Bone mineral density of the spine and radius shaft in children with X-linked hypophosphatemic rickets (XLH)

Evaluation of bone density and microarchitecture in adult patients with X-linked hypophosphatemic rickets: A pilot longitudinal study

X-linked Hypophosphatemia (XLH) is the most common type of inherited rickets. Although the clinical features are well characterized, bone structure, mineralization, and biomechanical properties are poorly known. Our aim was to analyze bone properties in the appendicular and axial skeleton of adults with XLH. In this observational case-control study, each affected patient (N = 14; 9 females; age 50 ± 15 years) was matched by sex, age and body mass index to a minimum of two healthy controls (N = 34). Dual-energy X-ray Absorptiometry (DXA) analyses revealed that areal bone mineral density (aBMD) was higher in XLH patients at the lumbar spine (Z score mean difference = +2.47 SD, P value = 1.4 × 10-3). Trabecular Bone Score was also higher at the lumbar spine (P value = 1.0 × 10-4). High Resolution peripheral Quantitative Computed Tomography (HRpQCT) demonstrated that bone cross-sectional area was larger at the distal radius (P value = 6 × 10-3). Total and trabecular volumetric BMD were lower at both sites. Trabecular bone volume fraction was also lower with fewer trabecular numbers at both sites. However, bone strength evaluated by micro-finite element analyzes revealed unaffected bone stiffness and maximum failure load. Evaluation of bone mineralization with aBMD by DXA at the distal radius correlated with vBMD by HRpQCT measurements at both sites. PTH levels were inversely correlated with trabecular vBMD and BV/TV at the tibia. We then followed a subset of nine patients (median follow-up of 4 years) and reassessed HRpQCT. At the tibia, we observed a greater decrease than expected from an age and sex standardized normal population in total and cortical vBMD as well as a trabecularization of the cortical compartment. In conclusion, in adult patients with XLH, bone mineral density is high at the axial skeleton but low at the appendicular skeleton. With time, microarchitectural alterations worsen. We propose that noninvasive evaluation methods of bone mineralization such as DXA including the radius should be part of the management of XLH patients. Larger studies are needed to evaluate the clinical significance of BMD changes in XLH patients under conventional or targeted therapies.

Disorders of phosphate homeostasis in children, part 2: hypophosphatemic and hyperphosphatemic disorders

Phosphorus, predominantly in the form of inorganic phosphate PO₄^-3, has many essential physiological functions. In the skeleton, phosphate and calcium form the mineral component and phosphate is also essential in regulating function of skeletal cells. Considerable advances have been made in our understanding of phosphate homeostasis since the recognition of fibroblast growth factor-23 (FGF23) as a bone-derived phosphaturic hormone. This second part of a two-part review of disorders of phosphate homeostasis in children covers hypophosphatemic and hyperphosphatemic disorders that are of interest to the pediatric radiologist, emphasizing, but not limited to, those related to abnormalities of FGF23 signaling.

Interactions between FGF23 and vitamin D

Fibroblast growth factor-23 (FGF23) controls the homeostasis of both phosphate and vitamin D. Bone-derived FGF23 can suppress the transcription of 1α-hydroxylase (1α(OH)ase) to reduce renal activation of vitamin D (1,25(OH)2D3). FGF23 can also activate the transcription of 24-hydroxylase to enhance the renal degradation process of vitamin D. There is a counter-regulation for FGF23 and vitamin D; 1,25(OH)2D3 induces the skeletal synthesis and the release of FGF23, while FGF23 can suppress the production of 1,25(OH)2D3 by inhibiting 1α(OH)ase synthesis. Genetically ablating FGF23 activities in mice resulted in higher levels of renal 1α(OH)ase, which is also reflected in an increased level of serum 1,25(OH)2D3, while genetically ablating 1α(OH)ase activities in mice reduced the serum levels of FGF23. Similar feedback control of FGF23 and vitamin D is also detected in various human diseases. Further studies are required to understand the subcellular molecular regulation of FGF23 and vitamin D in health and disease.

Bone microstructure evaluated by TBS and HR-pQCT in Chinese adults with X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is the most common form of heritable hypophosphatemic rickets. Although generalized mineralization defects have been observed, elevated areal bone mineral density (aBMD) in the lumbar spine measured by dual-energy X-ray absorptiometry (DXA) has also been found in XLH. In contrast, high-resolution peripheral quantitative computed tomography (HR-pQCT) revealed lower volumetric BMD (vBMD) and damaged bone microstructure in the peripheral bone in XLH. Trabecular bone score (TBS), which can assess the trabecular microstructure in the lumbar spine, has not been explored in XLH. This study aimed to explore TBS and its correlations with biochemical indices and HR-pQCT parameters in adult XLH patients. A total of 66 patients with XLH (26 men and 40 women) aged 29.6 ± 9.6 years and 66 age- and sex-matched healthy controls were included. Z score of lumbar spine aBMD was relatively high [2.0 (0.6, 3.7)], with normal TBS (1.475 ± 0.129) in the XLH patients. HR-pQCT revealed larger total and trabecular area in the peripheral bone in the XLH group compared with the control group. In addition, lower trabecular and cortical vBMD, lower trabecular number with greater separation, and lower bone strength at both the radius and tibia were found in the XLH group compared with the control group. Smaller cortical area, lower thickness and higher porosity in the XLH group compared with controls were only found at the radius. TBS was not associated with any biochemical indices, while better HR-pQCT parameters correlated with higher serum phosphate and lower ALP levels. TBS was positively related with aBMD but not HR-pQCT parameters. In conclusion, adult patients with XLH had high bone mass and normal TBS in the lumbar spine but compromised microarchitecture and bone strength in the peripheral bone. This finding indicated a site-specific effect of the disease on the skeleton in the XLH patients.

Maxillomandibular osteotomy healing in a patient with hypophosphatemic rickets

Hypophosphatemic rickets is a hereditary disease of childhood that affects bone metabolism. Patients with this disease can have poor bony healing at growth centers due to impaired mineralization of the cartilaginous bone plates. The orthopedic literature has shown appropriate long bone healing in patients with rickets who are undergoing concurrent medical management. However, there has never been a report of successful osteotomy or fracture of the maxillomandibular skeleton in a patient with rickets with documented radiographic and clinical resolution. This report describes a case of successful Le Fort I osteotomy with bilateral mandibular sagittal split osteotomies with concurrent medical management in an 18-year-old female with hypophosphatemic rickets. Before surgery the patient was consented and understood that because there has never been a documented case such as this, she might especially risk complications including non-union, malunion, or unfavorable osteotomy splits. The intraoperative quality of the patient's bone was of normal caliber and allowed routine osteotomy creation and internal fixation with no complications. She was followed for over 3 years and showed uneventful healing. While this disease is especially rare, practitioners should be aware that acceptable healing alongside appropriate medical management has been documented.

The Genetic Architecture of High Bone Mass

The phenotypic trait of high bone mass (HBM) is an excellent example of the nexus between common and rare disease genetics. HBM may arise from carriage of many 'high bone mineral density [BMD]'-associated alleles, and certainly the genetic architecture of individuals with HBM is enriched with high BMD variants identified through genome-wide association studies of BMD. HBM may also arise as a monogenic skeletal disorder, due to abnormalities in bone formation, bone resorption, and/or bone turnover. Individuals with monogenic disorders of HBM usually, though not invariably, have other skeletal abnormalities (such as mandible enlargement) and thus are best regarded as having a skeletal dysplasia rather than just isolated high BMD. A binary etiological division of HBM into polygenic vs. monogenic, however, would be excessively simplistic: the phenotype of individuals carrying rare variants of large effect can still be modified by their common variant polygenic background, and by the environment. HBM disorders-whether predominantly polygenic or monogenic in origin-are not only interesting clinically and genetically: they provide insights into bone processes that can be exploited therapeutically, with benefits both for individuals with these rare bone disorders and importantly for the many people affected by the commonest bone disease worldwide-i.e., osteoporosis. In this review we detail the genetic architecture of HBM; we provide a conceptual framework for considering HBM in the clinical context; and we discuss monogenic and polygenic causes of HBM with particular emphasis on anabolic causes of HBM.

Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia

X-linked hypophosphataemia (XLH) is the most common cause of inherited phosphate wasting and is associated with severe complications such as rickets, lower limb deformities, pain, poor mineralization of the teeth and disproportionate short stature in children as well as hyperparathyroidism, osteomalacia, enthesopathies, osteoarthritis and pseudofractures in adults. The characteristics and severity of XLH vary between patients. Because of its rarity, the diagnosis and specific treatment of XLH are frequently delayed, which has a detrimental effect on patient outcomes. In this Evidence-Based Guideline, we recommend that the diagnosis of XLH is based on signs of rickets and/or osteomalacia in association with hypophosphataemia and renal phosphate wasting in the absence of vitamin D or calcium deficiency. Whenever possible, the diagnosis should be confirmed by molecular genetic analysis or measurement of levels of fibroblast growth factor 23 (FGF23) before treatment. Owing to the multisystemic nature of the disease, patients should be seen regularly by multidisciplinary teams organized by a metabolic bone disease expert. In this article, we summarize the current evidence and provide recommendations on features of the disease, including new treatment modalities, to improve knowledge and provide guidance for diagnosis and multidisciplinary care.

Impact of Conventional Medical Therapy on Bone Mineral Density and Bone Turnover in Adult Patients with X-Linked Hypophosphatemia: A 6-Year Prospective Cohort Study

X-linked hypophosphatemia (XLH) is a rare, inheritable disorder manifesting as rickets in children and osteomalacia in adults. While conventional medical treatment with oral phosphate and alfacalcidol is recommended in childhood, it is undecided whether adults should continue therapy. The aim of this 6-year prospective study was to determine the impact of conventional medical treatment on areal bone mineral density (aBMD), bone turnover markers (BTMs) and measures of calcium homeostasis in 27 adult patients with XLH, 11 of whom received medical treatment. Lumbar spine and total hip aBMD, as assessed by DXA, and biochemical measures of calcium, phosphate, PTH, 1,25 dihydroxyvitamin D₂₊₃ (1,25(OH)₂D), fibroblast growth factor 23 (FGF23), P1NP and CTX were measured at baseline and at follow-up. The renal tubular reabsorption of PO₄ (TmPO₄/GFR) was calculated at both time points. Multilevel mixed-effects linear regression models were used for analyses. During the study period, spine and hip aBMD did not change significantly between treated and non-treated XLH patients. There was a trend towards a decrease in calcium, phosphate and TmPO₄/GFR in the treatment group (p = 0.057, p = 0.080 and p = 0.063, respectively), whereas PTH, FGF23, 1,25(OH)₂D and P1NP did not change significantly in either groups. However, CTX increased significantly in the treated compared to non-treated group (p = 0.044). Continuing conventional medical therapy in adulthood, although associated with increased bone resorption, does not promote or prevent loss of bone mass as evidenced from the stable aBMD of the hip and spine in XLH patients.

Serum phosphorus levels and fracture following renal transplantation

PURPOSE

Increased fracture rates are observed in renal transplant recipients (RTRs) compared with the general population. Risk factors include age, diabetes, dialysis vintage, immunosuppression and mineral and bone disorders.¹ Low serum phosphorus levels occur post-transplantation; however, its relationship with fracture risk has not been evaluated. The purpose of this study was to evaluate risk factors for fracture in RTRs at a single tertiary referral centre.

METHODS

A retrospective cross-sectional analysis of 146 patients (75 M, 71 F) who had been referred for dual energy X-ray densitometry (DXA) post-renal transplantation was performed. Aetiology of end stage kidney disease (ESKD), duration of dialysis, parathyroidectomy history, immunosuppression regimen, bone mineral density (BMD), biochemistry and fractures were documented. Statistical analyses included univariable and multivariable regression.

RESULTS

The mean age of patients was 54 years and mean time post-transplantation 6.7 years. A total of 79 fractures occurred in 52 patients (35%), with 40 fractures occurring post-transplantation. Ankle/foot fractures were most common (48%). Lower serum phosphorus levels and declining femoral neck (FN) T-score and were associated with fractures in both univariable and multivariable regression analyses after adjusting for age, gender, weight, estimated glomerular filtration rate and pre-transplant history of fracture (P=.011 and P=.042 respectively). The relationship between serum phosphorus and fracture remained significant independent of FN T-score, parathyroid hormone levels, parathyroidectomy status and prednisolone use.

CONCLUSION

Fracture was common post-renal transplantation. Lower serum phosphorus levels and declining FN T-scores were associated with fractures. The mechanism of this previously unreported observation requires further evaluation in prospective studies.

Evaluation of bone mineral density and microarchitectural parameters by DXA and HR-pQCT in 37 children and adults with X-linked hypophosphatemic rickets

In X-linked hypophosphatemic (XLH) rickets, dual-energy X-ray absorptiometry (DXA) measurements must be analyzed with caution. High-resolution peripheral quantitative computed tomography (HR-pQCT) analysis suggested that XLH primarily affects the cancellous compartment, with the tibia more affected than the radius. Effective treatment of XLH appears to positively affect bone mineralization, mainly in the bone cortex.

INTRODUCTION

The purpose of this study is to evaluate bone mineral density (BMD) and microarchitecture in 37 patients (13 children and 24 adults) with XLH confirmed by PHEX mutations from a tertiary center compared to healthy controls.

METHODS

Areal BMD (aBMD) was evaluated by DXA, whereas volumetric BMD (vBMD) and microarchitectural parameters were analyzed by HR-pQCT.

RESULTS

Adult XLH patients had higher lumbar aBMD (p < 0.01) than the controls. At the radius, the vBMD was similar between XLH patients and controls. At the tibia, XLH patients had lower total vBMD (p = 0.04), likely resulting from decreased trabecular vBMD (p < 0.01), and this difference was observed in the children and adult groups. Analysis based on metabolic status showed that the adult XLH patients with non-compensated disease had lower cortical vBMD at the tibia than the compensated XLH patients (p = 0.03). The microarchitectural differences at the radius and tibia included lower trabecular number (p < 0.01), greater trabecular separation (p < 0.01), and higher trabecular network inhomogeneity (p < 0.01) in XLH patients compared to their controls. At the radius, adults exhibited greater trabecular deficits than were seen in children.

CONCLUSIONS

In XLH patients, DXA measurements must be analyzed with caution due to the interference of anatomic and anthropometric factors. HR-pQCT analysis suggested that XLH primarily affects the cancellous compartment, with the tibia more affected than the radius. Effective treatment of XLH appears to positively affect bone mineralization, mainly in the bone cortex.

X-linked hypophosphatemia and growth

X-Linked hypophosphatemia (XLH) is the most common form of hereditary rickets caused by loss-of function mutations in the PHEX gene. XLH is characterized by hypophosphatemia secondary to renal phosphate wasting, inappropriately low concentrations of 1,25 dihydroxyvitamin D and high circulating levels of fibroblast growth factor 23 (FGF23). Short stature and rachitic osseous lesions are characteristic phenotypic findings of XLH although the severity of these manifestations is highly variable among patients. The degree of growth impairment is not dependent on the magnitude of hypophosphatemia or the extent of legs´ bowing and height is not normalized by chronic administration of phosphate supplements and 1α hydroxyvitamin D derivatives. Treatment with growth hormone accelerates longitudinal growth rate but there is still controversy regarding the potential risk of increasing bone deformities and body disproportion. Treatments aimed at blocking FGF23 action are promising, but information is lacking on the consequences of counteracting FGF23 during the growing period. This review summarizes current knowledge on phosphorus metabolism in XLH, presents updated information on XLH and growth, including the effects of FGF23 on epiphyseal growth plate of the Hyp mouse, an animal model of the disease, and discusses growth hormone and novel FGF23 related therapies.

Cervical ossification of posterior longitudinal ligament in x-linked hypophosphatemic rickets revealing homogeneously increased vertebral bone density

There is no report that describes in detail the radiological and intraoperative findings of rickets with symptomatic cervical ossification of the posterior longitudinal ligament. Here, we describe a case of X-linked hypophosphatemic rickets with cervical ossification of the posterior longitudinal ligament presenting unique radiological and intraoperative findings. The patient presented progressive tetraparesis. Magnetic resonance imaging studies revealed severe cervical spinal cord compression caused by ossification of the posterior longitudinal ligament. Computed tomography scans revealed homogeneously increased vertebral bone density. An expansive laminoplasty was performed. At surgery, homogeneously hard lamina bone was burdened in drilling and opening of the laminae. The patient's neurological symptoms were improved postoperatively. Bony fusion of the hinges occurred postoperatively. Therefore, expansive laminoplasty could be performed for symptomatic cervical ossification of the posterior longitudinal ligament with X-linked hypophosphatemic rickets. However, unusual bone characters should be taken into consideration for careful operation during surgery.

Bone geometry, volumetric density, microarchitecture, and estimated bone strength assessed by HR-pQCT in adult patients with hypophosphatemic rickets

Hypophosphatemic rickets (HR) is characterized by a generalized mineralization defect. Although densitometric studies have found the patients to have an elevated bone mineral density (BMD), data on bone geometry and microstructure are scarce. The aim of this cross-sectional in vivo study was to assess bone geometry, volumetric BMD (vBMD), microarchitecture, and estimated bone strength in adult patients with HR using high-resolution peripheral quantitative computed tomography (HR-pQCT). Twenty-nine patients (aged 19 to 79 years; 21 female, 8 male patients), 26 of whom had genetically proven X-linked HR, were matched with respect to age and sex with 29 healthy subjects. Eleven patients were currently receiving therapy with calcitriol and phosphate for a median duration of 29.1 years (12.0 to 43.0 years). Because of the disproportionate short stature in HR, the region of interest in HR-pQCT images at the distal radius and tibia were placed in a constant proportion to the entire length of the bone in both patients and healthy volunteers. In age- and weight-adjusted models, HR patients had significantly higher total bone cross-sectional areas (radius 36%, tibia 20%; both p < 0.001) with significantly higher trabecular bone areas (radius 49%, tibia 14%; both p < 0.001) compared with controls. In addition, HR patients had lower total vBMD (radius -20%, tibia -14%; both p < 0.01), cortical vBMD (radius -5%, p < 0.001), trabecular number (radius -13%, tibia -14%; both p < 0.01), and cortical thickness (radius -19%; p < 0.01) compared with controls, whereas trabecular spacing (radius 18%, tibia 23%; p < 0.01) and trabecular network inhomogeneity (radius 29%, tibia 40%; both p < 0.01) were higher. Estimated bone strength was similar between the groups. In conclusion, in patients with HR, the negative impact of lower vBMD and trabecular number on bone strength seems to be compensated by an increase in bone diameter, resulting in HR patients having normal estimates of bone strength. © 2014 American Society for Bone and Mineral Research.

Bone Mineral Density in Children From Anthropological and Clinical Sciences: A Review

Bone mineral density (BMD) is a frequent topic of discussion in the clinical literature in relation to the bone health of both adults and children. However, in archaeological and/ or anthropological studies the role of BMD is often cited as a possible factor in the poor skeletal preservation which can lead to an under-representation of juvenile skeletal remains. During skeletal development and growth throughout childhood and adolescence changes take place in both the size and shape of bones and these changes also result in the increasing of mineral content. BMD can be affected by many factors, which include, age, genetics, sexual maturation, amount of physical activity and dietary calcium. This paper aims to review the clinical and anthropological literature on BMD and discuss the numerous methods of measurement and how the availability of certain methods such as Dual-energy x-ray absorptiometry (DEXA) and quantitative computed tomography (QCT) can influence the study of bone density in archaeological skeletal collections and also the future potential for forensic anthropological studies.

Successful Medical Therapy for Hypophosphatemic Rickets due to Mitochondrial Complex I Deficiency Induced de Toni-Debré-Fanconi Syndrome

Primary de Toni-Debré-Fanconi syndrome is a non-FGF23-mediated hypophosphatemic disorder due to a primary defect in renal proximal tubule cell function resulting in hyperphosphaturia, renal tubular acidosis, glycosuria, and generalized aminoaciduria. The orthopaedic sequela and response to treatment of this rare disorder are limited in the literature. Herein we report a long term followup of a 10-year-old female presenting at 1 year of age with rickets initially misdiagnosed as vitamin D deficiency rickets. She was referred to the metabolic bone and genetics clinics at 5 years of age with severe genu valgum deformities of 24 degrees and worsening rickets. She had polyuria, polydipsia, enuresis, and bone pain. Diagnosis of hypophosphatemic rickets due to de Toni-Debré-Fanconi syndrome was subsequently made. Respiratory chain enzyme analysis identified a complex I mitochondrial deficiency as the underlying cause. She was treated with phosphate (50-70 mg/kg/day), calcitriol (30 ng/kg/day), and sodium citrate with resolution of bone pain and normal growth. By 10 years of age, her genu valgus deformities were 4 degrees with healing of rickets. Her excellent orthopaedic outcome despite late proper medical therapy is likely due to the intrinsic renal tubular defect that is more responsive to combined alkali, phosphate, and calcitriol therapy.

High bone mineral apparent density in children with X-linked hypophosphatemia

Bone mineral apparent density (BMAD) in children with X-linked hypophosphatemia (XLH) was evaluated, as they are unlikely to have extra-skeletal ossifications contributing to the elevated bone mineral density of the spine in adult patients. Children with XLH also had significantly higher BMAD of the spine compared to femoral neck.

INTRODUCTION

BMAD obtained by dual-energy X-ray absorptiometry scans in children with XLH was evaluated, as they are unlikely to have the extra-skeletal ossifications contributing to the elevated bone mineral density of the spine in adult patients.

METHODS

A total of 15 children with biochemically and genetically verified XLH were recruited. Anthropometric measurements were performed, and to correct for the short stature (small bones), the BMAD of the spine and the femoral neck was evaluated.

RESULTS

Z-scores of BMAD of the spine (mean (95 % CI); 2.0 (1.3-2.7); p < 0.001) were significantly elevated compared to reference children. Z-scores of the femoral neck (1.0 (-0.0 to 2.1); p = 0.059) tended to be elevated. Spine Z-scores were significantly higher than the Z-scores of the femoral neck, (paired t test, p = 0.02). BMAD of the spine was evaluated according to the Molgaard's approach; XLH children had normal bone size of the spine for age due to a normal sitting height Z-score of -0.4 (-1.0 to 0.1); p = 0.1. Z-scores of bone mineral content (BMC) of the spine for bone area were elevated (1.4 (0.8-2.1); p < 0.001). No reference data were available to allow evaluation of the BMAD of the femoral neck by the Molgaard's approach.

CONCLUSIONS

Children with XLH have an increased BMAD and a high BMC for bone area at the lumbar spine, and this was due to causes other than extra-skeletal ossifications and corrected for bone size. The BMAD of the spine was significantly higher compared to the femoral neck.

Cortical and trabecular bone density in X-linked hypophosphatemic rickets

CONTEXT

X-linked hypophosphatemic rickets is caused by mutations in PHEX. Even though the disease is characterized by disordered skeletal mineralization, detailed bone densitometric studies are lacking.

OBJECTIVE

The aim of the study was to assess volumetric bone mineral density (vBMD) in X-linked hypophosphatemic rickets using forearm peripheral quantitative computed tomography.

SETTING

The study was conducted in the metabolic bone clinic of a pediatric orthopedic hospital.

PATIENTS

Thirty-four patients (age, 6 to 60 years; 24 female) with PHEX mutations were studied, of whom 7 children (age, 6 to 11 years) were actively being treated with calcitriol and phosphate supplementation. Twenty-one patients (age, 16 to 40 years) had received the same therapy before but had discontinued the treatment; 6 patients (age, 12 to 60 years) had never received this treatment.

MAIN OUTCOME MEASURES

Trabecular and cortical vBMD of the radius.

RESULTS

Trabecular vBMD was elevated (mean age-specific and sex-specific z-score: +1.0) when all patients were analyzed together, due to very high results in currently treated patients (mean z-score: +2.4) and slightly above-average mean values in the other patients. Cortical vBMD was low when the entire cohort was analyzed together (mean z-score: -3.3), but was higher in currently treated patients (mean z-score: -1.3) than in patients who had discontinued therapy (mean z-score: -3.8) or who had never been treated (mean z-score: -4.1).

CONCLUSIONS

Patients with PHEX mutations have elevated trabecular vBMD at the distal radius while receiving calcitriol and phosphate supplementation, but low cortical vBMD at the radius diaphysis. Low cortical vBMD presumably reflects the underlying mineralization defect that is not entirely corrected by current treatment approaches.

Hypophosphatemic rickets: unraveling the role of FGF23

The classification of the various forms of hypophosphatemic rickets has been rationalized by the discovery of the central role that fibroblast growth factor 23 (FGF23) plays in the pathogenesis of a number of genetic and acquired forms of the disease. Although the details of the interaction of FGF23 with other osteoblast/osteocyte-derived proteins remain unclear at present, the measurement of circulating levels of FGF23 appears to be a useful biochemical test in determining the various causes of hypophosphatemic rickets. Furthermore, animal studies suggest that agents interfering in the action of FGF23 might play important roles in the future management of the FGF23-mediated forms of rickets. Phase 1 and phase 2 trials in humans with X-linked hypophosphatemic rickets are currently under way.

Clinical practice: diagnostic approach to the rachitic child

Rickets remains a common problem among infants and children in many countries worldwide. Although the classical presentation associated with bone abnormalities is well known, paediatricians need to be aware of atypical presentations, especially in the first 6 months of life. Furthermore, although vitamin D deficiency rickets remains the commonest form of rickets in most countries, health care providers need to be aware of other possible causes and their typical clinical and biochemical presentations. This article discusses these and highlights the characteristic features of various forms of rickets and possible pitfalls clinicians should be aware of when confronted with a patient with suspected rickets. In conclusion, the recent advances made in understanding the underlying pathogeneses of the various forms of rickets has helped to delineate the diagnostic tests that assist in the diagnosis and management of the disease in children.

Novel PHEX Mutation Associated with Hypophosphatemic Rickets

BACKGROUND

X-linked hypophosphatemia (XLH) is the most prevalent heritable form of rickets. It is a dominantly inherited disorder, characterized by renal phosphate wasting, abnormal vitamin D and PTH metabolism, and defective bone mineralization. Inactivating mutations in the gene encoding PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) have been found to be associated with XLH.

METHODS

We report about a 54-year-old male patient who exhibited the typical features of XLH, and in whom mutational analysis using PCR and sequencing was performed. Additionally, extensive laboratory and radiological investigations were carried out.

RESULTS

A 1-bp deletion in exon 2 of the PHEX gene was detected (177delC), which, to the best of our knowledge, has not been reported yet. This deletion results in a premature stop codon (C59X), suggesting a truncation of the PHEX protein. Furthermore, elevated FGF23 and PTH levels as well as an increased axial bone mineral density score were measured.

CONCLUSIONS

We present a male patient with XLH, who harbors a novel mutation in the PHEX gene, which might be the cause for his disease. Our data support previous findings and therefore contribute to the decipherment of the pathogenetic pathways of XLH.

Assessment of bone quality by quantitative ultrasound of proximal phalanges of the hand and fracture rate in children and adolescents with bone and mineral disorders

Bone quality by quantitative ultrasound and fracture rate were assessed in 135 (64 males) children and adolescents aged 3-21 y with bone and mineral disorders such as chronic anticonvulsants or glucocorticoids treatment, juvenile rheumatoid arthritis, celiac disease, paucity of intrahepatic bile ducts, autoimmune hepatitis, genetic diseases, idiopathic juvenile osteoporosis, disuse osteoporosis, beta-thalassemia major, survivors of acute lymphoblastic leukemia, liver transplantation, calcium deficiency, and nutritional or X-linked hypophosphatemic rickets. Amplitude-dependent speed of sound through the distal end of the first phalangeal diaphysis of the last four fingers of the hand was measured by an ultrasound device. In the majority of patients cortical area to total area ratio by metacarpal radiogrammetry (n = 120) and lumbar bone mineral density (BMD) by dual-energy x-ray absorptiometry (n = 99) were also assessed. In patients with X-linked hypophosphatemic rickets radial BMD by single-photon absorptiometry instead of lumbar BMD was measured. Mean values of amplitude-dependent speed of sound, cortical area to total area ratio, lumbar BMDarea, or lumbar BMD corrected for bone sizes estimated by a mathematical model (BMDvolume), as well as mean values of radial BMD in patients with X-linked hypophosphatemic rickets, expressed as z score, were significantly reduced (p < 0.0001) in comparison with their reference values (-1.7 +/- 1.0, -2.0 +/- 0.9, -3.0 +/- 1.3, -1.9 +/- 1.0, -2.7 +/- 0.7, respectively). A positive relationship was found between amplitude-dependent speed of sound and cortical area to total area ratio (r = 0.90, p < 0.0001), lumbar BMDarea (r = 0.62, p < 0.0001), or lumbar BMDvolume (r = 0.66, p < 0.0001). Fifty-two patients (38.5%) had suffered fractures in the 6 mo preceding the bone measurements, the radial distal metaphysis being the most frequent fracture site (28.8%). Mean values of amplitude-dependent speed of sound, cortical area to total area ratio, lumbar BMDarea, or lumbar BMDvolume, expressed as z score, of fractured patients were significantly lower (p < 0.0001) than those of fracture-free patients (-2.2 +/- 1.0 and -1.4 +/- 0.8, -2.6 +/- 0.9 and -1.7 +/- 0.7, -3.5 +/- 1.2 and -2.5 +/- 1.0, -2.5 +/- 1.0 and -1.3 +/- 0.7, respectively). Phalangeal quantitative ultrasound may be a useful method to assess bone quality and fracture risk in children and adolescents with bone and mineral disorders.

Bone densitometry of a patient with osteosclerosis

A 30-yr-old Caucasian man with a history of dorsal and lumbar back pain, which responded partially to antiinflammatory agents, was seen at our Unit. The biochemical bone markers showed an increment in bone alkaline phosphatase and urinary CTX. Serum phosphate tended to be low. Radiographic abnormalities were marked osteosclerosis in the pelvis and vertebral bodies without changes in size. Bone scintigraphy results were normal. The increase in bone mineral density (BMD) was greater in L2-L4 (+ 3.9 SDs) than in total skeleton (+ 1.4 SDs). Analysis of skeletal subareas showed a marked increase in axial skeleton BMD: trunk, +4.0 SDs; spine, +2.5 SDs and pelvis, +4.5 SDs. BMD of the remaining subareas was found to be normal: skull, +0.04 SDs; arms, -0.3 SDs and legs, -0.05 SDs. The patient refused to have a bone biopsy. The radiologic, densitometric, and biochemical findings in the patient presented herein are compatible with axial osteomalacia. Evaluation of total skeleton BMD, and especially skeletal subareas, clearly indicated that the abnormal BMD was restricted to the spine and pelvis whereas the rest of the skeleton was not affected.

Ossification of the posterior longitudinal ligament in vitamin D-resistant rickets: case report and review of the literature

STUDY DESIGN

A case report of cervical myelopathy caused by ossification of the posterior longitudinal ligament in a patient with vitamin D-resistant rickets is presented together with a review of literature.

OBJECTIVE

To report the diagnosis of ossification of the posterior longitudinal ligament in a white woman with vitamin D-resistant rickets.

SUMMARY OF BACKGROUND DATA

The association between ossification of the posterior longitudinal ligament and untreated vitamin D-resistant rickets has been reported in Japan, but infrequently in white populations. In whites, ossification of the posterior longitudinal ligament is closely associated with diffuse idiopathic skeletal hyperostosis. A clear association between ossification of the posterior longitudinal ligament and vitamin D-resistant rickets in white populations has not yet been established.

METHODS

The medical record and imaging studies of a patient treated at the authors' institution for cervical myelopathy caused by ossification of the posterior longitudinal ligament in the setting of treated vitamin D-resistant rickets were reviewed. A Medline search of the medical literature between 1966-1999 was performed to identify pertinent studies and similar case reports.

RESULTS

The occurrence of spinal stenosis in untreated adults with vitamin D-resistant rickets has been reported in all regions of the spine in Japanese patients. The association between ossification of the posterior longitudinal ligament and untreated vitamin D-resistant rickets was first reported in Japan, where ossification of the posterior longitudinal ligament is endemic. This association may be incidental, because reports on ossification of the posterior longitudinal ligament in whites are not as frequent as in Japanese, reflecting the higher prevalence of this condition in Japan.

CONCLUSION

Ossification of the posterior longitudinal ligament and ossification of the posterior longitudinal ligament associated with deranged calcium or phosphate metabolism may be different pathologic entities sharing a common outcome. Adequate treatment of vitamin D-resistant rickets may not always prevent or reverse ossification of the posterior longitudinal ligament.

Effect of growth hormone treatment on final height, phosphate metabolism, and bone mineral density in children with X-linked hypophosphatemic rickets

OBJECTIVE

To evaluate the effect of growth hormone treatment on final height, phosphate metabolism, bone markers, and bone mineral density in children with X-linked hypophosphatemic rickets.

STUDY DESIGN

Six patients (aged 7.8 +/- 1.8 years; height z score, -3.4 +/- 0.5) received conventional treatment (1,25-dihydroxyvitamin D(3) plus phosphate salts) combined with growth hormone (0.6-0.9 IU/kg per week, subcutaneously) (group A); 6 patients (aged 7.9 +/- 2.5 years; height z score, -2.1 +/- 0.6, P <.01 with respect to group A) received only conventional treatment (group B).

RESULTS

Final height z score significantly improved in group A (-2.4 +/- 0.5, P <.03 with respect to the value at entry), whereas it did not change in group B (-2.8 +/- 0.5, P = NS). At final height, degree of body disproportion was similar between the groups (group A, 1.3 +/- 0.2; group B, 1.2 +/- 0.1; P = NS). Phosphate retention, bone markers, and radial bone mineral density increased only in group A. No adverse effects were observed.

CONCLUSIONS

In poorly growing patients with X-linked hypophosphatemic rickets, growth hormone therapy combined with conventional treatment improves final height, phosphate retention, and radial bone mineral density, but it does not influence degree of body disproportion.

[Growth hormone treatment of familial hypophosphatemic rickets]

X-linked hypophosphatemic rickets (XLHR) is frequently associated with growth retardation and short adult stature, even with an appropriate conventional treatment associating phosphate and calcitriol or 1 alpha-hydroxyvitamin D. Its pathogenesis is unclear; growth hormone (GH) secretion is usually normal. Six children with XLHR and growth retardation were treated with GH during 6 years. In addition, they received the conventional treatment. At the beginning of the treatment mean age was 7.8 +/- 1.8 years, and height mean Z score was -3.4 +/- 0.5. A control group was composed of six children with XLHR (age: 7.9 +/- 2.5 years) receiving the conventional treatment only. Under GH treatment statural growth was improved, with significant increase in Z score and predicted adult height; the height gain was significantly higher in the GH treated group as compared with the group receiving the conventional treatment only. In addition, radial bone mineral density increased significantly under GH treatment. GH treatment thus appears to be a useful treatment to improve statural growth in children with XLHR.

Bone metabolism and bone mineral density in childhood hypophosphatasia

Childhood hypophosphatasia (HP) is an inborn error of bone metabolism, characterized by a reduced tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). Bone mineral density (BMD) in childhood HP has not been reported so far. We measured BMD, in addition to markers of bone metabolism, in 6 boys with childhood HP (age 2-13 years) and in 10 parents and one sibling, each carrying the autosomal-recessive trait. BMD, measured by peripheral quantitative computed tomography (pQCT) and by dual-energy X-ray absorptiometry (DXA), was expressed as standard deviation from the normal mean (Z scores) and compared with bone mineralization by standard X-rays of the same locations. HP was confirmed by low TNSALP and its elevated substrates. In patients, BMD of the total body or spine, measured by DXA, was in the lower normal range. Total BMD of the distal metaphyses of the radius, measured by pQCT, was normal (Z score: mean +1.5). However, trabecular BMD of both radius (Z score: mean +4) and femur was grossly elevated. In parallel, X-rays showed reduced transradiancy of the radial and femoral metaphyses in all patients. In parents of patients with HP, BMD was in the lower normal range. Only one father had a decreased lumbar BMD. The discrepancy in BMD between cortical and trabecular bone of the metaphyses in patients was not present in their relatives. TNSALP levels in all first degree relatives were just below or in the lower normal range. The markers of bone turnover hydroxyproline and free deoxypyridinoline in urine were normal in patients and relatives. Hypermineralization/sclerosis of trabecular bone might serve as a compensation for a mechanically incompetent bony structure due to an impaired mineralization of cartilage caused by the genetic deficiency of alkaline phosphatase.

Long-term growth hormone treatment in children with renal hypophosphatemic rickets: effects on growth, mineral metabolism, and bone density

OBJECTIVE

To evaluate the effects of treatment with recombinant human growth hormone (rhGH) on growth, mineral metabolism, and bone density in children with renal hypophosphatemic rickets (RHR).

DESIGN

Long-term rhGH treatment combined with conventional therapy with 1,25-dihydroxyvitamin D3 plus inorganic phosphate salts.

SETTING

Endocrine unit, department of pediatrics, university hospital.

SUBJECTS

Twelve patients (5 boys; age range 4.6 to 12.5 years, median 7.0 years) were subdivided into two groups of six patients on the basis of the median of height z score (-2.41) and the median bone age/statural age (BA/SA) ratio (1.23). Group A included patients with a severe degree of short stature (height z score -3.4 +/- 0.5) (mean +/- SD) and altered BA/SA ratio (1.26 +/- 0.08); group B included patients with a lesser degree of short stature (height z score -2.1 +/- 0.6, p < 0.001 vs group A) and more normal BA/SA ratio (1.04 +/- 0.15, p < 0.01 vs group A).

INTERVENTION

Group A received rhGH treatment (0.6 IU/kg per week subcutaneously) combined with conventional therapy; group B received conventional therapy alone.

MEASUREMENTS

Height, growth velocity, predicted adult height, serum values of calcium, phosphate, bone alkaline phosphatase isoenzyme, osteocalcin, propeptides of type I and type III procollagen, intact parathyroid hormone, 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, and urinary calcium/urinary creatinine ratio and tubular maximum for phosphate reabsorption normalized to the glomerular filtration rate (TmP/GFR), as well as radial bone density, were measured at baseline and for 3 years.

RESULTS

Height z score, growth velocity z score, predicted adult height, serum values of phosphate, bone alkaline phosphatase isoenzyme, osteocalcin, propeptides of type I and type III procollagen, intact parathyroid hormone 1,25-dihydroxyvitamin D, and TmP/GFR, as well as radial bone density, improved significantly only in group A. Serum calcium and 25-hydroxyvitamin D, and urinary calcium/urinary creatinine ratio did not change in either group.

CONCLUSIONS

Long-term rhGH administration may benefit growth, phosphate retention, and bone density in patients with RHR, without evidence of side effects.

Autonomous hyperparathyroidism in X-linked hypophosphataemia

Four patients with familial hypophosphataemic rickets developed significant hypercalcaemia which persisted after discontinuation of vitamin D therapy. They had increased PTH levels and were operated for hyperparathyroidism at the ages of 18, 20, 24 and 45 years, respectively. Three of the patients had previously received phosphate treatment and one patient developed hyperparathyroidism 7 years after treatment with calcitriol. Histological evaluation revealed different degrees of parathyroid hyperplasia in all patients, with persistently increased PTH and/or calcium levels after surgery. The possibility of autonomous hyperparathyroidism should be evaluated in the follow-up of patients with X-linked hypophosphataemic rickets.

Bone metabolism and mineralisation after cytotoxic chemotherapy including ifosfamide

Lumbar spine bone mineral density and bone mineral metabolism were studied in 13 children three months or more after completion of cytotoxic chemotherapy that included ifosfamide given for different malignancies. Blood and urine were analysed for calcium, phosphorus, and magnesium and blood for alkaline phosphatase activity, parathyroid hormone, and 1,25(OH)2 vitamin D3. Bone mineral density (BMD) was measured at the lumbar spine (L1-L4) using a commercial dual x ray absorptiometer. Serum concentrations of calcium, phosphorus, and magnesium and alkaline phosphatase activity, as well as plasma 1,25(OH)2 vitamin D3 concentrations were normal in all children. Slightly raised parathyroid hormone concentrations were seen in two children. An increased urinary excretion of calcium was found in five children. Mean (SD) BMD of the children was -0.88 (1.44). Three children had osteopenia, as defined by a BMD lower than -2 SD for age and sex related standards. No significant relation was found between the BMD and the biochemical parameters. In conclusion, a normal BMD was found in most patients who had received ifosfamide, even in those with persisting hypercalciuria.