Clinical features of hypophosphatemic osteomalacia induced by long-term low-dose adefovir dipivoxil.

OBJECTIVE: To investigate the predictors of hypophosphatemic osteomalacia induced by adefovir dipivoxil (ADV) and to monitor for early detection. PATIENTS AND METHODS: Hospitalized patients who were diagnosed with ADV-related hypo-phosphatemic osteomalacia were recruited and retrospectively analyzed in our hospital from January 2012 to December 2016. A telephone interview was conducted at 1, 3, 6, 9, 12, and 24 months after cessation of ADV. RESULTS: In the 8 patients enrolled in the study, the hypophosphatemic osteomalacia symptoms developed at an average of 5.14 (4-7) years since ADV treatment (10 mg/d). The average alkaline phosphatase (ALP) level was 279.50 (137-548) U/L, which was significantly higher than the normal level (45-125 U/L). The serum phosphorus level was an average of 0.59 (0.43-0.69) mmol/L, which was lower than the normal range (2.06-2.60 mmol/L). Serum calcium levels of the enrolled patients remained within normal limits. Reduced estimated glomerular filtration rate (eGFR <29 mL/min/1.73 m2) was seen in 4 cases. The clinical manifestations were mainly progressive systemic bone and joint pain, frequent fractures, trouble in walking, height reduction (4-6 cm), and so on. After cessation of ADV, symptoms like bone pain resolved gradually. Serum phosphorus level restored to normal in 4.5 months after the withdrawal of ADV. However, in 4 patients, renal function failed to return to normal in 24 months. CONCLUSION: More attention should be paid to the duration of ADV treatment. The level of serum phosphorus and ALP, as well as renal function, should be monitored for early detection of potential adverse drug reactions.

Pathologic Femoral Neck Fracture Due to Fanconi Syndrome Induced by Adefovir Dipivoxil Therapy for Hepatitis B.

In Fanconi syndrome, hypophosphatemic osteomalacia is caused by proximal renal tubule dysfunction which leads to impaired reabsorption of amino acids, glucose, urate, and phosphate. We present a rare case of a 43-year-old Korean male who was found to have insufficiency stress fracture of the femoral neck secondary to osteomalacia due to Fanconi syndrome. He had been receiving low-dose adefovir dipivoxil (ADV, 10 mg/day) for the treatment of chronic hepatitis B virus infection for 7 years and he subsequently developed severe hypophosphatemia and proximal renal tubule dysfunction. The incomplete femoral neck fracture was fixed with multiple cannulated screws to prevent further displacement of the initial fracture. After cessation of ADV and correction of hypophosphatemia with oral phosphorus supplementation, the patient's clinical symptoms, such as bone pain, muscle weakness, and laboratory findings improved.

Osteomalacia with low alkaline phosphatase: a not so rare condition with important consequences.

Hypophosphatasia is a genetic disorder, characterised by a dysfunctional tissue-non-specific isoenzyme of alkaline phosphatase that impacts bone metabolism and predisposes to osteomalacia or rickets. The clinical presentation is very diverse, depending on the age of onset and the severity of the disease. Several forms of hypophosphatasia are recognised. We present a case of a 50-year-old woman with low impact fractures and loss of teeth at a young age. She also had a low alkaline phosphatase and was diagnosed with adult hypophosphatasia. Although the severe forms of hypophosphatasia are rather rare, the adult form is thought to occur quite frequently. As this condition is not well known by healthcare professionals, the time to diagnosis and initiation of adequate treatment is often postponed. When encountering a patient with low alkaline phosphatase, low bone density or a history of bone fractures, the possibility of hypophosphatasia should be considered.

Hypophosphatemic rickets: lessons from disrupted FGF23 control of phosphorus homeostasis.

Fibroblast growth factor-23 (FGF23) regulates phosphate reabsorption in the kidney and therefore plays an essential role in phosphate balance in humans. There is a host of defects that ultimately lead to excess FGF23 levels and thereby cause renal phosphate wasting and hypophosphatemic rickets. We describe the genetic, pathophysiologic, and clinical aspects of this group of disorders with a focus on X-linked hypophosphatemia (XLH), the best characterized of these abnormalities. We also discuss autosomal dominant hypophosphatemic rickets (ADHR), autosomal recessive hypophosphatemic rickets (ARHR) and tumor-induced osteomalacia (TIO) in addition to other rarer FGF23-mediated conditions. We contrast the FGF23-mediated disorders with FGF23-independent hypophosphatemia, specifically hypophosphatemic rickets with hypercalciuria (HHRH). Errant diagnosis of hypophosphatemic disorders is common. This review aims to enhance the recognition and appropriate diagnosis of hypophosphatemia and to guide appropriate treatment.

[Metabolic bone disease osteomalacia]. / Metabolische Knochenkrankheit Osteomalazie.

Osteomalacia is a rare disorder of bone metabolism leading to reduced bone mineralization. Underlying vitamin D deficiency and a disturbed phosphate metabolism (so-called hypophosphatemic osteomalacia) can cause the disease. Leading symptoms are dull localized or generalized bone pain, muscle weakness and cramps as well as increased incidence of falls. Rheumatic diseases, such as polymyalgia rheumatica, rheumatoid arthritis, myositis and fibromyalgia must be considered in the differential diagnosis. Alkaline phosphatase (AP) is typically elevated in osteomalacia while serum phosphate and/or 25-OH vitamin D3 levels are reduced. The diagnosis of osteomalacia can be confirmed by an iliac crest bone biopsy. Histological correlate is reduced or deficient mineralization of the newly synthesized extracellular matrix. Treatment strategies comprise supplementation of vitamin D and calcium and for patients with intestinal malabsorption syndromes vitamin D and calcium are also given parenterally. In renal phosphate wasting syndromes substitution of phosphate is the treatment of choice, except for tumor-induced osteomalacia when removal of the tumor leads to a cure in most cases.

Osteocyte regulation of phosphate homeostasis and bone mineralization underlies the pathophysiology of the heritable disorders of rickets and osteomalacia.

Although recent studies have established that osteocytes function as secretory cells that regulate phosphate metabolism, the biomolecular mechanism(s) underlying these effects remain incompletely defined. However, investigations focusing on the pathogenesis of X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets (ADHR), and autosomal recessive hypophosphatemic rickets (ARHR), heritable disorders characterized by abnormal renal phosphate wasting and bone mineralization, have clearly implicated FGF23 as a central factor in osteocytes underlying renal phosphate wasting, documented new molecular pathways regulating FGF23 production, and revealed complementary abnormalities in osteocytes that regulate bone mineralization. The seminal observations leading to these discoveries were the following: 1) mutations in FGF23 cause ADHR by limiting cleavage of the bioactive intact molecule, at a subtilisin-like protein convertase (SPC) site, resulting in increased circulating FGF23 levels and hypophosphatemia; 2) mutations in DMP1 cause ARHR, not only by increasing serum FGF23, albeit by enhanced production and not limited cleavage, but also by limiting production of the active DMP1 component, the C-terminal fragment, resulting in dysregulated production of DKK1 and ß-catenin, which contributes to impaired bone mineralization; and 3) mutations in PHEX cause XLH both by altering FGF23 proteolysis and production and causing dysregulated production of DKK1 and ß-catenin, similar to abnormalities in ADHR and ARHR, but secondary to different central pathophysiological events. These discoveries indicate that ADHR, XLH, and ARHR represent three related heritable hypophosphatemic diseases that arise from mutations in, or dysregulation of, a single common gene product, FGF23 and, in ARHR and XLH, complimentary DMP1 and PHEX directed events that contribute to abnormal bone mineralization.

High bone turnover persisting after vitamin D repletion: beware of calcium deficiency.

Treatment of vitamin D deficiency with vitamin D is a common procedure when taking care of elderly patients, calcium supplementation being added only when calcium dietary intake is insufficient. Here, we report the case of a 58-year-old female who was referred to our unit because of suspicion of Paget's disease of the skull, based on elevated serum alkaline phosphatase and high skull methylene diphosphonate-technetium uptake. She had been prescribed cholecalciferol (100,000 IU/month) and calcium salts for the past 7 months after discovery of severe vitamin D deficiency by her primary care physician. No specific skull bone lesions were observed on both X-ray and computerized tomography. Serum calcium, phosphate and 25(OH) vitamin D levels were normal, while serum C-terminal cross-linked telopeptide, bone alkaline phosphatase and calcitriol were high and daily urinary calcium excretion was low. We found that she had not been compliant with the calcium prescription while vitamin D had been thoroughly taken. We suspected osteomalacia due to calcium deficiency. Both skull uptake and biological abnormalities normalised in few months after adding calcium supplementation to the vitamin D treatment, and spine bone mineral density increased by 9.5 % after 14 months of full treatment. The present case illustrates the necessity for adequate calcium intake during vitamin D repletion to normalise bone mineralisation and turnover and maintain the skeletal integrity.

The effect of vitamin D on bone and osteoporosis.

The main effect of the active vitamin D metabolite 1,25(OH)2D is to stimulate the absorption of calcium from the gut. The consequences of vitamin D deficiency are secondary hyperparathyroidism and bone loss, leading to osteoporosis and fractures, mineralization defects, which may lead to osteomalacia in the long term, and muscle weakness, causing falls and fractures. Vitamin D status is related to bone mineral density and bone turnover. Vitamin D supplementation may decrease bone turnover and increase bone mineral density. Several randomized placebo-controlled trials with vitamin D and calcium showed a significant decrease in fracture incidence. However, very high doses of vitamin D once per year may have adverse effects. When patients with osteoporosis are treated with a bisphosphonate, they should receive a vitamin D and calcium supplement unless the patient is vitamin D replete. These subjects are discussed in detail in this review. Finally, the knowledge gaps and research agenda are discussed.

Hypophosphatemic rickets and osteomalacia.

The hypophosphatemic conditions that interfere in bone mineralization comprise many hereditary or acquired diseases, all of them sharing the same pathophysiologic mechanism: reduction in the phosphate reabsorption by the renal tubuli. This process leads to chronic hyperphosphaturia and hypophosphatemia, associated with inappropriately normal or low levels of calcitriol, causing osteomalacia or rickets in children and osteomalacia in adults. X-linked hypophosphatemic rickets, autosomal-dominant hypophosphatemic rickets, and tumor-induced osteomalacia are the main syndromes involved in the hypophosphatemic rickets. Although these conditions exhibit different etiologies, there is a common link among them: increased activity of a phosphaturic factor, being the fibroblast growth factor 23 (FGF-23) the most studied one and to which is attributed a central role in the pathophysiology of the hyperphosphaturic disturbances. Activating mutations of FGF-23 and inactivating mutations in the PHEX gene (a gene on the X chromosome that codes for a Zn-metaloendopeptidase proteolytic enzyme which regulates the phosphate) involved in the regulation of FGF-23 have been identified and have been implicated in the pathogenesis of these disturbances. Genetic studies tend to show that the phosphorus homeostasis depends on a complex osteo-renal metabolic axis, whose mechanisms of interaction have been poorly understood so far. This paper reviews the current knowledge status concerning the pathophysiology of phosphate metabolism regulation and the pathophysiologic basis of hypophosphatemic rickets. It also analyzes the clinical picture and the therapeutic aspects of these conditions as well.

Development of renal bone disease.

Renal osteodystrophy (ROD) develops as the early stages of chronic renal failure (CRF) and covers a spectrum of bone changes observed in the uraemic patient, which extend from high remodelling bone disease (frequently known as osteitis fibrosa) to low turnover, or adynamic disease. Between these two extremes there are also cases of bone mineralization compromised in variable degrees, as is the case of 'mixed bone disease' and osteomalacia. The dynamic process of bone remodelling is compromised in CRF, and a positive or negative bone balance can be observed in uraemic patients. In addition to the classic modulators of bone remodelling, like parathyroid hormone, calcitriol and calcitonin, other factors were recently identified as significant modulators of osteoblast and osteoclast activation in uraemic patients. In fact, different cytokines and growth factors, acting at an autocrine or paracrine level, seem to play a relevant role in the bone and mineral changes observed in uraemia. Recently, observations have been made of the development of more sensitive and specific techniques to assay different biochemical markers of bone turnover and mineral metabolism. Analogously, new contributions of conventional bone histology, bone immunocytochemistry and molecular biology, which enabled the understanding of some etiopathogenic mechanisms of ROD, were observed.

Hypophosphatemic rickets and osteomalacia / Raquitismo hipofosfatêmico e osteomalácia

The hypophosphatemic conditions that interfere in bone mineralization comprise many hereditary or acquired diseases, all of them sharing the same pathophysiologic mechanism: reduction in the phosphate reabsorption by the renal tubuli. This process leads to chronic hyperphosphaturia and hypophosphatemia, associated with inappropriately normal or low levels of calcitriol, causing osteomalacia or rickets in children and osteomalacia in adults. X-linked hypophosphatemic rickets, autosomal-dominant hypophosphatemic rickets, and tumor-induced osteomalacia are the main syndromes involved in the hypophosphatemic rickets. Although these conditions exhibit different etiologies, there is a common link among them: increased activity of a phosphaturic factor, being the fibroblast growth factor 23 (FGF-23) the most studied one and to which is attributed a central role in the pathophysiology of the hyperphosphaturic disturbances. Activating mutations of FGF-23 and inactivating mutations in the PHEX gene (a gene on the X chromosome that codes for a Zn-metaloendopeptidase proteolytic enzyme which regulates the phosphate) involved in the regulation of FGF-23 have been identified and have been implicated in the pathogenesis of these disturbances. Genetic studies tend to show that the phosphorus homeostasis depends on a complex osteo-renal metabolic axis, whose mechanisms of interaction have been poorly understood so far. This paper reviews the current knowledge status concerning the pathophysiology of phosphate metabolism regulation and the pathophysiologic basis of hypophosphatemic rickets. It also analyzes the clinical picture and the therapeutic aspects of these conditions as well.

Os distúrbios hipofosfatêmicos que comprometem a mineralização óssea englobam várias doenças, hereditárias e adquiridas, as quais compartilham um mesmo mecanismo fisiopatológico: a diminuição da reabsorção de fosfato nos túbulos renais. Este processo promove hiperfosfatúria e hipofosfatemia crônicas, associadas a níveis inapropriadamente normais ou baixos de 1,25 (OH)2D3, com conseqüente desordem do metabolismo ósteo-mineral, resultando em raquitismo e osteomalácia na faixa etária pediátrica e em osteomalácia nos adultos. O raquitismo hipofosfatêmico ligado ao X, o raquitismo hipofosfatêmico autossômico dominante e a osteomalácia induzida por tumor são as principais síndromes que constituem os raquitismos hipofosfatêmicos. Apesar de estas doenças apresentarem etiopatogenias distintas, as evidências bioquímico-moleculares indicam uma base fisiopatológica em comum: maior atividade de um agente fosfatúrico, sendo o fator de crescimento do fibroblasto 23 (FGF-23) o mais estudado e ao qual é atribuído um papel central na fisiopatologia destes distúrbios. Várias mutações ativadoras do gene do FGF-23 e mutações inativadoras do gene localizado no cromossomo X que codifica uma enzima proteolítica Zn-metaloendopeptidase reguladora do fosfato (PHEX), implicada na regulação do FGF-23, já foram identificadas, e sua participação reconhecida na gênese destes distúrbios. Os dados dos estudos genéticos nesta área convergem para a hipótese de que a homeostase do fósforo estaria vinculada a um complexo eixo metabólico ósteo-renal, cujos mecanismos de interação entre seus vários componentes têm sido aos poucos elucidados. Este artigo revisa o atual estado de conhecimento dos mecanismos fisiológicos envolvidos na regulação do metabolismo do fosfato, das bases fisiopatológicas dos raquitismos hipofosfatêmicos e analisa aspectos clínicos e de tratamento disponíveis para estas condições.

The vitamin D questions: how much do you need and how should you get it?

UV radiation is a well-documented human carcinogen, indisputably linked to the current continued increased rate of skin cancer. UV radiation is also responsible for cutaneous synthesis of vitamin (vit) D3, a substance that is then sequentially hydroxylated in the liver and kidney to yield 1,25(OH)2 vit D, a hormone critical for calcium homeostasis and skeletal maintenance. Because the UV action spectra for DNA damage leading to skin cancer and for vit D photosynthesis are virtually identical, the harmful and beneficial effects of UV irradiation are inseparable. This has given rise to the argument that sun avoidance, with a goal of skin cancer prevention, may compromise vit D sufficiency. Public interest in this matter has been heightened in recent years by multiple studies correlating the level of 25-OH vit D, the readily measurable "storage" precursor form of the vit, with a variety of benefits separate from skeletal health. Although the studies are of variable quality and all alleged treatment benefits are based on dietary supplementation with vit D, not on increased sun exposure, they have been interpreted by some as support for advocating increased sun exposure of the public at large. The goal of this review is to provide a detailed, balanced, and referenced discussion of the complex literature underlying the current popular interest in vit D and sun exposure for the purpose of increasing vit D photosynthesis. We review the nomenclature, metabolism, and established functions of vit D; the evidence supporting the less well-established but purported vit D effects; the concept of vit D insufficiency; populations at risk for vit D deficiency; and finally the risk/benefit of obtaining vit D from cutaneous photosynthesis versus diet or supplementation.

"Phosphatonins" and the regulation of phosphorus homeostasis.

Phosphate ions are critical for normal bone mineralization, and phosphate plays a vital role in a number of other biological processes such as signal transduction, nucleotide metabolism, and enzyme regulation. The study of rare disorders associated with renal phosphate wasting has resulted in the discovery of a number of proteins [fibroblast growth factor 23 (FGF-23), secreted frizzled related protein 4 (sFRP-4), matrix extracellular phosphoglycoprotein, and FGF 7 (FGF-7)] that decrease renal sodium-dependent phosphate transport in vivo and in vitro. The "phosphatonins," FGF-23 and sFRP-4, also inhibit the synthesis of 1alpha,25-dihydroxyvitamin D, leading to decreased intestinal phosphate absorption and further reduction in phosphate retention by the organism. In this review, we discuss the biological properties of these proteins, alterations in their concentrations in various clinical disorders, and their possible physiological role.

Decreased cutaneous vitamin D-synthesis in heavily melanized individuals: a rare cause for pathologic fractures of the hip.

Painful pathological fractures of the femoral neck and the subtrochanteric region of the femur are reported in two women originating from India. After exclusion of renal or intestinal causes, laboratory data on bone metabolism, scintigraphic and radiographic examinations were characteristic for the presence of secondary hyperparathyroidism. Based on vitamin deficiency and low calcium absorption, disturbed mineralization of bone and increased osteoclastic resorption have apparently led to osteomalacia and subsequent fracturing. Fracture localization necessitated surgical fixation in one patient; conservative treatment including protected weightbearing was effective in the other women. After supplementation of calcium and vitamin D3, levels of parathyroid hormone and scintigraphic alterations returned to normal in both patients. In these two cases, pathological fractures of the hip could be attributed to the presence of secondary hyperparathyroidism based on decreased cutaneous vitamin D synthesis.

Tertiary hyperparathyroidism after high-dose phosphate therapy in adult-onset hypophosphatemic osteomalacia.

OBJECTIVE: To describe a case of adult-onset hypophosphatemic osteomalacia treated with orally administered phosphate and complicated by tertiary hyperparathyroidism. METHODS: We present pertinent clinical, radiologic, and laboratory details of the study patient for a period of more than 20 years and discuss the few reported cases of tertiary hyperparathyroidism attributable to prolonged phosphate therapy. RESULTS: A 49-year-old Jordanian man, who had been diagnosed at age 26 years as having sporadic adult-onset hypophosphatemic vitamin D-resistant osteomalacia, presented with severe right hip pain, severe osteopenia with lytic bone lesions, and hypercalcemia after prolonged oral treatment with phosphate and vitamin D. These clinical, radiologic, and biochemical findings, in conjunction with a very high serum parathyroid hormone level, indicated the diagnosis of tertiary hyperparathyroidism, which was substantiated histopathologically. CONCLUSION: Physicians should be aware of the potential for development of tertiary hyperparathyroidism in patients receiving prolonged oral phosphate therapy.

Bone histology in patients with nephrotic syndrome and normal renal function.

BACKGROUND: The prevalence of metabolic bone disease in patients with nephrotic syndrome (NS) at normal level of renal function remains uncertain. METHODS: To address this issue, we studied 30 patients (20 men and 10 women, mean age 27.3 +/- 11.7 years) with NS who had normal renal function (mean creatinine clearance 103 +/- 4 ml/min). We evaluated their serum calcium, phosphorus, alkaline phosphatase, immunoreactive parathyroid hormone (iPTH), vitamin D metabolites, urinary calcium, and skeletal survey. The extent of bone mineralization was analyzed by histomorphometric analysis of iliac crest bone biopsy specimens in all patients. The findings on bone histology were correlated with biochemical parameters. RESULTS: The mean duration of NS was 35.5 +/- 26.9 months, with a protein excretion of 7.3 +/- 3.2 g/24 hr and a serum albumin of 2.2 +/- 0.8 g/dl. Total serum calcium was 7.8 +/- 0.8 mg/dl, whereas ionized calcium was 5.7 +/- 0.7 mg/dl, phosphorus 3.2 +/- 1.2 mg/dl, and alkaline phosphatase 149 +/- 48.6 U/liter. Serum iPTH levels were normal in all except two patients. The mean serum 25-hydroxyvitamin D [25(OH)D] level was 3.9 +/- 1.2 ng/ml (normal 15 to 30 ng/ml), whereas 1,25-dihydroxyvitamin D was 24 +/- 4.7 pg/ml (normal 16 to 65). There was an inverse correlation between serum levels of 25(OH)D and the magnitude of proteinuria (r = -0.42, P < 0.05). The mean 24-hour urinary calcium excretion was 82 +/- 21 mg/day. The skeletal survey was normal in all patients. Bone histology was normal in 33.3% of the patients, whereas 56.7% had isolated osteomalacia (OSM), and 10% had an increased bone resorption in association with defective mineralization. The severity of OSM measured by mineralization lag time correlated linearly with the duration (r = 0.94, P < 0.0001) and the amount (r = 0.97, P < 0.0001) of proteinuria. All patients with NS for more than three years had histological changes. Patients with OSM had lower 25(OH)D and serum albumin as compared with those with normal histology (P < 0.005). Bone mineralization had no significant correlation with serum iPTH, divalent ions, or vitamin D levels. CONCLUSIONS: OSM is a frequent finding in adult patients with NS, even at a normal level of renal function. Its severity correlates with the amount and duration of proteinuria.