Renal osteodystrophy in predialysis patients without stainable bone aluminum. A cross-sectional bone-histomorphometric study

Changing bone patterns with progression of chronic kidney disease

It is commonly held that osteitis fibrosa and mixed uremic osteodystrophy are the predominant forms of renal osteodystrophy in patients with chronic kidney disease. Osteitis fibrosa is a high-turnover bone disease resulting mainly from secondary hyperparathyroidism, and mixed uremic osteodystrophy is in addition characterized by a mineralization defect most often attributed to vitamin D deficiency. However, there is ancient and more recent evidence that in early chronic kidney disease stages adynamic bone disease characterized by low bone turnover occurs first, at least in a significant proportion of patients. This could be due to the initial predominance of bone turnover-inhibitory conditions such as resistance to the action of parathyroid hormone (PTH), reduced calcitriol levels, sex hormone deficiency, diabetes, and, last but not least, uremic toxins leading to repression of osteocyte Wnt/β-catenin signaling and increased expression of Wnt antagonists such as sclerostin, Dickkopf-1, and sFRP4. The development of high-turnover bone disease would occur only later on, when serum PTH levels are able to overcome peripheral PTH resistance and the other inhibitory factors of bone formation. Whether FGF23 and Klotho play a direct role in the transition from low- to high-turnover bone disease or participate only indirectly via regulating PTH secretion remains to be seen.

Role of bone biopsy in stages 3 to 4 chronic kidney disease

Secondary hyperparathyroidism develops relatively early in chronic kidney disease as a consequence of impaired phosphate, calcium, and vitamin D homeostasis. The disease state in chronic kidney disease, which includes the histologic features of bone disease, defined as renal osteodystrophy, and the hormonal and biochemical disturbances, have recently been redefined as a disease syndrome and is referred to as "chronic kidney disease-mineral and bone disorder." As chronic kidney disease progresses, specific histologic disturbances in the bone develop, which may or may not be predictable from the biochemical and hormonal changes that are associated with chronic kidney disease. In addition, patients may have had underlying bone disease before developing kidney failure or may have been treated with agents that will alter the classical pathologic findings of the bones in chronic kidney disease and their relation to parathyroid hormone. Thus, in stage 5 chronic kidney disease, bone biopsy with quantitative histomorphometric analysis is considered the gold standard in the diagnosis of renal osteodystrophy. In contrast to stage 5 chronic kidney disease, there are very few data on the histologic changes in bone in earlier stages of chronic kidney disease. There also is no adequate information on the etiopathogenesis of bone disease in stages 3 and 4 chronic kidney disease. Thus, because biochemical data cannot predict bone pathology in stages 3 and 4 chronic kidney disease, bone biopsy should be used to define these bone changes and to allow appropriate therapeutic approaches.

Renal Osteodystrophy in Peritoneal Dialysis: Special Considerations

Bone disease is one of the most challenging complications in patients with chronic kidney disease. Today, it is considered to be part of a complex systemic disorder manifested by disturbances of mineral metabolism and vascular calcifications called chronic kidney disease – mineral bone disorder (CKD-MBD).

The term renal osteodystrophy is reserved to define the specific bone lesion in CKD-MBD, whose spectrum ranges from high turnover to low turnover disease. Phosphate retention, decreased serum calcium, and 1,25-dihydroxy vitamin D synthesis are involved in the pathogenesis of high bone turnover. However, the various therapeutic approaches (calcium supplements, phosphate binders, and vitamin D metabolites, among others), the renal replacement modality (hemodialysis or continuous ambulatory peritoneal dialysis), and the types of patients to whom dialysis is offered (more patients who are diabetic or older, or both) may influence the evolution of the bone disorder. As a result, recent studies have reported a greater prevalence of adynamic forms of renal osteodystrophy, especially in diabetic and peritoneal dialysis patients.

The present article reviews, for patients treated with peritoneal dialysis, the pathophysiologic mechanisms involved in the evolution and perpetuation of this bone disease and the therapeutic modalities for treating and preventing adynamic bone.

Osteoporosis and chronic kidney disease

Osteoporosis (OP), the most frequent bone disease affecting the general population, is associated with high fracture risk. Patients with impaired kidney function have bone and mineral disturbances leading to extraskeletal calcifications and complex changes in bone turnover that predispose them to increased fracture risk accompanied by increased morbidity and mortality. The combination of these two bone disorders seems to have an additive effect with regard to fracture risk and its outcome, so that appropriate diagnosis and treatment of this disorder should be of primary concern when approaching patients with kidney disease. Nevertheless, the clinical and laboratory diagnostic tools used to identify OP in the general population do not suit the requirement for detecting the complex bone and metabolic changes that occur with chronic kidney disease, leading to the lack of or the initiation of inappropriate therapy. This review will focus on the bone pathophysiologic processes involved in OP and renal osteodystrophy and address some of the problems associated with our current diagnostic tools and aspects of the therapeutic approaches.

Bone health in chronic kidney disease-mineral and bone disease

Chronic kidney disease (CKD) is accompanied by disturbances in calcium, phosphate, vitamin D, and parathyroid hormone (PTH) homeostasis that play an important role in the pathophysiology of renal bone disease. The increased cardiovascular morbidity and mortality observed among patients with CKD has recently been recognized to be associated with these disturbances in mineral metabolism. Thus, disturbances in mineral metabolism observed in renal failure results in a multisystem disorder, making the development of a standardized definition of these disorders a top priority. Therefore, the Board of Directors of Kidney Disease: Improving Global Outcomes proposed to define the broader category of mineral disorders associated with CKD as CKD-mineral and bone disorder (CKD-MBD). This newly proposed definition will include the disorders of mineral metabolism, bone histology (renal osteodystrophy), and the extraskeletal manifestations such as vascular calcification. This new definition and stratification of disease should result in improvement not only in the clinical management of patients but also will facilitate the interpretation and translation of clinical research. Renal osteodystrophy is now considered as 1 component of this disorder and will be defined as a morphologic alteration only, based on unification of the histomorphometric definitions that will include parameters of turnover, mineralization, and volume. An internationally accepted classification system will enable the consensus for bone biopsy evaluation as well as for the role of biomarkers. This article will focus on the newly proposed definitions of bone disease as part of CKD-MBD, based on the complex pathophysiologic process underlying bone disease in CKD stages 2 to 5.

Vascular calcification and renal osteodystrophy relationship in chronic kidney disease

Cardiovascular disease and stroke account for 60-70% of all deaths in patients with end-stage renal disease (ESRD), at a risk that is 10-20-fold the age- and sex-matched general population. There is also increased coronary artery calcification and increased cardiovascular mortality in chronic kidney disease (CKD) and dialysis patients compared with the general population. Bone is similarly abnormal in CKD. There is an increased incidence of low bone mass and fractures in dialysis patients compared with the general population. Furthermore, a hip fracture in a dialysis patient is associated with a doubling of the mortality observed in nondialysis patients with a hip fracture. These two problems may be linked, as cross-sectional studies have demonstrated an inverse relationship between osteoporosis and coronary artery calcification in the general population and in ESRD patients. In vitro and ex vivo, there is clear evidence that vascular calcification is an active cell-mediated process, made worse by disorders of mineral metabolism. Many factors known to be associated with cardiovascular disease in CKD patients can directly increase calcification in vitro. In addition, in CKD, there are many mechanisms by which bone may adversely affect vascular calcification including disorders of bone remodelling, altered secretion of parathyroid hormone (PTH), hyperphosphatemia, hypercalcaemia, use of calcium based binders, and excessive vitamin D therapy. The coexistence of vascular risk factors and abnormal bone represent a double threat to the well being of patients with CKD.

Spectrum of renal bone disease in end-stage renal failure patients not yet on dialysis

BACKGROUND

During the last few years the spectrum of renal osteodystrophy (ROD) in dialysis patients has been studied thoroughly and the prevalence of the various types of ROD has changed considerably. Whereas until a decade ago most patients presented with secondary hyperparathyroidism (HPTH), adynamic bone (ABD) has become the most common lesion within the dialysis population over the last few years. Much less is known about the spectrum of ROD in end-stage renal failure (ESRF) patients not yet on dialysis.

METHODS

Transiliac bone biopsies were taken in an unselected group of 84 ESRF patients (44 male, age 54+/-12 years) before enrolment in a dialysis programme. All patients were recruited within a time period of 10 months from various centres (n=18) in Macedonia. Calcium carbonate was the only prescribed medication in patients followed up by the outpatient clinic.

RESULTS

HPTH was found in only 9% of the patients, whilst ABD appeared to be the most frequent renal bone disease as it was observed in 23% of the cases next to normal bone (38%). A relatively high number of patients (n=10; 12%) fulfilled the criteria of osteomalacia (OM). Mixed osteodystrophy (MX) was diagnosed in 18% of the subjects. There was no significant difference between groups in age, creatinine, or serum and bone strontium and aluminium levels. Patient characteristics associated with ABD included male gender and diabetes, whilst OM was associated with older age (>58 years).

CONCLUSIONS

In an unselected population of ESRF patients already, 62% of them have an abnormal bone histology. ABD is the most prevalent type of ROD in this population. In the absence of aluminium or strontium accumulation the relatively high prevalence of a low bone turnover as expressed by either normal bone or ABD and OM is striking.

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.