Paul Cockwell - Review Date July 2015 (Senior Editor Paul Cockwell)
Chronic Kidney Disease – Mineral Bone Disorder (CKD-MBD) is the term used to describe the pathophysiological changes that occur in the vascular and skeletal system in association with chronic kidney disease (CKD) (Moe et al). It has been classified by KDIGO in August 2009.CKD-MBD has replaced the terms renal osteodystrophy and renal bone disease in routine clinical practice. The changes that occur in CKD-MBD include biochemical abnormalities, bone changes and extra-osseus (including vascular) calcification.
CKD- MBD can be diagnosed if a patient with CKD has evidence of one or more of:
1. Abnormalities of calcium, phosphate, PTH or vitamin D metabolism
2. Vascular and/or soft tissue calcification
3. Abnormalities in bone turnover, metabolism, volume, linear growth or strength
The development of CKD-MBD starts early in the course of CKD. Changes in serum levels of parathyroid hormone (PTH), 25-hydroxyvitamin D and 1,25 dihydroxyvitamin D are often present in patients with an estimated glomerular filtration rate (eGFR) of <60mL/min/1.73m2. Significant changes in serum calcium and phosphate levels can be seen with an eGFR <40mL/min. Whilst there are strong associations between CKD-MBD and adverse outcomes (both cardiovascular disease (CVD) morbidity and mortality and progressive CKD) (Eddington et al) there is little good quality evidence for interventions to address CKD-MBD in patients with less severe CKD (stages 3 and 4). Thus most clinicians focus treatment on patients with stage 5 CKD (eGFR <15mL/min/1.73m2).
To understand the pathophysiology of CKD-MBD requires an understanding of the central role of the kidney in calcium and phosphate homeostasis.
Calcium and Phosphate Homeostasis
Pathways that regulate calcium and phosphate homeostasis are shown in figures 1 and 2. Calcium and phosphate are of major importance in most biological processes within the body and their regulation is complex. Intestinal absorption of calcium occurs in the duodenum and more distally in the colon.
In the kidney, passive reabsorption of calcium occurs in the proximal tubule and thick ascending limb of the loop of Henle. Furthermore calcium is reabsorbed against an electrochemical gradient via an active process in the distal convoluted tubule and collecting duct; this is regulated by a number of factors including PTH, 1,25 dihydroxy vitamin D (1,25(OH)2 vitamin D), insulin, oestrogen, acidity, calcium ions, magnesium ions and klotho. Medications including diuretics and calcineurin inhibitors can also affect cellular calcium transport within the kidney and therefore also have an impact on calcium regulation.
Figure 1 - Calcium Homeostasis
There are many organs involved in phosphate regulation and these include the kidney, gastrointestinal tract, parathyroid glands and bone. The fine regulation of phosphate occurs in the proximal tubule of the kidney with 80-97% of the phosphate filtered load reabsorbed through Sodium-Phosphate co-transporters. Gastrointestinal tract absorption of phosphate is increased in the presence of 1,25(OH)2 vitamin D. As phosphate levels rise, phosphaturic hormones are released to promote increased renal phosphate excretion; the best characterised of these is fibroblast growth factor 23 (FGF23; a phosphatonin), one of a number of molecules that regulate phosphate excretion and reabsorption. Other mediators of phosphate metabolism include PTH, acidosis, glucocorticoids, calcium levels, oestrogens and medications such as diuretics.
Figure 2 - Phosphate Homeostasis
FGF23 and Klotho
FGF23 is 32Kda protein secreted by osteocytes in bone in response to increasing phosphate concentration (Rodriguez et al). FGF23 is dependent on the presence of klotho to allow interaction with the FGF receptor(Martin et al).Klotho is predominantly expressed in the distal tubule and can be found in a membrane bound or in a soluble form.In the kidney the majority of FGF-Klotho complexes are found in the distal tubule, although phosphate excretion occurs at the proximal tubules. The mechanism of action is currently unknown. though a possible paracrine pathway has been hypothesised. FGF23 also acts as a counter regulatory hormone to 1,25 dihydroxyvitamin D by inhibiting 1α hydroxylase and therefore the levels of active 1,25 dihydroxyvitamin D in circulation.
In addition to a role as a cofactor with FGF23 for FGF receptor activation, soluble klotho inhibits internalisation of a calcium channel within the kidney and is important in calcium regulation. Soluble klotho may also affect calcium by partly regulating PTH secretion; klotho deficient mice were found to have less PTH secretion on hypocalcaemic stimulation than wild type.
The role of FGF23 in CKD–MBD and cardiovascular disease has been reviewed by Larsson in 2009.
Normal Parathyroid Regulation
Parathormone (PTH) is an 84 amino acid single chain protein that is released from the chief cell in the parathyroid glands. This secretion is regulated via the calcium sensing receptor and the vitamin D receptor by mediating changes in gene transcription and hormone synthesis. FGF23 receptors and klotho are present on parathyroid cells and FGF23 has been shown to have a suppressive effect on PTH release. Furthermore some studies have shown that 1,25 dihydroxyvitamin D and PTH stimulate FGF23 expression and secretion therefore closing a feedback loop.
Magnesium, adrenaline, and histamine can also affect PTH release; longstanding hypomagnesaemia both inhibits PTH synthesis and impairs PTH actions in target tissues. Adrenaline and histamine both stimulate PTH release via specific receptors.
The main actions of PTH (figure 4) are to increase plasma calcium and decrease plasma phosphate; this is accomplished through a direct effect on bone and kidneys and an indirect effect on the gastrointestinal tract.
In the kidney, PTH blocks the reabsorption of phosphate at the proximal tubule stimulating an increase in phosphate excretion and increases reabsorption of calcium from the distal tubule.
In bone, PTH promotes recruitment and differentiation of osteoclasts, therefore increasing breakdown of bone mineral and degradation of bone collagen. Osteoclasts, however, have no PTH receptors and the PTH effect is mediated via cytokines and other factors released from osteoblasts and other local cells. PTH also promotes bone formation by direct action on osteoblasts, increasing both the number and activity of these cells.
In the gastrointestinal tract, PTH stimulates 1α hydroxylation of 25 hydroxyvitamin to produce active 1,25 dihydroxyvitamin D. This then acts to increase gastro-intestinal absorption of calcium and phosphate. High calcium and active vitamin D levels and low phosphate levels fall act as negative feedback on the PTH gland and therefore homeostasis is maintained.
Figure 3 - Regulation of PTH Production
The single term ‘Vitamin D’ is used to describe vitamin D at varying stages of hydroxylation; the stage of hydroxylation has a profound effect of the activity of the hormone and therefore is important to state which ‘vitamin D’ is being referred to.
Cholecalciferol (Vitamin D3) is converted from 7-dehydrocholesterol in the skin on exposure to Ultraviolet-B radiation. Native vitamin D, which can include cholecalciferol and ergocalciferol (vitamin D2), can be converted in the liver to 25 hydroxyvitamin D (aka calcidol) and stored. The 25 hydroxyvitamin D is then hydroxylated to 1,25 dihydroxyvitamin D (aka calcitriol) for local autocrine and paracrine pathways as well as the endocrine pathway already mentioned. The two main feedback mechanisms are shown in figure 4.
Figure 4 - As CKD develops pertubation in the homeostatic pathways described above lead to hypocalcaemia, hyperphosphataemia and Vitamin D deficiency and the development of secondary hyperparathyroidism.
Hypocalcaemia stimulates release of parathyroid hormone(PTH) directly by inactivation of the calcium sensing receptors (CaR) on parathyroid cells. The plasma PTH concentration increases within minutes of fall in serum calcium levels. Reduction of extracellular calcium levels for weeks or months promotes the development of parathyroid gland hyperplasia, which is the characteristic ofhyperparathyroidism (Goodman, 2003).
Metastatic calcification has been attributed to hyperphosphataemia since the early 1960s (Parfitt, 1969). However, the clinical impact and toxicity of hyperphosphataemia was not widely emphasised until the groundbreaking studies carried out of Bricker et al(Bricker, 1969). Using animal models, this group delineated the pathophysiological cascade triggered by hyperphosphatemia, leading to hypocalcaemia, secondary hyperparathyroidism, reduced 1,25 vitamin D3, and progressive metabolic bone disease.
Hyperphosphataemia does not become evident until the glomerular filtration rate (GFR) has decreased to between 25 and 40% of normal (CKD4).
Until that stage, in the course of CKD, compensatory hyperparathyroidism results in increased phosphate excretion and maintains serum phosphate levels within the normal range (Alfrey, 2004). Due to declining GFR, phosphate retention leads to a decrease in serum free calcium levels, which in turn stimulates PTH secretion (Drueke, 1995).This is the 'trade-off' hypothesis (Bricker, 1972). It also leads to decreased production of calcitriol by the kidney, which in turn decreases intestinal calcium absorption leading to hypocalcaemia and consequently, stimulation of PTH secretion (Tallon, 1996).
Hyperphosphataemia is associated with resistance to the actions of calcitriol on the parathyroid glands, which also leads to increased PTH secretion - andcauses resistance to the action of PTH on bone (Llach, 1995). Also high phosphate levels have a direct stimulatory effect on PTH secretion, independent of changes in calcium and 1,25 vitamin D3 levels (Almaden, 1998).
Vitamin D Deficiency
The kidney is a major site for calcitriol production. In CKD, calcitriol production falls. Calcitriol has several direct and indirect effects on parathyroid glands. In CKD, vitamin D receptors (VDR) in parathyroid glands are down regulated as a consequence of low levels of calcitriol. This direct mechanism leads to stimulation of PTH gene expression and increases PTH secretion.
Administration of calcitriol increases vitamin D receptors in the parathyroid glands and suppresses PTH secretion. Also, low circulating calcitriol levels may facilitate parathyroid cell proliferation (Drueke, 2000). Thus, calcitriol deficiency indirectly leads to secondary hyperparathyroidism(Friedman, 2005)in addition to causing skeletal resistance to the calcaemic actions of PTH in CKD.
This is a common complication of CKD and is caused by decreased renal ammonia synthesis and hydrogen ion excretion. There are a number of consequences of chronic metabolic acidosis (Ortega and Arora 2012), including increasing bone turnover due to compensatory mechanisms activated as a consequence of the buffering effect of bone in acidosis.
Hyperparathyroidism in CKD
Parathyroid glands develop nodular hyperplasia, in which is situ VDR expression and CaR expression are significantly decreased. This contributes to the reduced capacity of the parathyroid glands to respond to therapy such as vitamin D, as well as changes in serum calcium levels (Locatelli, 2002). Hyperparathyroidism starts early in the course of CKD (figure 5):
Figure 5: The relationship between CKD and PTH
Clinical Features of CKD MBD
Early bone disease in patients with CKD is usually asymptomatic. Musculoskeletal symptoms usually appear late in the course of CKD-MBD (Llach, 2003). Skin symptoms are also important for patients with advanced CKD-MBD who are receiving treatment with dialysis; in this group pruritis is very common.
Clinical signs and end-organ damage is associated both with bone and cardiovascular disease. Where CKD-MBD has contributed to the development of cardiovascular disease, the presentations of cardiac disease, cerebrovascular disease and peripheral vascular disease are similar to those seen in patients where CKD-MBD does not have a pathopysiological role. Calciphylaxis is a rare but important skin complication of CKD-MBD.
Musculoskeletal symptoms and signs. Most of the data is derived from dialysis patients. Bone pain, which occurs in the low back, hips and legs and is aggravated by weigh bearing. Bone deformities are common in patients with severe hyperparathyroidism; this can be due to verterbral fractures which can lead to kypho-scoliosis or chest wall deformity. There is also an increased incidence of fractures of major joints and major long bones. A significant fracture risk is present with increasing PTH levels.
Cardiovascular calcification. Coronary artery calcification is commoner and more severe in patients on haemodialysis than in persons without renal failure (Braun, 1996) and is probably due to excessive use of calcium-containing phosphate binders and vitamin D analogues.
Coronary artery calcification is found in the majority of patients on RRT and can be detected by non-invasive electron-beam computed tomography (EBCT). In addition to coronary artery calcification, calcium deposits on the heart valves (especially mitral and aortic valves), and in the myocardium - causing arrhythmias, left ventricular dysfunction, aortic and mitral stenosis, ischaemia, congestive heart failure, and death.
A relation between left ventricular hypertrophy (LVH) and PTH has been described in patients with CKD and secondary hyperparathyroidism. PTH induces hypertrophic growth of cardiomyocytes and smooth muscle cells through the activation of the cardiomyocyte protein kinase (De Fransisco, 2004).
Pruritus. Pruritus occurs in advanced CKD, especially in patients on dialysis, and has a major impact on the quality of life of patients on dialysis. It mayrelate to deposition of calcium and phosphate in the skin.
Calciphylaxis. Another serious complication of secondary hyperparathyroidism is soft tissue calcification. This is called calcific uraemic arteriolopathy (CUA), also known as calciphylaxis. It is a syndrome of calcification of small arterioles and venules with severe intimal hyperplasia. It is often complicated by thrombosis and recanalisation, which results in painful skin necrosis. It carries a high mortality rate due to secondary infection, sepsis and ischaemia.
Calciphylaxis is caused by high PTH levels, hyperphosphatemia and hypercalcaemia; induced by high calciumdialysate and use of calcium-containing phosphate binders. Obesity, hypo-albuminaemia and diabetes increase the risk of calciphylaxis (Llach, 2003); as does warfarin usage and hypercoagulable states (protein C and protein S deficiency) (Willmer, 2002). The management of CUA includes: stopping oral calcium; using non-calcium containing binders; and parathyroidectomy if PTH levels are high and cannot be controlled.
Bone biopsy remains the gold standard for the definitive diagnosis of CKD-MBD, however this is not carried out in routine clinical practice in the large majority of centres, and the diagnosis is usually based on biochemical parameters. Some of the histological features seen in the bone in CKD-MBD are demonstrated in figure 6.
PTH levels greater than 50 pcmol/L are highly indicative of osteitis fibrosa, whereas an adynamic lesion is suspected when levels are below 10 pcmol/L. The serum alkaline phosphatase level may be elevated in hyperparathyroidism indicating increased osteoblastic activity (Gonzalez, 2001). The radiological manifestations of bone disease are shown in figure 7 and figure 8.
Pathology of Secondary Hyperparathyroidism
Bone involvement in CKD-MBD can be divided into high turnover states and low turnover states.
High turnover states (osteitis fibrosa) are a consequence of hyperparathyroidism which increases bone turnover and bone resorption. This then contributes to bone pain and fractures at skeletal sites and systemic vascular injury including vascular calcification that is associated with an increased cardiovascular mortality.
Low turnover states are a consequence of PTH oversuppression and also vitamin D deficiency and (uncommonly) aluminium deposition in bone. Low turnover states are also associated with bone pain and an increased fracture rate and cardiovascular disease progression.
High turnover and low turnover changes can co-exist. Whilst the pathogenic pathways associated with this are different, ultimately they have the same clinical consequences for the patient.
Figure 6: A bone biopsy obtained from a patient with secondary hyperparathyroidism - thisbiopsy shows zones of decalcification andincreased numbers of osteoclasts
Radiology of Secondary Hyperparathyroidism
Figure 7: Resorption of distal clavicles; usually bilateral and symmetrical
Figure 8: Subperiosteal resorption along the radial surface of the proximal and middle phalanges of the second and/or third digits
Adynamic bone disease (ABD) is a common skeletal lesion in adult patients with CKD (figure 9) and a manifestation of low bone turnover. It is characterised histopathologically by a marked decrease in osteoblasts and osteoclasts with an increase in osteoid formation (Salusky, 2001). Adynamic bone disease is also caused by over-suppression of PTH levels by aggressive use of high-calcium dialysate, calcium-containing phosphate binders and vitamin D analogues. It is frequently seen in patients on peritoneal dialysis and those withdiabetes.
Figure 9: Adynamic bone disease in a patient on long-term haemodialysis
Osteomalacia is characterised by a reduction in the number of osteoblasts and osteoclasts, with an increase in the amount of osteoid. It is related to aluminum accumulation due to use of aluminum-containing phosphate binders. Long standing severe metabolic acidosis may also cause osteomalacia.
DIalysis (β2-Microglobulin) Amyloidosis
ESRD patients typically have serum concentrations of β2-microglobulin (β2M) 30-50 fold higher than normal, and are risk of developing dialysis amyloidosis (Aβ2M) affecting the skeletal system. Risk factors include: older patients; duration on dialysis; and, loss of residual renal function.
Dialysis amyloidosis can affect any joint, but especially affects the shoulder joint. So the disease often presents as shoulder pain in a dialysis patient, who has had many years on dialysis. Other clinical manifestations include carpal tunnel syndrome, tendon rupture/contracture, spondyloarthropathy, osseous involvement, subcutaneous masses and renal calculi.
MRI is the best radiological test. Histological evidence is required to make a definite diagnosis. Although of unproven benefit, it is reasonable to consider using biocompatible HD membranes, haemofiltration or haemodiafiltration at high flow rates, with ultrapure dialysate. Successful transplantation will restore β2M levels to the normal range. But it is unclear whether this causes the lesions to regress.
Principles of Treatment
Key Point: The aim of treatment is is to reduce: (1) the occurrence and/or severity of uraemic bone disease; and, (2) cardiovascular morbidity and mortality caused by elevated serum levels of PTH and high phosphate levels and calcium overload.
Key Point: Treatment includes control of phosphate retention, maintaining serum calcium concentration within the normal range and prevention of excess PTH secretion. However the evidence base within this area is poor and the monitoring and interventions are based on observational associations and our understanding of pathophysiology. Few high quality randomised controlled trials have been performed that show a benefit of intervention.
Key Points: Renal Association Treatment Guidelines
- The UK Renal Association recommends measuring serum calcium, phosphate and PTH levels when GFR is < 60ml/min/1.73m 2 (CKD stage 3 and above). It also recommends, in dialysis patients:
- Serum calcium, should be maintained within the normal range and be between 2.2 and 2.5 mmol/L, with avoidance of hypercalcaemic episodes
- Serum phosphate should be maintained between 1.1 and 1.7 mmol/L
- The target range for parathyroid hormone (measured using an intact PTH assay) should be 2-9 times the upper limit of normal for the assay used
Dietary Phosphate Restriction
Restricting phosphate intake can be achieved by reducing intake of dairy products, certain vegetables, and colas. Many physicians recommend phosphate restriction (to 800-1000 mg daily) when the serum phosphate level is > 1.8 mmol/L in patients with stage 5 CKD. Phosphate restriction can be challenging for the patient, and is best delivered by an individualised prescription designed and implemented by renal dieticians.
Phosphate Binders are long established in the management of hyperphosphataemia in patients with CKD-MBD. The figure below shows the site of action of phosphate binders and also the target for calcitriol or active vitamin D analogs (discussed in detail under other treatments).
Figure 10: Therapeutic targets for phosphate binders and calcitriol
Aluminium-Containing Phosphate Binders
Aluminium hydroxide was first used as a phosphate binder by Freeman and Freeman in 1941. Aluminium-containing phosphate binders were, for many years, the phosphate binders of choice, forming insoluble and nonabsorbable aluminium phosphate precipitates in the intestinal lumen. They are still the most effective phosphate binders but due to their renal elimination, patients with impaired renal function have a gradual tissue accumulation of absorbed aluminium with resultant toxicity (Ritz, 2004). The major manifestations of aluminium toxicity areinskeletal muscle, CNS and bone (leading to low-bone turnover osteomalacia, adynamic bone disease). Clinically, refractory microcytic anaemia, bone and muscle pain, and dementia develop (Salusky, 1991).
However, some physicians still prescribe low doses of aluminium hydroxide. The K/DOQI guidelines recommend the use of aluminium-containing phosphate binders only in patients with serum phosphorus levels >7.0 mg/dL (2.26 mmol/L). If used, one course of short-term therapy (4 weeks) is recommended; then replaced by another phosphate binder.
Calcium-Containing Phosphate Binders
Problems with aluminium-containing phosphate binders led to the development ofcalcium salts to bind intestinal phosphate. In addition to lowering plasma phosphate concentration, absorption of somecalcium canraise plasma calcium concentration, providing an additional mechanism by which PTH secretion can be reduced.
Calcium carbonate has been the most widely used calcium salt. However calcium acetate is a more efficient phosphate binder than calcium carbonate, as it dissolves in both acid and alkaline environments; whilst calcium carbonate dissolves only in acid pH and many patients with advanced renal failure have achlorhydria or are taking H2-blockers or proton pump inhibitors (Emmett, 2004).
The required dose of calcium carbonate to control phosphate level ranges from 6 gm up to 15 gm/day (40 % of which is elemental calcium). Calcium salts are most effective if taken with meals, because they bind dietary phosphate - and therefore, leave less free calcium available for absorption.
However calcium salts in patients with ESRD can lead to hypercalceamia and metastatic calcification including coronary artery calcification, which are in turn is associated with cardiovascular mortality. To avoid metastatic calcification, K/DOQI guidelines suggest that the total dose of elemental calcium (including dietary sources) should not exceed 2000 mg/day. The identification of problems associated with calcium overload - and a total suggested calcium intake well below the level required for effective phosphate control - has led to a new generation of phosphate binders being increasingly used in clinical practice.
Non-Calcium, Non-Aluminium Phosphate Binders
This wasthe first synthetic non-calcium, non-aluminum phosphate binder to become widely available in the USA and Europe for the treatment of hyperphosphataemia in patients with CKD (Hutchinson, 2004). This cross linked poly allylamine hydrochloride exchange resin binds phosphate and releases chloride.
While the potency of sevelamer is low when compared with aluminium, beneficial effects of this drug include attenuation of the progression of coronary and aortic calcification, seen withcalcium-containing phosphate binders. Also, a significant improvement in lipid profile, with reduction intotal and low-density lipoprotein (LDL) cholesterol, has been noted with sevelamer(Sturtevant, 2004).
Sevelamer hydrochloride causes metabolic acidosis, which can exacerbate secondary hyperparathyroidism and renal osteodystrophy. Each 800 mg tablet of sevelamer hydrochloride could theoretically lead to an acid load equivalent to 4 mEq hydrochloric acid (Brezina, 2004). More recently, Sevelamar Carbonate has been introduced to avoid this problem with metabolic acidosis.
Sevelamer hydrochloride is considered a moderate phosphate binder because its optimal phosphate binding capacity occurs at a pH of 7; whereas the pH of the stomach and first part of the duodenum is much lower than this level. Also, high doses of this drug may reduce the absorption of vitamin D from the gut. Additionally, sevelamer hydrochloride is not selective for phosphate ions only, as it can bind other negatively charged ions such as chloride and bicarbonate. The dose range of 2.4g to 4.8g daily provides effective phosphate control without hypercalcaemia.
This is amore recent non-calcium, non-aluminium phosphate binder. It is a salt of a rare earth metal, and isa highly effective phosphate binder (Hutchison, 2004). It is minimally absorbed from the gastrointestinal tract and is not excreted by the kidneys (D'Haese, 2003). Lanthanum carbonate has been shown to be as effective as aluminium in binding phosphate but without the associated toxic effects.
It has minimal tissue accumulation when compared to aluminium, but long-term toxicity with bone accumulation cannot be ruled out. A dose between 1350 and 2250mg dailyis effective in reducing and maintaining phosphate levels in most patients. And the effect is seen within three weeks of treatment. Also, the incidence of hypocalcaemia is lower, and the calcium x phosphorus product reduced, when compared to calcium carbonate (De Broe, 2004).
Both calcium-based phosphate binders and other non-calcium, non-aluminium containing phosphate-binding agents are effective in lowering serum phosphate levels and may be used as the primary therapy.
Other Phosphate Binders
Magnesium salts may be used as an alternative to calcium-containing phosphate binders in patients who develop hypercalcemia. Magnesium carbonate reduces PTH and phosphorus levels when used with magnesium-free dialysate (O'Donovan, 1986).
Summary of Phosphate Binders
|Calcium or Aluminium Containing||Non-Calcium, Non-Aluminium Containing|
|Aluminium Hydroxide||Lanthanum Carbonate|
|Calcium Acetate||Magnesium Carbonate|
|Calcium Carbonate||Sevelamar Hydrochloride and Carbonate|
Correction of Metabolic Acidosis
Correction of metabolic acidosis may be useful because studies of alkali therapy in patients who are not in renal failure suggest an improvement in bone parameters (Domrongkitchaiporn, 2002). Current dialysis prescriptions are tailored so that patients do not sustain significant acidosis.
Calcitriol is the most active metabolite of vitamin D and direct effects on the parathyroid gland by suppressing the synthesis and secretion of PTH and limiting parathyroid cell growth. It was firstshown to be effective by Berl in 1978.
It can be administered orally and intravenously for the treatment of secondary hyperparathyroidism. It may cause hyperphosphataemia and hypercalcaemia by increasing absorption of both calcium and phosphate. Intravenous 'pulse' therapy has a greater effect in reducing bone turnover by diminishing the number of active ostoblasts rather than the reduction in PTH levels. There is indirect evidence that pulse therapy with calcitriol, combined with use of calcium-containing phosphate binding agents, increases the prevalence of adynamic bone disease (Coburn, 2003).
Due to the potential of calcitriol to cause hyperphosphataemia, hypercalcaemia and an increase in calcium x phosphate product, new vitamin D analogues have been developed. These analogues are relatively selective for theparathyroid gland with lesser effect on intestinal absorption of calcium and phosphate. 22-oxacalcitriol has a decreased affinity for vitamin D binding protein and a short plasma half-life - resulting in rapid clearance from the circulation. This may be the mechanism for its lesser effect on calcium and phosphate levels. It also decreases PTH secretion without affecting bone turnover (Tsukamoto, 2000).
The effects of calcitriol and 22-oxacalcitriolon serum calcium and phosphate levels are similar, andthe suppressive effects of these drugs on PTH secretion are not significantly different (Hayashi, 2004).
Paricalcitol adequately controls PTH secretion with minimal hypercalcaemia and hyperphosphataemia compared to calcitriol (Llach, 2001). Also, doxercalciferol has the same suppressive effect on PTH with minimal changes in serum calcium and phosphate levels (Salusky, 2005). 22-oxacalcitriol and doxercalciferol are not yet available in the UK.
The American Food and Drug Administration (FDA) has indicated that there is no difference in the ability of intravenous paricalcitol and calcitriol to suppress PTH, and control calcium and phosphate levels, in paediatric haemodialysis patients (Cunningham, 2004). Of interest, other studies have suggested that none of the newer vitamin D analogues is superior to calcitriol or alfacalcidol in suppressing PTH; or controlling calcium, phosphate, or calcium x phosphate product.
For stages 3 and 4 CKD the efficacy of paricalcitol had been assessed in a randomised controlled trial with cardiovascular surrogates for long-term outcome as the study end-points. This showedno difference in left ventricular mass index or left ventricular diastolic dysfunction for patients who received paracalcitol compared to patients who received treatment with placebo (Thadani et al, 2012).
According to KDIGO guidelines, both haemodialysis and peritoneal dialysis patients with serum PTH levels > 300 pg/ml (31.9 pmol/L), should receive an active vitamin D sterol such as calcitriol, alfacalcidol, paricalcitol, or doxercalciferol to reduce the serum levels of PTH to the target range.
Therapy with calcimimetics is another, more recent, approach for the treatment of secondary hyperparathyroidism. These agents act at the level of the CaR, which is found in the parathyroid and C thyroid glands as well as renal tubular cells (Urena, 2003). Activation of this receptor by calcimimetics increases intracellular calcium concentration, which causes rapid reduction in PTH secretion, serum phosphate, and the calcium x phosphate product, which remain suppressed for up to three years (Block, 2003).
Cinacalcet is a CaR sensitiser which is usually used in dialysis patients when parathyroid hormone exceeds85 pmol/L, or at lower levels with hypercalcaemia, despite conventional treatment. Doses are titrated from 30 mg to 180 mg daily. Cinacalcet has the advantage of lowering levels of parathyroid hormone, and serum calcium and phosphate ; whereas vitamin D tends to increase serum calcium. It should be taken at the same hour every day to avoid overdose and adverse effects.
The patient should be started with a low dose, which is increased progressively every two weeks until target PTH levels are achieved. The serum PTH, phosphate and calcium levels should be checked regularly. Transient hypocalcaemia may occur which can be corrected by increasing the dialysate calcium levels, ordose of calcium-containing phosphate binders and vitamin D (Block, 2004).
Transient episodes of nausea, vomiting have occurred in patients who were treated with cinacalcet. Also, upper respiratory tract infections, hypotension, diarrhoea and headache have been observed(Lindberg, 2005).
There was great interest in the renal community in the results of a large randomised trial of cinacalcet in patients requiring chronic dialysis treatment. The study reported recently and showed that cinacalcet did not significantly reduce the risk of death or major cardiovascular events in patients wtih hyperparathyroidism who were undergoing chronic dialysis treatment (EVOLVE, 2012).
The need for surgical parathyroidectomy in patients with secondary hyperparathyroidism has decreased significantly in recent years. This is due to increased efficacy of medical measures that can suppress parathyroid hormone (PTH) secretion, especially vitamin D (Kestenbaum, 2004).
Parathyroid sestamibi scan (with technetium Tc 99m-MIBI) demonstrating uptake in all 4 glands consistent with 4-gland hyperplasia
The main indications for PTx include: therapy-resistant hypercalcaemia and/or hyperphosphatemia in the presence of a high PTH level; pruritus that does not respond to medical or dialysis therapy; progressive extra-skeletal calcification or calciphylaxis; unexplained symptomatic myopathy; and in renal transplant recipients with persistent hyperparathyroidism and hypercalcaemia.
Indications for Parathyroidectomy = High PTH levels +
- Uncontrollable hyercalcaemia
- Uncontrollable hyperphosphataemia
- Unresponsive to phosphate binders and Vitamin D
- Metastatic calcification
- Potential renal transplant recipient
There are two main types of parathyroidectomy: subtotal PTx; or total PTx with or without re-implantation of parathyroid tissue in the forearm. The latter is considered the procedure of choice in patients with metastatic calcification. Subtotal or total PTx with re-implantationis performed to avoid hypoparathyroidism, particularly after renal transplantataion.
Dialysis dose is one of the major determinants of phosphate levels and hyperparathyroidism in patients with stage 5 CKD requiring chronic dialysis therapy. High phosphate levels are associated with inadequate dialysis dosing. Indeed, in patients who have daily or extended nocturnal dialysis, phosphate control is usually excellent; and there is less requirment for dietary phosphate restriction or the use of phosphate binders and vitamin D analogues.
Top Tip: Utilise dietary phosphate restriction, a phosphate binder and calcitriol in all ESRD patients. Tailor dosing to guideline targets.
- Secondary hyperparathyroidism occurs as a result of hyperphosphataemia, hypocalcaemia and impaired synthesis of renal vitamin D with reduction in serum calcitriol levels
- Patients with secondary hyperparathyroidism have a range of symptoms
- The aim of treatment is is to reduce: (1) the occurrence and/or severity of uraemic bone disease; and, (2) cardiovascular morbidity and mortality caused by elevated serum levels of PTH and 'calcium x phosphate' product
- Treatment includes control of phosphate retention, maintaining serum calcium concentration within the normal range and prevention of excess PTH secretion
- The UK Renal Association recommends measuring serum calcium, phosphate and PTH levels when GFR is < 60ml/min/1.73m 2 (CKD stage 3 and above). Italso recommends, in dialysis patients:
- Serum calcium, should be maintained within the normal range and be between 2.2 and 2.5 mmol/L, with avoidance of hypercalcaemic episodes
- Serum phosphate should be maintained between 1.1 and 1.7 mmol/L
- The target range for parathyroid hormone (measured using an intact PTH assay) should be 2-9 times the upper limit of normal for the assay used
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This is a very good summary of phosphate binding agents, and worth reading
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Good practical guide to prescribing in CKD-MBD
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