of patients and monitoring of treatment. However, ALP isoenzyme determination will only provide clinically useful information if the patterns are correctly interpreted. In this respect, care should be taken to use the proper reference ranges, taking into account the age and sex of the patient. A normal total ALP activity does not rule out the presence of an abnormal isoenzyme pattern, particularly in children. Separating ALP into its isoenzymes adds considerable value to the mere assay of total ALP activity.