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X-Linked Hypophosphatemia

Hypophosphatemic Rickets X Linked Dominant

X-linked hypophosphatemia (XLH) is the most common form of familial hypophosphatemia.


Presentation

Generalized muscle weakness is the most common symptom across all types of familial hypophosphatemia, while myalgia and fatigue are invariably present [2] [8]. Additionally, XLH is distinguished by a childhood onset of progressive bowing deformities and short stature [8], as well as calcification of tendons, joint capsules and ligaments (known as enthesopathy) and a limited range of motion at the hips, spine and other larger joints [2] [3]. Dentinal clefts, enlarged pulp chambers, and appearance of dental abscesses are some of the most common dental abnormalities seen in hypophosphatemia patients [7]. In adults, bone pain, osteomalacia and pseudofractures, rachitis-like characteristics (genu valgum, frontal bossing, widening of the ends of long bones), rhabdomyolysis, and and hearing loss were observed.

Short Stature
  • Thus, the short stature in patients with X-linked hypophosphatemia is not due to a GH/IGF-I secretory defect.[ncbi.nlm.nih.gov]
  • Abstract Patients with X-linked hypophosphatemic rickets exhibit clinically evident derangements that include bowed legs and short stature.[ncbi.nlm.nih.gov]
  • Patients with X-linked hypophosphatemia have short stature, rickets, bone pain and dental abscesses. Current therapy is oral phosphate and vitamin D which effectively treats the rickets and bone pain but does not adequately improve short stature.[ncbi.nlm.nih.gov]
  • Anthropometric measurements were performed, and to correct for the short stature (small bones), the BMAD of the spine and the femoral neck was evaluated.[ncbi.nlm.nih.gov]
  • Short stature and rachitic osseous lesions are characteristic phenotypic findings of XLH although the severity of these manifestations is highly variable among patients.[ncbi.nlm.nih.gov]
Camping
  • Dipyridamole therapy also had no effect on serum PTH, serum 1,25 (OH)2 vitamin D, alkaline phosphatase, osteocalcin levels, urinary calcium or cAMP excretion.[ncbi.nlm.nih.gov]
  • Camp RL, Peritt D., Panettieri RA Jr, Lazaar AL, Navak S. ( 1995 ). Defective phosphorylation and hyaiuronate binding of CD44 with point mutations in the cytoplasmic domain. J Exp Med 181: 55 -62.[doi.org]
Atrial Septal Defect
  • RESULTS: The proband showed a wide spectrum of systemic anomalies, including bilateral ectopia lentis, atrial septal defect, ventricular septal defect, widening of tibial metaphysis with medial bowing, and dolichostenomelia in digits, while her mother[ncbi.nlm.nih.gov]
Intravenous Administration
  • In severe cases, intravenous administration of phosphate is necessary. In addition to supplementation, various surgical, orthopedic and dental procedures may be indicated, depending on the extent of damage caused by hypophosphatemia.[symptoma.com]
Malocclusion
  • However, only patients with XLH had Class III malocclusions and mild enamel defects, and males with XLH had more severe occlusal and enamel defects than females with XLH.[ncbi.nlm.nih.gov]
Ectopia Lentis
  • RESULTS: The proband showed a wide spectrum of systemic anomalies, including bilateral ectopia lentis, atrial septal defect, ventricular septal defect, widening of tibial metaphysis with medial bowing, and dolichostenomelia in digits, while her mother[ncbi.nlm.nih.gov]
Bone Pain
  • Patients with X-linked hypophosphatemia have short stature, rickets, bone pain and dental abscesses. Current therapy is oral phosphate and vitamin D which effectively treats the rickets and bone pain but does not adequately improve short stature.[ncbi.nlm.nih.gov]
  • Bone pain was associated with a relative osteoid volume in excess of 25%.[ncbi.nlm.nih.gov]
  • In adulthood, therapy is usually continued in symptomatic patients and generally aims at reducing bone pain. Corrective surgery of skeletal deformities may be required in some cases.[orpha.net]
  • As the affected patients mature, pseudofractures, skeletal deformities, osteomalacic bone pain, progressive ankylosis, and dental caries occur, which may be alleviated and even prevented with adequate medical therapy.[ncbi.nlm.nih.gov]
Muscle Weakness
  • Generalized muscle weakness is the most common symptom across all types of familial hypophosphatemia, while myalgia and fatigue are invariably present.[symptoma.com]
  • Muscle weakness and hypotonia are absent. Etiology The disease is caused by various mutations in the PHEX gene (Xp22.1) and is transmitted as an X-linked dominant trait with complete penetrance, but variable expressivity.[orpha.net]
  • Hypotonia and muscle weakness are absent. Causes - X-linked hypophosphatemia Prevention - X-linked hypophosphatemia Not supplied.[checkorphan.org]
Rachitic Rosary
  • Clinical description XLH manifests during childhood with typical clinical features of rickets such as short stature, bone pain, and skeletal deformities (bowed legs, genu varum, rachitic rosary...).[orpha.net]
  • Symptoms - X-linked hypophosphatemia XLH manifests during childhood with typical clinical features of rickets such as skeletal deformities (bowed legs, genu varum, rachitic rosary...), short stature, and bone pain.[checkorphan.org]
  • In infants and children, the signs and symptoms include: Abnormal "waddling" walk due to knock-knees (genu valgum) or bowed legs (genu varum) Thick wrists Ribs of the chest may show small knots, similar to beads; this is called rachitic rosary Hip deformities[dovemed.com]
Cesarean Section
  • Fifteen of these were by cesarean section, although radiologic evidence of pelvic narrowing was not found in any subject. Serum ALP was elevated in all but 3 of the 18 untreated subjects.[ncbi.nlm.nih.gov]
Seizure
  • More severe cases can present with neurological complaints (altered speech, confusion, seizures, hearing impairment), bone pain, poor teeth status, and fatigue.[symptoma.com]
  • Acute hypercalcemia (with resulting irritability, confusion, and potential seizures) can occur during treatment. Nephrocalcinosis, the long-term result of aggressive therapy, may be more damaging.[boneandspine.com]
  • MacGregor (1995) Mice lacking tissue non-specific alkaline phosphatase die from seizures due to defective metabolism of vitamin B-6. Nat Genet 11: 45–51. Weiner, S., P.M.[doi.org]
  • MEF2C Rhizomelic chondrodysplasia punctata, type 1 PEX7 Rieger-Syndrom PAX6 Roberts-Syndrom ESCO2 Rubinstein-Taybi-Syndrom CREBBP Rubinstein-Taybi-Syndrom EP300 Saethre-Chotzen Syndrom TWIST1 Seckel Syndrom ATR, CEP152, PCNT Segawa-Syndrom TH, GCH1 Seizures[meduniwien.ac.at]

Workup

Clinical suspicion of X-linked hypophosphatemia should be supported by data from patient history and findings during physical examination. A positive family history for similar symptoms or confirmed hypophosphatemia in the presence of skeletal-related complaints in childhood are highly suggestive, in which case laboratory assessment is the next step. Evaluation of serum phosphate, calcium, alkaline phosphatase (ALP), PTH, 1,25 vitamin D3, as well as kidney function tests, urinalysis to detect proteinuria, glycosuria, calciuria, and determination of renal phosphate absorption through the ratio of the maximum rate of tubular phosphate reabsorption to the glomerular filtration rate (TmP/GFR) is recommended in the initial workup [2] [9]. In addition to laboratory workup, imaging studies such as radiography and bone scintigraphy can be used to assess skeletal changes [2]. Although clinical and laboratory clinical findings are usually sufficient to confirm the diagnosis, genetic testing to confirm FGF23 mutations and further molecular studies may be performed [9].

Parathyroid Hormone Increased
  • Biochemical findings include elevated circulating levels of FGF-23 associated with hypophosphatemia, hyperphosphaturia, normal serum levels of calcium and parathyroid hormone, increased or normal plasma levels of alkaline phosphatise, normal plasma calcidiol[orpha.net]

Treatment

The goals of therapy are to reduce or abolish symptoms and complaints caused by reduced phosphate concentrations in serum, as directed therapy does not exist yet. Oral phosphorus supplementation, however together with active vitamin D analogs (for ex. alfacalcidiol or calcitriol) can be quite effective and is the mainstay of therapy for all types of hereditary hypophosphatemia [10]. The dosage of oral phosphorus ranges between 30-60 mg/kg [8] [10], while calcitriol is given in doses of 20-30 ng/kg/24h [8], but factors such as age, levels of PTH, body weight, ALP levels and the extent of skeletal changes can lead to marked variations in the dosage, ranging between 10-80 ng/kg/24h for vitamin D and 30-180 mg/kg/24h for phosphorus [10]. Because of rapid changes in phosphate concentrations due to impaired reabsorption, multiple daily doses are recommended [3]. In severe cases, intravenous administration of phosphate is necessary [8]. In addition to supplementation, various surgical, orthopedic and dental procedures may be indicated, depending on the extent of damage caused by hypophosphatemia. Metaphyseal–diaphyseal osteotomies, epiphysiodesis, physiotherapy, applications of topical fluoride on teeth as well as pit and fissure sealants have all been described as potential therapeutic measures [7] [8] [10].

Prognosis

Recognition of hypophosphatemia in early life and determination of the underlying cause is of vital importance in terms of prognosis, as the effects of debilitating sequelae such as growth impairment and dental alterations may be seen [5] [7], but more importantly, severe cases may develop life-threatening cardiorespiratory failure, which illustrates the importance of an early diagnosis.

Etiology

Genetic mutations affecting regulation of phosphate absorption and excretion in the body are the underlying cause of several forms of familial hypophosphatemia. Inactivating PHEX mutations trigger X-linked hypophosphatemia (XLH), leading to increased FGF23 concentrations, leading to decreased absorption of phosphate in the proximal renal tubules.

Epidemiology

XLS, described as the most common form, is seen in approximately 1 in 20,000 individuals [4]. Countries such as Norway, however, report XLS in 1 in 60,000 children [5].

Sex distribution
Age distribution

Pathophysiology

Phosphate is an integral electrolyte of numerous physiological functions and together with calcium, it plays a pivotal role in growth and development of the skeletal system. This electrolyte is absorbed from the gastrointestinal tract by passive and active transcellular pathways, the latter involving sodium phosphate cotransporter 2b, after which it is filtered and reabsorbed in the proximal renal tubule through the activity of two additional sodium phosphate cotransporters - NaPi2a and NaPi2c [6]. This mechanism is tightly regulated by dietary intake of phosphate, PTH, 1,25 vitamin D3, and FGF23, one of the main targets of X-linked hypophosphatemia. Mutations of FGF23 lead to reduced activity of NaPi2a and NaPi2c, causing increased phosphate wasting through urine as they are not able to reabsorb it [6].

Prevention

Genetic counselling for couples or families in whom a diagnosis of familial hypophosphatemia is made should be carried out, as other prevention strategies are not available.

Summary

X-linked hypophosphatemia (XLH) is considered as the most common form of familial hypophosphatemia (seen in 1 in 20,000 children) [1]. XLH arises due to mutations in the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (or PHEX), leading to increased expression of the fibroblast-growth-factor 23 (FGF23), which is responsible for decreased absorption of phosphate in the proximal renal tubules [2]. The onset of lower-extremity bowing, calcification of tendons, joint capsules and ligaments (known as enthesopathy) in the first few years of life is typically encountered [3].

Patient Information

Together with parathyroid hormone (PTH), vitamin D, and calcium, phosphate is one of the most important molecules involved in skeletal growth, but in dozens of other metabolic pathways as well. X-linked hypophosphatemia (XLH) impairs the ability of the body to maintain normal levels of phosphate. Symptoms usually occur in early childhood, most common being generalized muscle weakness and skeletal alterations - short stature, bowing of the limbs, and reduced range of motion in the spine, hips and shoulders. More severe cases can present with neurological complaints (altered speech, confusion, seizures, hearing impairment), bone pain, poor teeth status, and fatigue. The diagnosis can be made by obtaining a thorough patient history that will determine the presence of hypophosphatemia within the family and by observing symptoms and signs during physical examination. Laboratory studies are needed to confirm the disorder, however, and assessment of serum phosphate and calcium levels, parathyroid hormone (PTH), vitamin D, kidney function tests and urinalysis is necessary. In some cases, genetic testing can be conducted. Treatment aims to correct the levels of phosphate through long-term oral supplementation with phosphorus, but it must be administered simultaneously with vitamin D, which is necessary for its adequate absorption. If patients are compliant, treatment can alleviate the majority of symptoms, but an early recognition of hypophosphatemia carries a much better prognosis, emphasizing the need for physicians to consider this group of disorders in the differential diagnosis of skeletal abnormalities in both children and adults.

References

Article

  1. Cho HY, Lee BH, Kang JH, Ha IS, Cheong HI, Choi Y. A clinical and molecular genetic study of hypophosphatemic rickets in children. Pediatr Res. 2005;58(2):329-333.
  2. Imel EA, Econs MJ. Approach to the Hypophosphatemic Patient. J Clin Endocrinol Metab. 2012;97(3):696-706.
  3. Ruppe MD. X-Linked Hypophosphatemia. 2012 Feb 9 [Updated 2014 Oct 16]. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2016.
  4. Pavone V, Testa G, Gioitta Iachino S, Evola FR, Avondo S, Sessa G. Hypophosphatemic rickets: etiology, clinical features and treatment. Eur J Orthop Surg Traumatol. 2015;25(2):221-226.
  5. Rafaelsen S, Johansson S, Ræder H, Bjerknes R. Hereditary hypophosphatemia in Norway: a retrospective population-based study of genotypes, phenotypes, and treatment complications. Eur J Endocrinol. 2015;174(2):125-136.
  6. Gattineni J. Inherited Disorders of Calcium and Phosphate Metabolism. Curr Opin Pediatr. 2014;26(2):215-222.
  7. Souza MA, Valente Soares Junior LA, dos Santos MA, Vaisbich MH. Dental abnormalities and oral health in patients with Hypophosphatemic rickets. Clinics. 2010;65(10):1023-1026.
  8. Carpenter TO, Imel EA, Holm IA, Jan de Beur SM, Insogna KL. A Clinician's Guide to X-linked Hypophosphatemia. J Bone Miner Res. 2011;26(7):1381-1388.
  9. Lee JY, Imel EA. The Changing Face of Hypophosphatemic Disorders in the FGF-23 Era. Pediatr Endocrinol Rev. 2013;10(0 2):367-379.
  10. Linglart A, Biosse-Duplan M, Briot K, et al. Therapeutic management of hypophosphatemic rickets from infancy to adulthood. Endocr Connect. 2014;3(1):R13-R30.

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Last updated: 2019-07-11 20:32