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Bone metabolism and disease in chronic kidney disease
Protein Interactions. Protein Families. Nucleotide Sequences. Functional Genomics Experiments. Protein Structures. Gene Ontology GO Terms. Data Citations. Proteomics Data. Menu Formats. Full Text. Eckardt et al. Richalet et al. Levine and Stray-Gundersen, Chapman et al. Casas H. Stray-Gundersen et al.
Ge et al. Julian et al. Gore et al. Wilber et al. Richalet and Gore, Ramos-Campo et al. Hauser et al. Bailey et al. Burtscher et al. Panisello et al. Better physical fitness, metabolic risk markers, and body composition. Wiesner et al. Male Wistar rats hyperlipidemia induced by 8 wk high-fat diet. Ischemia: 30 min LAD occlusion followed by min of reperfusion. Li et al. Marticorena, Asemu et al.
Marticorena et al. Reynafarje and Marticorena, Chouabe et al. Valle et al. Korcarz et al. Kusunose et al. Turban and Spengler, Simon et al. Allegra et al. Christie et al. Cogo et al. Gourgoulianis et al. Harrison et al. Yangzong et al. Karagiannidis et al. Beneficial effect, in particular in steroid-dependent patients. Schultze-Werninghaus, Droma et al. Kiechl-Kohlendorfer et al. Sy et al. Beneficial effects beyond the effects of allergen avoidance. Ohta et al. Fourth, a very high fractional excretion of phosphate is present in patients with CKD, even after total parathyroidectomy.
The latter may occur if the adaptive response of the parathyroid glands is exaggerated, as in the case of nutritionally induced secondary hyperparathyroidism in horses. However, available data show that the mean levels of both serum phosphorus and calcium in most patients with moderate loss of kidney function are actually lower than the values in normal subjects.
These observations cannot be explained by the phosphate retention theory alone. Thus, other factors must also be operative and contribute to the genesis of the hypocalcemia in the early course of CKD. These considerations do not necessarily mean that phosphate retention is not an important factor in the pathogenesis of the hypocalcemia and secondary hyperparathyroidism of CKD. Rather, they suggest that phosphate retention in the course of CKD may contribute to the hypocalcemia by mechanisms other than a direct effect of hyperphosphatemia on serum calcium. It should be mentioned that with more advanced loss of kidney function Stages 4 and 5 when hyperphosphatemia develops, the elevated blood levels of phosphorus may suppress blood levels of calcium and contribute to the hypocalcemia.
Hyperphosphatemia has a direct effect on post-transcriptional mechanisms that increase PTH synthesis and secretion. Role of skeletal resistance to the calcemic action of PTH. Skeletal resistance to the calcemic action of PTH occurs in patients with acute kidney failure as well. Hypocalcemia is almost always observed in these patients.
The degree of hypocalcemia is moderate to marked range, 7. The hypocalcemia occurs early in the course of the oliguric phase of the disease and persists through the diuretic period. This hypocalcemia is observed in patients with low, normal, or elevated serum concentrations of phosphorus, indicating that the hyperphosphatemia of acute kidney failure is not the major determinant of the hypocalcemia. Also, the hypocalcemia cannot be attributed to a failure in the function of the parathyroid glands because the blood levels of PTH are elevated and display an inverse correlation to the concentrations of serum calcium.
Further, the infusion of PTH fails to elicit a normal rise in serum calcium. All these derangements are reversed after the return of kidney function to normal. These observations indicate that there is a skeletal resistance to the calcium-mobilizing action of PTH, an abnormality that occurs early in the course of both acute and chronic kidney disease and is not reversed by hemodialysis.
This derangement is an important factor contributing to the hypocalcemia in kidney disease and to the pathogenesis of secondary hyperparathyroidism in these patients. A series of studies in thyroparathyroidectomized dogs with diverse models of acute kidney failure bilateral ureteral ligation, bilateral nephrectomy, or diversion of both ureters into the jugular veins has demonstrated that the skeletal resistance to the calcemic action of PTH is partially due to a deficiency of 1,25 OH 2D3 and its complete correction requires adequate amounts of both 1,25 OH 2D3 and 24,25 OH 2D3.
Other studies suggest that the skeletal resistance to the calcemic action of PTH is, at least in part, due to downregulation of PTH receptors. Indeed, several studies have shown that the PTH-PTHrP receptors are downregulated in many organs in uremia; these include the kidney, liver, and heart. Indeed, in kidney failure the basal levels of cytosolic calcium is elevated in all organs, and the correction of this abnormality by treatment with calcium channel blockers is associated with reversal of the downregulation of the PTH-PTHrP receptors.
Role of altered vitamin D metabolism. As mentioned earlier, the number of VDRs decreases as the loss of kidney function progresses, resulting in resistance to vitamin D action. It is intriguing that, despite the presence of adequate functioning kidney mass in patients with moderate reduction in kidney function Stage 2 , the production of l,25 OH 2D3 does not increase adequately to meet the needs of the target organs for vitamin D. The mechanisms through which dietary phosphate restriction in patients with Stage 2 CKD is associated with increased production of l,25 OH 2D3 are not evident.
This effect does not seem to be mediated by changes in the serum levels of phosphorus because no significant changes in this parameter were found in adults. Indeed, studies in rats have shown that the level of inorganic phosphorus in the kidney cell is reduced during the feeding of a phosphate-restricted diet. Interaction between 1,25 OH 2D3 and parathyroid glands. Available data indicate that l,25 OH 2D3 may have a direct effect on the parathyroid glands.
First, exposure to 1,25 OH 2D3 both in vivo and in vitro may directly suppress the activity of the parathyroid glands. Second, l,25 OH 2D3 renders the parathyroid glands more susceptible to the suppressive action of calcium. Such an effect of l,25 OH 2D3 may correct the abnormal shift in set point for calcium of the parathyroid glands in patients with CKD. Thus, it is possible that deficiency of 1,25 OH 2D3 may initiate secondary hyperparathyroidism even in the absence of overt hypocalcemia; this has been demonstrated in dogs with reduced kidney function.
Regulation of the parathyroid hormone gene by vitamin D, calcium, and phosphorus. Secondary hyperparathyroidism in CKD is due to increased synthesis and secretion of PTH secondary to an increase in PTH gene expression and parathyroid cell proliferation. Thus, the effects of hypercalcemia and hypophosphatemia on PTH synthesis is post-transcriptional. It appears that phosphate retention, which may develop with loss of kidney function, interferes with the ability of patients with CKD to augment the production of 1,25 OH 2D3 by the kidneys to meet the increased need for this metabolite.
Thus, a state of absolute or relative vitamin D deficiency develops, leading to defective intestinal absorption of calcium and impaired calcemic response to PTH. These 2 abnormalities produce hypocalcemia which in turn causes secondary hyperparathyroidism. Although this formulation still assigns an important role to phosphate retention in the genesis of secondary hyperparathyroidism in CKD, the pathway through which such phosphate retention mediates its effect is different from that originally proposed.
The original theory maintained that phosphate retention in the early course of CKD is associated with a rise in levels of serum phosphorus and a consequent fall in the levels of serum ionized calcium, which in turn stimulates the parathyroid gland activity. However, it must be emphasized that if marked hyperphosphatemia does develop in a patient with CKD, it could directly lower the level of serum calcium and contribute to the severity of the hypocalcemia and the secondary hyperparathyroidism.
In addition, hyperphosphatemia per se may stimulate parathyroid hormone synthesis by a post-transcriptional effect on PTH gene expression. An Na-P cotransporter is present in the parathyroid gland, and this transporter may play a role in the process that allows the parathyroid gland to sense the level of extracellular phosphorus.
An additional pathway through which an absolute or relative deficiency of 1,25 OH 2D3, independent of hypocalcemia, may mediate second- S33 ary hyperparathyroidism is related to its direct effect on the parathyroid glands, as discussed earlier. This integrated formulation for the pathogenesis of secondary hyperparathyroidism has important clinical implications. It is consistent with the hypothesis that dietary phosphate restriction in proportion to the fall in GFR in patients with CKD is adequate to reverse and correct secondary hyperparathyroidism and other abnormalities in mineral metabolism.
However, achieving the proper and adequate dietary phosphate restriction and successful patient compliance with the dietary regimen may prove difficult. Because the available data indicate that dietary phosphate restriction exerts its effect through the increased production of 1,25 OH 2D3 and because this vitamin D metabolite also exerts a direct effect on the parathyroid glands, an alternative therapeutic approach would be supplementation of 1,25 OH 2D3. Indeed, treatment of patients with Stage 3 CKD with 1,25 OH 2D3 for 12 months was associated with improvement or normalization of the disturbances in mineral metabolism, including secondary hyperparathyroidism and bone disease.
Structure and Function of the Parathyroid Glands Hyperplasia of the parathyroid glands is almost always present in patients with CKD, but the increase in volume and mass of the glands varies among patients and among the 4 glands in the same patient. The size of the glands may reach 10 to 50 times normal. Occasionally, the parathyroid glands may be of normal size in patients with CKD. Histologically, the glands show chief cell hyperplasia with or without oxyphil cell hyperplasia.
Nodular or adenomatous-like masses may be found within the hyperplastic glands. These nodules are well circumscribed and surrounded by a fibrous capsule. The cells in the nodular hyperplasia have less VDR and calcium-sensing receptor CaR density and a higher proliferative potential than the cells of diffuse hyperplasia. The change in the structure of the parathyroid glands begins as polyclonal diffuse hyperplasia.
Pathogenesis of abnormalities in mineral metabolism and bone disease in CKD. CaR start to proliferate monoclonally early nodularity in diffuse hyperplasia and form nodules. Several monoclonal nodules of different size may develop resulting in multinodular hyperplasia. Alternatively, the cells of 1 of the nodules may proliferate faster and more vigorously giv- ing rise to a very large nodule that almost occupies the entire gland single nodular gland. Molecular changes are implicated in the tumorigenesis of the parathyroid gland in CKD.
Hypocalcemia, relative or absolute deficiency of 1,25 OH 2D3, and phosphate retention or hyperphosphatemia are the most important factors responsible for the hyperplasia of the parathyroid glands. Because hypocalcemia and relative or absolute deficiency of 1,25 OH 2D3 vitamin D resistance may develop early in the course of CKD, hyperactivity of the parathyroid glands is also encountered in the early stages of kidney disease. The appearance of spontaneous and persistent hypercalcemia in some uremic patients Stage 4 and 5 CKD has led to the suggestion that the parathyroid glands in these patients may ultimately become autonomous.
However, after calcium infusion, the blood levels of PTH of these patients invariably fall, but not to normal levels. Thus, the parathyroid glands in these patients are suppressible at higher levels of serum calcium. The appearance of spontaneous hypercalcemia and the failure of the blood levels of PTH to fall to normal values after calcium infusion in uremic patients Stage 4 and 5 CKD are most likely due to the large mass of the parathyroid glands in these individuals.
Malregulation of PTH release at the cellular level may also be present. Indeed, in vitro studies of dispersed cells from the parathyroid glands of such patients show that a higher concentration of calcium is required to achieve a suppression of PTH secretion. This has been interpreted to indicate a shift in the set point for calcium; this abnormality is at least in part due to deficiency in 1,25 OH 2D3. True adenomas may develop and function autonomously in certain cases of secondary hyperparathyroidism, but such cases are not common. The use of the term tertiary hyperparathyroidism should be limited to those cases in which it is documented that a true adenoma has developed in a previously hyperplastic gland.
Multiple factors control the release of PTH from the glands, and they do so by inducing changes in the cellular function of the parathyroid glands. Calcium is the most important regulator of PTH secretion, and its effect is mediated by changes in intracellular concentration of cal- S35 cium. The CaR is located in the membrane of the cells of the parathyroid glands.
This receptor protein plays an important role in the ability of parathyroid glands to recognize changes in the concentration of calcium ion in the blood and as such, CaR mediates the effect of calcium on the secretion of PTH from the parathyroid glands. The levels of serum calcium and l,25 OH 2D3 as well as dietary phosphate do not appear to regulate the synthesis of CaR.
The relationship between the serum levels of calcium and the parathyroid gland in the modulation of PTH secretion is altered in CKD patients. In normal subjects, this relationship is sigmoidal over a narrow range of calcium concentration, but in patients with CKD, higher levels of serum calcium are needed to suppress the secretion of PTH compared to normal subjects.
Also, in CKD patients, the susceptibility of parathyroid adenyl cyclase to the inhibitory effect of calcium is reduced. Such an effect would impair the ability of calcium to inhibit PTH secretion. These abnormalities in calcium and PTH secretion could be evaluated by the changes in set-point for calcium. The latter is defined as the calcium concentration that produces half the maximal inhibition of PTH and that is the midpoint between the maximal and minimal PTH secretions. Administration of 1,25 OH 2D3 to dialysis patients was associated with suppression of PTH secretion and with a shift of the set-point to the left, supporting the hypothesis that deficiency of this vitamin D metabolite plays an important role in the genesis of secondary hyperparathyroidism in CKD.
Thus, the available evidence suggests that in patients with CKD, the structural changes in the parathyroid glands increase in their mass due to diffuse and nodular hyperplasia and its func- S36 tional abnormality shift in set point of calcium to the right are responsible to the increase production and release of PTH. After its secretion from the parathyroid gland, intact PTH is cleaved by the liver into an N- and a C-terminal fragment. The half-life of both the intact hormone and its N-terminal fragment is short about 5 minutes , whereas that of the C-terminal fragment is much longer.
Both the hepatic removal of the intact hormone and the kidney clearance of the C-terminal fragment are impaired. The major component of the elevated blood levels of the immunoreactive PTH in these patients is the C-terminal fragments and particularly the midmolecule or midregion of C-terminal fragments.
Hyperplasia of the parathyroid glands in patients with CKD is not easily reversed, even after the correction of its causes. Some investigators found that parathyroid gland hyperplasia regresses in all patients in whom PTH secretion was successfully suppressed. The mechanisms underlying this regression are not well understood. Apoptosis has been proposed, and certain in vitro studies indicate that very high concentrations of 1,25 OH 2D3 induce apoptosis of parathyroid gland cells. Such an effect may occur in vivo as well. In some patients, spontaneous hemorrhage in the hyperplastic glands occurs and may be responsible for the regression of the hyperplastic glands in occasional cases.
Hyperphosphatemia Although phosphate retention occurs early in the course of CKD Stage 2 , hyperphosphatemia becomes evident in patients with marked loss of kidney function Stage 4. Several factors may affect the level of serum phosphorus in patients with CKD Table 3. The dietary intake of phosphate and the fraction of the ingested phosphate absorbed by the intestine have an important effect on the serum levels of phosphorus in patients with CKD.
These patients have only mild impairment in intestinal absorption of phosphate, but their kidneys are unable to adequately handle phosphate loads. Intestinal absorption of phosphate is enhanced by 1,25 OH 2D3, and its administration to patients in Stages 4 and 5 of CKD may produce or worsen hyperphosphatemia. In patients who have substantial osteomalacia, the levels of serum phosphorus may remain unchanged or even fall during therapy with 1,25 OH 2D3.
Phosphate-binding compounds render dietary phosphate and phosphate contained in swallowed saliva and intestinal secretions unabsorbable. Thus, patients receiving these compounds may have normal levels of serum phosphorus or develop modest hypophosphatemia. It should be emphasized that these compounds are most effective when dietary intake of phosphate is below 1. With higher phosphate intake more than 2. An important factor determining the level of serum phosphorus in Stage 4 and 5 CKD is the degree of hypersecretion of PTH and the response of the skeleton to the high levels of this hormone.
Normally, PTH decreases the tubular reabsorption of phosphate by the kidney, increases urinary phosphate excretion, and consequently maintains serum phosphorus levels. In such patients, the severely damaged kidneys cannot respond to further increments in PTH with additional augmentation in phosphate excretion. The enhanced bone resorption, which is induced by the high levels of PTH, liberates calcium and phosphorus from the skeleton into the extracellular fluid.
This phosphorus cannot be excreted by the kidney and hence serum phosphorus concentration rises. The same phenomenon occurs in dialysis patients. Several clinical observations support this view. First, the levels of serum calcium and phosphorus are higher in patients with advanced kidney failure Stage 5 and severe secondary hyperparathyroidism than in other patients with comparable kidney failure but without severe hyperparathyroidism. Second, following total or subtotal parathyroidectomy in patients with kidney failure and severe secondary hyperparathyroidism, the serum concentrations of calcium and phosphorus fall Fig 2.
Third, when patients with chronic kidney disease and overt secondary hyperparathyroidism are treated with hemodialysis, the serum phosphorus levels not only may remain above normal but may rebound rapidly after dialysis to predialysis levels. A shift in the balance between protein synthesis and breakdown toward catabolism, as occurs S37 Fig 2.
Changes in total serum calcium and inorganic phosphorus observed in 11 uremic patients before and after subtotal parathyroidectomy. Reproduced with permission. The parenteral administration of solutions containing large quantities of glucose and amino acids to such patients cause an abrupt reduction in serum phosphorus levels. Also, the concentration of serum phosphorus may fall during refeeding after a period of calorie or protein malnutrition. The use of calcium compounds in patients with Stage 4 and 5 CKD results in the reduction in the serum levels of phosphorus due to the ability of these compounds to bind phosphate in the intestine.
In addition, these calcium compounds cause a rise in serum calcium levels which would inhibit the parathyroid gland and results in a fall in blood PTH levels. This would be followed by a reduction in serum levels of serum phosphorus as discussed above. In S38 anephric patients and in those treated with dialysis, the blood levels of 1,25 OH 2D3 are usually undetectable. Thus, in such patients, there is vitamin D deficiency and vitamin D resistance as well. Low levels of 25 OH D3 may be encountered in patients who have nephrotic-range proteinuria due to loss of 25 OH D3 in urine, those who are treated with peritoneal dialysis due to loss of 25 OH D3in peritoneal fluid, or those who have nutritional vitamin D deficiency.
The biological consequences of vitamin D deficiency are multiple and are manifested by disturbances in the function of its target organs: parathyroid glands, bone, intestine, and skeletal muscle Table 4. Because other organs such as testes, myocardium, and pancreas have receptors for 1,25 OH 2D3, it is possible that a deficiency of this vitamin D metabolite plays a role in the dysfunction of these organs in kidney failure. The action of vitamin D is mediated by its binding to its cytosolic receptor, VDR. Both of these events, in addition to the reduced number of VDR, are responsible for the vitamin D-resistant state of severe kidney dysfunction Stages 4 and 5.
Bone Disease The nature and type of bone disease that develops in CKD may vary from one patient to another. Multiple reasons may account for these variations Table 5. The 2 major types of bone disease that are commonly encountered in patients with CKD are enhanced bone resorption osteitis fibrosa and adynamic bone disease. Some patients may have 1 of these types predominantly, whereas others may have a mixed type of bone disease. Mild forms of these derangements in bone metabolism may be observed in the early stages of CKD Stage 2 and they become more severe as kidney function deteriorates.
Osteosclerosis may also occur, and osteoporosis may be encountered. Bone lesion of excess PTH high-turnover bone disease. The elevated blood levels of PTH are responsible for the enhanced number and activity of osteoclasts leading to increased bone resorption. As this process increases in severity, marked fibrosis involving the marrow space develops, with the histological picture of osteitis fibrosa becoming evident. In this condition, there is also increased bone formation as evidenced by increased amounts of osteoid.
This osteitis fibrosa is a high-turnover bone disease.
The manifestations of excess PTH in the bone of uremic patients include increased numbers of osteoclasts and osteoblasts, osteoclastic bone resorption, enlarged haversian lacunae, endosteal fibrosis, and accumulation of woven osteoid and woven bone. Bone lesion of defective mineralization. Defective mineralization of osteoid leads to rickets in children and osteomalacia in adults.
Histologically, osteomalacia can be accurately diagnosed only by the evaluation of undecalcified bone specimens. Osteomalacia is due to a delay in the rate of bone mineralization resulting in accumulation of excess unmineralized osteoid. However, it must be emphasized that the presence of excess osteoid does not necessarily mean osteomalacia. Excess osteoid may be a secondary to abnormalities in normal mineralization osteomalacia ; or b caused by an increased rate of synthesis of bone collagen, which exceeds normal mineralization.
The use of double tetracycline labeling can differentiate between these 2 possibilities and is thus critical for the diagnosis of osteomalacia. The skeleton in osteomalacia is weakened, and patients with this bone disease have skeletal deformities, bone pain, fractures, and musculoskeletal disabilities.
Several mechanisms may underlie the defective mineralization of osteoid and hence the development of osteomalacia in CKD patients. The most important factor in the development of osteomalacia is aluminum overload. This latter effect is the result of the action of vitamin D on intestinal absorption of these minerals. It is not evident whether a deficiency in one or more of the vitamin D metabolites is critical. For example, few anephric patients with undetectable blood levels of 1,25 OH 2D3 did not show histological evidence of osteomalacia.
On the other hand, long-term therapy with 1,25 OH 2D3 improved or healed osteomalacia in many patients with advanced CKD. Osteomalacia may be more frequently encountered in uremic patients with low blood levels of 25 OH D3. Second, abnormalities in the formation and maturation of collagen have been found in rats with experimental uremia and in patients with advanced CKD.
These derangements result in a defect in collagen cross-linking and may affect bone mineralization. These abnormalities in collagen metabolism are most likely due to vitamin D deficiency. Indeed, treatment with 25 OH D3 reversed these defects.
Third, inhibition of maturation of amorphous calcium phosphate to its crystalline phase is another defect participating in the genesis of the osteomalacia. The magnesium content of the bones of these patients is increased, and this may interfere with the process of normal mineraliza- S40 tion. Magnesium stabilizes the amorphous calcium phosphate and inhibits its transformation into hydroxyapatite. The bone content of pyrophosphate is also increased in these patients, and pyrophosphate may inhibit mineralization. Fourth, aluminum toxicity may be responsible for a certain type of mineralization defect that is resistant to vitamin D therapy.
This type of bone disease has been called low-turnover bone disease or low-turnover osteomalacia. This is mainly seen in dialysis patients who have a large content of aluminum in bone and in whom the aluminum is localized in the mineralization front ie, the limit between osteoid and calcified tissue. With a decrease in the use of aluminum-containing compounds for the control of hyperphosphatemia, the incidence and prevalence of osteomalacia have been decreasing.
Increased burden of iron, alone or in combination with aluminum, can cause osteomalacia in kidney failure patients. Adynamic bone disease. The exact mechanisms underlying adynamic bone disease ABD are not fully elucidated. It is seen in kidney failure patients before and after treatment with peritoneal dialysis or hemodialysis. This entity is characterized by a defect in bone matrix formation and mineralization, increased osteoid thickness, and a decrease in the number of both osteoblast and osteoclast on bone surfaces.
There are no excessive amounts of aluminum in the mineralization front. ABD is also encountered after parathyroidectomy, in CKD patients with diabetes, and in those with increased aluminum burden; in all these clinical settings, the blood levels of PTH are low. Patients with ABD have increased rates of overt fractures and microfractures. The latter causes bone pain. Calcium uptake by the adynamic bone is reduced, and therefore patients with ABD may develop hypercalcemia if calcium intake is increased or if dialysate calcium is high.
Osteosclerosis and osteoporosis. Osteosclerosis appears as increased bone density in roentgenographic studies. Histologically, osteosclerosis is most likely due to accumulation of unmineralized trabecular bone with an increase in total bone mass. Because osteosclerosis affects trabecular bone, it is most evident in the vertebrae, pelvis, ribs, clavicles, and metaphyses of long bones, which are made predominantly of cancellous trabecular bone.
In patients with osteosclerosis, no correlation is found between the bone lesion and any specific pattern of change in serum levels of calcium, phosphorus, or alkaline phosphatase. Certain experimental and clinical evidence suggests that osteosclerosis could be induced by excess PTH. Indeed, patients with primary hyperparathyroidism may display radiographic evidence of osteosclerosis. Osteoporosis is defined as a decrease in the mass of normally mineralized bone.
Immobilization, calcium deficiency per se, and chronic protein depletion may be causes of the osteoporotic component of kidney osteodystrophy. In patients older than 50 years, factors that cause postmenopausal, idiopathic, or senile osteoporosis may contribute to the skeletal abnormalities of CKD. Role of Acidosis in Bone Disease Acute acidosis produces a significant loss of the acid-soluble calcium carbonate from bone and is usually associated with negative calcium balance.
Rats fed a diet rendering them permanently acidotic were found to have less calcified bone than control animals, despite adequate intake of calcium. Patients with CKD show a persistent positive retention of hydrogen ion which is partially buffered by bone. Indeed, evaluation of the composition of bone in CKD reveals a loss of calcium carbonate.
These observations imply that acidosis may contribute to negative calcium balance and the development of skeletal demineralization. Although there may be a slight improvement in negative calcium balance following treatment of the chronic acidosis with alkali, a positive balance for calcium usually does not occur, and hypocalcemia, bone pain, and radiographic abnormalities are not corrected.
Moreover, there is no convincing evidence suggesting that chronic acidosis can cause defective mineralization. It appears that the chronic acidosis of CKD may not play a major role in the pathogenesis of bone disease in adult patients with CKD. An increase in the calcium-phosphorus product in the extracellular fluid is probably the most important pathogenetic factor. These breakpoints not withstanding, and because of the biological variations in range of calcium-phosphorus product over which calcification may occur and because of other contributing factors including age, it is recommended that the product be maintained below Alkalemia, which often occurs after hemodialysis, may persist during the interdialytic period and may predispose to precipitation of calcium salts in soft tissues.
An increase in local pH due to loss of CO2 from the exposed part of the eye may bring about the observed conjunctival and corneal calcification. PTH enhances movement of calcium into cells, and the state of secondary hyperparathyroidism may play an important part S41 in the genesis of soft-tissue calcification in kidney failure. Certain factor s that may act locally to inhibit calcification and are present in the blood of these patients may possibly be removed during hemodialysis. Local tissue injury may also predispose to calcification when the calciumphosphorus product is normal or only slightly elevated.
The expression of genes coding for certain proteins involved in prevention of calcification has been demonstrated in macrophages and smooth muscle cells of blood vessel walls. One of these proteins is matrix gla protein MGP. Its deficiency permits medial calcification of blood vessels. Indeed, MGP knockout homozygous mice displayed extensive and severe vascular calcification.
It is possible that downregulation of the production of this protein occurs in uremia and participates in the genesis of the vascular calcification seen in patients with kidney failure. The chemical nature of soft-tissue calcification may vary in different tissues. Thus, the calcification found in nonvisceral tissue periarticular and vascular calcification consists of hydroxyapatite, with a molar Ca:Mg:P ratio similar to that of bone. In contrast, the calcification found in visceral organs skeletal and myocardial muscle is made of amorphous CaMg 3 PO4 2 which has a much higher magnesium content.
These observations suggest that the mechanisms responsible for the calcification of various tissues in uremic patients may be different. Soft-tissue calcification constitutes a serious problem in CKD patients. These extraskeletal calcification may be localized in the arteries vascular calcification , in the eyes ocular calcification , in the visceral organs visceral calcification , around the joints periarticular calcification , and in the skin cutaneous calcification. S42 Vascular calcification. Vascular calcification is detected radiographically.
The calcification appears as a fine, granular density outlining a portion of the entire artery, giving a radiographic appearance of a pipestem due to deposition of calcium within the media and the internal elastic membrane of the artery. The lumen of the vessel is usually not involved. This medial calcification may first be seen in the dorsalis pedis as a ring or a tube as it descends between the first and second metatarsals. Calcification can also occur in atherosclerotic plaques in the intima of large vessels whose radiographic appearance is that of discrete, irregular densities.
It is possible that uremic patients are more prone to this type of calcification because of the presence of hypertension and a propensity to accelerated atherosclerosis. Arterial calcification is rare in children, uncommon between 15 and 30 years of age, and common in those older than Vascular calcifications are seen in kidney failure patients and in those treated with hemodialysis, and they persist after kidney transplantation.
In general, the reported incidence of arterial calcification increases with duration of dialysis treatment. Vascular calcification may involve almost every artery and has been seen in arteries of the forearm, wrist, hands, eyes, feet, abdominal cavity, breasts, pelvis, and brain.
The calcification may be very extensive, rendering the artery so rigid that the pulse is not palpable and the Korotkoff sounds may be difficult to hear during the measurement of the blood pressure. Such calcification may also present difficulties during surgery for the creation of arteriovenous shunts or fistulas for maintenance hemodialysis or during renal transplantation.
Arterial calcification shows little tendency to regress; in some patients, improvement or disappearance of arterial calcification occurs within months to years after subtotal parathyroidectomy or renal transplantation. Ocular calcification. Calcium deposition in the eye may produce visible inflammation and local irritation, resulting in the red eye of uremia. This is a transient phenomenon and may last only a few days. Recurrence of the red eye phenomenon is not infrequent, and it becomes apparent each time a new calcium deposition occurs in the conjunctiva.
More commonly, conjunctival calcium deposits are asymptomatic and are seen as white plaques or as small punctate deposits on the lateral or medial segment of the bulbar conjunctiva. Also, calcium deposits may occur within the cornea at the lateral or medial segments of the limbus, the so-called band keratopathy.
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Slit-lamp examination permits easier recognition of these lesions. The loss of CO2 through the conjunctival surface into the air increases the local pH of the ocular tissue, and this rise in pH predisposes to calcium deposition. Visceral calcification. Deposits of calcium may be found in the lungs, stomach, myocardium, skeletal muscles, and kidney. The Hereditary Amyloidoses. Familial Amyloidotic Polyneuropathies. Senile Amyloid Affecting the Heart. Amyloid Arthropathy.
Peripheral Angiopathy. Animal Models. Pathology of Experimental Amyloidosis. Kisilevsky, A. Snow, L. Subrahmanyan, L.
Boudreau, R. Amyloidogenic Proteins in Mice. Dorothea Zucker-Franklin, Alejandro Fuks. SAA Kinetics in Animals. Endocrine Amyloid in Animals. Edgar S. Cathcart, Crystal A. Leslie, Simin M.