The treatment of adult phosphate diabetes and Fanconi syndrome with neutral sodium phosphate

Autosomal dominant hypophosphatemic rickets in an 85 year old woman: characterization of her disease from infancy through adulthood

BACKGROUND

Autosomal dominant hypophosphatemic rickets (ADHR) is a rare genetic disorder of phosphate homeostasis characterized, when severely expressed, by osteomalacia, suppressed levels of calcitriol, and renal phosphate wasting due to elevated levels of fibroblast growth factor 23 (FGF23). The disease is caused by heterozygous FGF23 mutations at the RXXR site that prevent cleavage of the intact hormone.

OBJECTIVES

An FGF23 mutation was identified in the proband an 85-year-old woman with elevated FGF23 levels, and her clinical course was characterized. Medical records revealed she was treated for rickets as an infant. She was then asymptomatic until soon after her 4th pregnancy, when she suffered incapacitating bone pain and weakness, age 37. Symptoms remitted with brief treatment.

RESULTS

The proband and one son, but not other family members, were found to be heterozygous for the R176Q mutation in FGF23. Expression of this germ line mutation was strikingly different in both individuals in terms of skeletal health, FGF23 levels and disease activity.

CONCLUSIONS

The identified FGF23 mutation in two members of this family raises questions about molecular mechanisms that have led to intermittent increases in FGF23 synthesis and secretion, and disease expression.

Hyperaminoaciduria in mild phosphate diabetes in adults

Quantitative urinary amino acid excretion has been studied in 23 adult patients with mild phosphate diabetes (MPD), in 22 adult control patients with various renal disorders and in 15 children, 7--19 years old, with atopic disorders (normal controls). Statistically significant increases in urinary excretion of glutamine (p < 0.01), glycine (p < 0.01) and cystine (p < 0.05) were found in the MPD patients compared to the normal controls. The urinary excretion of glutamine was significantly increased while the increases in cystine and glycine excretions were non-significant when MPD patients were compared to the control patients. In addition to clinical signs and analyses of plasma and urinary phosphate, a quantitative evaluation of urinary amino acids might be a tool in the diagnosis of MPD. The significance of the increased urinary amino acid excretion in MPD is discussed.

Mild phosphate diabetes in adults

Phosphate diabetes has been considered as rare and to occur almost exclusively in children. Upon examination of adult patients with rheumatic or kidney diseases it has, however, been found that the combination of hypophosphataemia and hyperphosphaturia is not so rare. This paper deals with 24 adult patients of this type, whom we have found during 6 months. Their mean serum phosphorus concentration was 0.7 mmol/l (range 0.5--0.8). Mean phosphate clearance was 31 ml/min/1.73 m2 (range 16--51). The diagnoses were myalgia, dorsalgia (n = 7), papillitis calcificans (n = 5), prostatitis or prostate accretions (n = 4), dizziness (n = 2), kidney stones, tubular defect, interstitial nephritis, medullary sponge kidney (1 case each), two patients had transplanted kidneys. Asthenia was a common additional diagnosis. The patients' complaints have been pain in the muscles, joints, bones (18 cases), tiredness (10 cases), dizziness (8 cases), shakyness, numbness, burning sensation (7 cases), tenderness in the muscles and bones ("the princess-on-the-pea syndrome") (7 cases). The most common findings upon examination were bone tenderness (13 cases), reduced manual power (8 cases), positive Romberg test (3 cases), slight muscle atrophy (2 cases), waddling gait (2 cases). The most common findings encountered in the laboratory, besides hypophosphataemia and hyperphosphaturia, were high pH in the urine, hyperaminoaciduria, and phosphate crystals in dried urine.

Osteomalacia

Osteomalacia is a generalized bone disorder characterized by impairment of mineralization, leading to accumulation of unmineralized matrix or osteoid in the skeleton. The classical clinical features of osteomalacia include musculoskeletal pain, skeletal deformity, muscle weakness and symptomatic hypocalcaemia. In childhood the features of osteomalacia are accompanied by rickets, with widening of the epiphyses and impaired skeletal growth. The major cause of osteomalacia is vitamin D deficiency, which is most often due to reduced cutaneous production of vitamin D in housebound elderly people, immigrants to Northern countries and women who adopt strict dress codes which prohibit exposure of uncovered skin. Vitamin D deficiency osteomalacia may also occur with malabsorption, liver disease and anticonvulsant therapy. Less commonly, osteomalacia may result from abnormal vitamin D metabolism, resistance to the action of vitamin D, hypophosphataemia or toxic effects on osteoblast function.

Medical reversal of acquired hypophosphatemic osteomalacia

Hypophosphatemic osteomalacia may present as severe disability from bone disease. This report describes a patient with long-standing disease and multiple fractures. Medical management of the phosphate loss may be successful in promoting bone healing when it is not possible to establish the cause of the phosphaturia. Judicious increases in calcium, 1,25-dihydroxyvitamin D, and phosphorus supplements were carefully monitored to avoid failure of therapy or hypercalcemic complications from pharmacologic amounts of these supplements.

Reflex sympathetic dystrophy syndrome: an unusual mode of presentation of osteomalacia

We report 2 patients with reflex sympathetic dystrophy syndrome (RSDS) associated with renal tubular osteomalacia. In both patients, RSDS was the presenting manifestation. Treatment of the underlying condition with calcitriol resulted in healing in both patients. These cases prompted us to undertake a retrospective study of 24 consecutive patients with osteomalacia (6 of renal tubular origin, 18 related to vitamin D deficiency), and of 259 consecutive patients with RSDS. Among those with osteomalacia, radiologic lesions strongly suggestive of RSDS were found in 5 patients (21%), 4 of whom had renal tubular defects, and only 1 had vitamin D deficiency. In the group with RSDS, 1 patient (0.4%) was found to have osteomalacia of renal tubular origin. Thus, radiologic findings suggestive of RSDS are frequent in patients with osteomalacia, especially that of renal tubular origin. On rare occasions, the full-blown clinical picture of RSDS may obscure the diagnosis of osteomalacia.

Adult sporadic hypophosphatemic osteomalacia presenting as regional migratory osteoporosis

Osteoarticular manifestations associated with adult sporadic renal phosphate diabetes have previously been reported. We describe a 36-year-old man with phosphate diabetes who presented with migratory regional osteoporosis of the lower limbs. Our data suggest a cause-effect relationship between regional migratory osteoporosis and adult phosphate diabetes.

Adult hypophosphatemic osteomalacia: report of two cases

Two cases of late hypophosphatemic osteomalacia are described: a male aged 30 who had the disease since he was 22 and a woman of 23 who had the disease since she was 14. Both presented with myopathy and bone pain, and showed hypophosphatemia, hyperglycinuria, reduced tubular phosphate reabsorption (TPR), increased hydroxyprolinuria and normal iPTH and iCT values. Radiologically the male had no Looser's zones and the woman did. Bone biopsy confirmed hypophosphatemic osteomalacia. Both cases were treated with vitamin D and oral phosphate and no improvement was observed. When treatment with 25(OH)D3 was initiated, no improvement was seen and afterwards this was combined with treatment using 1.25(OH)2D3 and from this time on a clinical improvement of the myopathy became evident in both patients. In the woman, healing of the bone lesions occurred at the same time as that of the myopathy, whereas in the male the bone lesions became worse. Healing of the myopathy was only obtained when treatment with 1.25(OH)2D3 was begun. Both patients had reduced values of 2.3 erythrocytic DPG and low level of serum phosphorus when the myopathy was cured, which suggests a lack of effect of 2.3 DPG or serum phosphorus as a cause of the myopathy. Although this had been attributed to a deficiency in the function of 25(OH)D3, the response to 1.25(OH)2D3 and due to the effects of this metabolite on calcium transport in muscle, suggests that the myopathy which occurs in late hypophosphatemic osteomalacia is a result of deficiency or resistance to the muscular effect of this metabolite. We cannot explain the lack of bone healing in the man and further therapeutic studies are required.

Calcification of entheses associated with X-linked hypophosphatemic osteomalacia

We undertook a retrospective analysis of 26 patients with X-linked hypophosphatemic osteomalacia (or rickets), whose ages ranged from 1 to 62 years and who were from 11 different kindreds, to determine the prevalence and clinical characteristics of a unique disorder of the entheses (tendons, ligaments, and joint capsules). We found a generalized involvement of the entheses, with exuberant calcification of tendon and ligament insertions and of joint capsules, in 69 per cent of the subjects. The prevalence and extent of disease increased with age but were not correlated with sex. Commonly affected sites included the hand and sacroiliac joints. Histologic evaluation in a selected patient revealed intratendinous lamellar bone but no inflammatory cells. Our observations indicate that this disorder is an integral part of X-linked hypophosphatemic osteomalacia and exhibits clinical, radiographic, and histologic characteristics that differentiate it from degenerative disorders of these tissues and seronegative spondyloarthropathies.

Development of hypercalcemic hyperparathyroidism after long-term phosphate supplementation in hypophosphatemic osteomalacia. Report of two cases

Orally administered phosphate supplements are the mainstay of therapy for hypophosphatemic osteomalacia of diverse causes and are generally believed to be free from harmful side effects. Two cases are reported, however, in which long-term therapy (14 and 10 years, respectively) resulted in hypercalcemic hyperparathyroidism associated with surgically proved adenomatous hyperplasia. This complication occurred despite concomitant treatment with pharmacologic doses of vitamin D. Thus, long-term oral phosphate therapy can produce tertiary hyperparathyroidism in susceptible patients.

Nonosteomalacic osteopathy associated with chronic hypophosphatemia

We studied bone histomorphometry in 19 patients with chronic hypophosphatemia related to an idiopathic renal phosphate wasting and without histological osteomalacia. Nine patients had renal lithiasis (group 1), three had radiological osteoporosis (group 2), and seven had lumbar pain (group 3). In the whole group of 19 patients, serum phosphate levels were low (24.9 +/- 2.1 mg/l), calcium in blood was normal, calcium in urine was increased, and iPTH was low. Histomorphometric data showed decreased osteoblastic surfaces with normal resorption surfaces, normal osteoid volume and calcification front. There was no correlation between serum phosphate level and histomorphometric parameters. There was no statistical difference between the data of the 3 groups of hypophosphatemic patients. We concluded that chronic hypophosphatemia in the adult doses not always lead to osteomalacia but to an unusual osteopathy characterized by an osteopenia due to an isolated decrease in bone formation. The respective importance of phosphate deficiency and of decreased iPTH level in the pathogenesis of this osteopathy is uncertain.

Differences between the effects of phosphate deficiency and vitamin D deficiency on bone metabolism

It has been widely believed that phosphate deficiency causes osteomalacia. Based on this belief, the rickets of familial hypophosphatemia has been attributed to phosphate deficiency associated with the hypophosphatemia. The present studies on rats have, however, demonstrated significant differences between the effects of phosphate deficiency on bone metabolism and the characteristic features of rickets. Weanling rats, maintained on a mildly phosphate deficient diet, had hypercalcemia and hypophosphatemia, and impairment of body growth, bone growth, and bone mineralization. The maximum effect was observed at 5 wk; between 5 and 20 wk the rats improved despite persistent hypophosphatemia. Histologically, at 5 wk the bone showed thick unmineralized osteoid seams covering most bone surfaces, but the epiphyseal cartilage was normal. In addition, the excess osteoid readily incorporated tetracycline indicating normal mineralization and, based on a new sequential pulse labeling technique, the linear bone apposition rate (LBA) was significantly (p < 0.001) increased above control values. This increase was observed within the initial 4 days of phosphate (P) deficiency and persisted up to 15 wk. This effect of P deficiency on LBA was dependent on vitamin D activity. At 4 wk, the mean LBA was 0.106 +/- 0.003 (1 SE) in control rats, 0.149 +/- 0.008 microns/hr in P deficient rats, 0.083 +/- 0.004 microns/hr in vitamin D deficient rats and 0.086 +/- 0.006 microns/hr in rats deficient in both P and vitamin D. We have reported a similar increase in LBA with parathyroid hormone activity. With vitamin D deficiency, phosphate deficient rats showed all the characteristic features of rickets; disorganization of epiphyseal cartilage, excessive unmineralized osteoid, and reduced mineralization based on the incorporation of tetracycline. We conclude that the effects of phosphate deficiency on bone metabolism more closely resembles the effects of PTH activity than the characteristic effects of osteomalacia and rickets.

Renal responsiveness to parathyroid hormone in a case of nonfamilial hypophosphatemic osteomalacia

Plasma immunoreactive parathyroid hormone level, urinary excretion of adenosine cyclic 3',5'-monophosphate (cyclic AMP) and the sensitivity of the renal tubule to calcium infusion and to parathyroid extract were investigated in a patient with nonfamilial hypophosphatemic osteomalacia. Plasma immunoreactive parathyroid hormone concentration was normal and basal urinary excretion of cyclic AMP was increased. Renal cortical adenylate cyclase, as measured by urinary cyclic AMP excretion, was certainly as sensitive to exogenous parathyroid extract as in normal subjects. After a previous calcium infusion, a greater parathyroid-hormone-sensitive component of phosphorus transport in the kidney was present than in two control subjects. Our results indicate that in nonfamilial hypophosphatemic osteomalacia the renal tubule could be hyperresponsive to parathyroid hormone.

Rheumatic syndromes in endocrine disease

It is not widely appreciated that endocrine disease may present primarily as rheumatic syndromes, sometimes spectacular in onset, more commonly insidious and subtle, making their true recognition difficult. The underlying hormonal, biochemical, and metabolic events have understandable reflection in the structure and function of bone, joint, and muscle.

Osteomalacic dialysis osteodystrophy: a trial of phosphate-enriched dialysis fluid

To assess whether phosphate depletion is an aetiological factor in osteomalacic dialysis osteodystrophy we undertook a prospective trial of phosphate-enriched dialysis fluid, in association with oral 1alpha-hydroxycholecalciferol, for this condition. Thirty patients started the trial; of the 27 who completed more than 6 months' treatment, 14 had iliac crest bone biopsies at the beginning and end of the treatment period. Side effects included pruritus, stiffness, and increase in corneal and vascular calcification. Only one patient showed histological improvement of osteomalacia, and eight deteriorated; in seven the osteitis fibrosa worsened. Myopathy showed some improvement in four patients, but became worse in four. This treatment does not seem to have a place in the routine management of non-hypophosphataemic patients on dialysis.

Stimulation of bone formation in vivo by phosphate supplementation

The effect of phosphate supplementation on bone remodeling was assessed in six mature, healthy beagle dogs. The phosphate supplement was given in divided doses orally, daily for 12 weeks in the form of a neutral potassium phosphate preparation. The dose averaged 108 mg P/kg per day, which is double the normal canine phosphorus intake. Bone remodeling was assessed by measurement, at sacrifice, of areas of cortical bone containing different color-coded tetracyclines which had been continuously administered during 12-week control and treatment periods; remodeling was assessed kinetically during the control and treatment periods by replicate studies employing 47Ca intravenously. Both techniques demonstrated that the principal effect of phosphate supplementation was a significant stimulation of bone formation. Within cortical bone, formation was doubled, from an average of 2.7% to 5.3% per year. The major location of new bone deposits was endosteal. Whole skeletal mineral accretion, measured kinetically, increased 45% above an average control value of 0.154 g/day. These studies suggest that, in the adult dog, "normal" plasma phosphate levels are suboptimal for new bone formation. Even with this short duration of administration, phosphate produced microscopic calcification of the renal parenchyma. However, there was no biochemical evidence of renal functional impairment.

Muscle weakness in osteomalacia

The muscle weakness that frequently accompanies osteomalacia and rickets may arise from a variety of causes. Particularly in patients with muscle weakness, identification of the metabolic disorder is important, since effective treatment is often possible.

The use of high calcium dialysate in the treatment of renal osteomalacia

Four chronic haemodialysis patients suffering from osteomalacia were treated by increasing their dialysate calcium concentration from 1-40 to 2-15 mM/I. Bone biopsies were taken before and after 22 weeks of this treatment and a further biopsy was taken in one patient after 52 weeks. Symptomatic cure occurred in one patient with mild osteomalacia and some improvement osteomalacia and hyperparathyroidism. Bone biopsies showed slight improvement in the patient with mild osteomalacia after 22 weeks, and considerable improvement after 25 weeks. In the patient with mixed histology, the osteitis fibrosa subsided and the osteomalacia became a little worse after 22 weeks. There was no histologic improvement in the two patients with severe osteomalacia after 22 weeks. It is concluded that high calcium dialysis is not an effective treatment for renal osteomalacia.

Myopathy in hypophosphataemic osteomalacia presenting in adult life

Three cases of hypophosphataemic osteomalacia presenting in adult life, in which a myopathy was a prominent presenting feature, are described. In one, a nasopharyngeal haemangioma was also present. Possible mechanisms underlying the myopathy are discussed briefly.

Iatrogenic osteomalacia and myopathy due to phosphate depletion

In a patient receiving regular dialysis prolonged hypophosphataemia due to aluminium hydroxide therapy resulted in osteomalacia and severe proximal myopathy. Both osteomalacia and myopathy responded to correction of hypophosphataemia without vitamin D therapy.

Loss of a parathyroid hormone-sensitive component of phosphate transport in X-linked hypophosphatemia

Mutant hemizygotes with X-linked hypophosphatemia lack a parathyroid hormone-sensitive component of inorganic phosphate transport in kidney; female heterozygotes retain a variable proportion of this type of transport. The residual mechanism for reabsorption in affected males allows inorganic phosphate efflux from the kidney to urine so that net "secretion" is sometimes observed; the latter is directly proportional to the serum concentration of inorganic phosphate. Calcium acts on the kidney tubule to enhance net reabsorption by this component of inorganic phosphate transport.

Formation, mineralization, and resorption of bone in hypophosphatemic rats

Quantitative morphologic methods were used to measure the effects of feeding a low phosphorus diet to intact and thyroparathyroidectomized rats on several processes of bone mineralization and turnover. In severely hypophosphatemic animals, the matrix formation rate was decreased, the osteoid maturation rate was decreased, which indicated a delay in the onset of mineralization, the initial rate of mineralization was decreased, and the endosteal osteoclastic bone resorption rate was increased. In moderately hypophosphatemic animals, there was a substantial increase in bone resorption but no change in formation or in mineralization. The increase in endosteal bone resorption was due to an increase in the linear rate of bone resorption and particularly to an increase in the length of the endosteal resorbing surface. The magnitude of the increase in bone resorption was similar in thyroparathyroidectomized and intact rats indicating that neither parathyroid hormone nor calcitonin is involved in this change. This, together with the finding that there was a strong negative correlation (r = -0.99) between the per cent endosteal resorbing surface and the serum phosphorus, supports the view that the increased resorption was due to hypophosphatemia. This inverse relationship between endosteal resorbing surface and serum phosphorus appeared to hold for values of serum phosphorus above normal. The resorptive response to hypophosphatemia, as previously shown for the resorptive response to excess endogenous parathyroid hormone, was partially inhibited by vitamin D deficiency. Increased resorption occurred at levels of serum phosphorus where no changes were observed in bone formation, mineralization, or growth, suggesting that this resorptive response functions as a homeostatic mechanism to maintain serum and intracellular phosphorus concentrations.

The effect of supplemental oral phosphate on the bone mineral changes during prolonged bed rest

Five healthy young men were studied during 24-30 wk of continuous bed rest. During the first 12 wk of bed rest, untreated subjects increased calcium excretion in the urine by 109 mg/day and in the feces by 147 mg/day. The rate of total body calcium loss was 0.5-0.7% per month. Losses of central calcaneus mineral, assessed by gamma ray transmission scanning, occurred at a tenfold higher rate, whereas the mineral content of the radius did not change. Changes in phosphorus balance resembled the calcium pattern, and increased excretion of nitrogen and hydroxyproline also occurred during bed rest. Upon reambulation, the subjects' calcium balance became positive in 1 month and recovery of their calcaneus mineral was complete within 10-20 wk. Treatment with potassium phosphate supplements (1327 mg P/day) entirely prevented the hypercalciuria of bed rest, but fecal calcium tended to increase. During the first 12 wk, calcium balance was slightly less negative (mean - 193 mg/day) than during bed rest without added phosphate (mean - 267 mg/day). This effect was not seen during the second 12 wk of bed rest. The patterns of magnesium excretion were similar to those of calcium. Fecal and urinary phosphorus excretions were doubled, and phosphorus balance became positive (+ 113 mg/day). Mineral loss from the central calcaneus was similar to that of untreated subjects. It is concluded that this form of phosphate supplementation reduces urinary calcium excretion but does not prevent bone loss during bed rest.

Serum parathyroid hormone in X-linked hypophosphatemia

Serum immunoreactive parathyroid hormone(IPTH) is normal in patients with X-linked hypophosphatemic rickets who are not treated with phosphate salts. Phosphate raises IPTH in these patients. Endogenous IPTH does not influence the existing defect in tubular reabsorption of phosphate in male patients.

An acceptable slow-release phosphate preparation "slow P"

Slow P has been an acceptable form of oral phosphate therapy in a series of patients with a variety of bone disease. Therapeutic responses have been achieved in a number of patients without side effects together with minor elevation of the serum and urinary phosphate in the majority of patients. Some patients have continued with such supplements for several months which encourages an extended trial under more stringent conditions.

Urinary polypeptides related to collagen synthesis

Of the total urinary hydroxyproline in normal subjects and those with skeletal disorders, between 4 and 20% was nondialyzable. In some patients with Paget's disease of bone, hyperparathyroidism with osteitis fibrosa, hyperphosphatasia, and extensive fibrous dysplasia the total urinary hydroxyproline was sufficiently high to permit purification of this polypeptide hydroxyproline by gel filtration and ion exchange chromatography. The partially purified polypeptides had molecular weights between 4500 and 10,000 and amino acid compositions and physical properties resembling those of gelatin. The polypeptide fractions also contained neutral sugar and glucosamine. These fragments had been shown to be susceptible to cleavage by purified bacterial collagenase suggesting the presence of the sequence-Pro-X-Gly-Pro-Y-. After administration of proline-(14)C to patients with Paget's disease hydroxyproline-(14)C was excreted in the urine. The hydroxyproline-(14)C specific activity reached a peak in 2-4 hr and declined rapidly. The specific activity of the polypeptide (retentate) portion was severalfold greater than that of the raw urine and diffusate. When the labeled urines were subjected to gel filtration the hydroxyproline-(14)C fractions of highest molecular weight which were eluted first from the columns had the highest specific activities. Exposure of the hydroxyproline-(14)C-containing polypeptides to bacterial collagenase rendered them dialyzable. Four patients with hyperparathyroidism and osteitis fibrosa were studied before and after removal of a parathyroid adenoma, a period of transition from a predominance of bone collagen resorption to one of relatively increased bone collagen synthesis. The total urinary hydroxyproline fell rapidly after operation whereas the ratio of the polypeptide fraction to the total rose three- to fourfold. The results of these studies suggest that the urinary polypeptides represent fragments of collagen related to collagen synthesis. Changes in the ratio of these peptides to total hydroxyproline in the urine may serve as an index of new bone formation in patients with skeletal disorders.