Invited Mini Review Metabolic Bone Disease of Prematurity: Overview and Practice Recommendations.

Metabolic bone disease of prematurity (MBDP) is defined by undermineralization of the preterm infant skeleton arising from inadequate prenatal and postnatal calcium (Ca) and phosphate (PO4) accretion. Severe MBDP can be associated with rickets and fractures. Despite advances in neonatal nutrition, MBDP remains prevalent in premature infants due to inadequate mineral accretion ex-utero. There also remain significant knowledge gaps regarding best practices for monitoring and treatment of MBDP among neonatologists and pediatric endocrinologists. Preventing and treating MBDP can prevent serious consequences including rickets or pathologic fractures. Postnatal monitoring to facilitate early recognition of MBDP is best done by first-tier laboratory screening by measuring serum calcium, phosphorus, and alkaline phosphatase to identify infants at risk. If these labs are abnormal, further studies including assessing parathyroid hormone and/or tubular resorption of phosphate can help differentiate between Ca and PO4 deficiency as primary etiologies to guide appropriate treatment with mineral supplements. Additional research into optimal mineral supplementation for the prevention and treatment of MBDP is needed to improve long-term bone health outcomes and provide a fuller evidence base for future treatment guidelines.

Mineralized tissues in hypophosphatemic rickets.

Hypophosphatemic rickets is caused by renal phosphate wasting that is most commonly due to X-linked dominant mutations in PHEX. PHEX mutations cause hypophosphatemia indirectly, through the increased expression of fibroblast growth factor 23 (FGF23) by osteocytes. FGF23 decreases renal phosphate reabsorption and thereby increases phosphate excretion. The lack of phosphate leads to a mineralization defect at the level of growth plates (rickets), bone tissue (osteomalacia), and teeth, where the defect facilitates the formation of abscesses. The bone tissue immediately adjacent to osteocytes often remains unmineralized ("periosteocytic lesions"), highlighting the osteocyte defect in this disorder. Common clinical features of XLH include deformities of the lower extremities, short stature, enthesopathies, dental abscesses, as well as skull abnormalities such as craniosynostosis and Chiari I malformation. For the past four decades, XLH has been treated by oral phosphate supplementation and calcitriol, which improves rickets and osteomalacia and the dental manifestations, but often does not resolve all aspects of the mineralization defects. A newer treatment approach using inactivating FGF23 antibodies leads to more stable control of serum inorganic phosphorus levels and seems to heal rickets more reliably. However, the long-term benefits of FGF23 antibody treatment remain to be elucidated.