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Early Treatment Improves Growth and Biochemical and Radiographic Outcome in X-Linked Hypophosphatemic Rickets

Division of Endocrinology (O.M., S.W.K., E.S.), Diagnostic Imaging (A.Do., A.Da.), and Orthopedic Surgery (W.G.C.), The Hospital for Sick Children, University of Toronto, Toronto, Ontario, Canada M5G 1X8

Address all correspondence and requests for reprints to: Outi Mäkitie, M.D., Ph.D., Division of Endocrinology, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8. E-mail: outi.makitie{at}helsinki.fi.

X-Linked hypophosphatemic rickets (XLH) is characterized by hypophosphatemia, rickets, and impaired growth. Despite oral phosphate and 1,25-dihydroxyvitamin D₃ treatment, many patients have suboptimal growth and bone healing. The aim of this study was to assess whether age at treatment onset impacts the outcome. Growth data, biochemistry, and radiographs of 19 well-controlled patients with XLH were analyzed retrospectively. Patients were divided into two groups based on the age at treatment onset (group 1, <1.0 yr; group 2, 1.0 yr). The median height z-score was higher in group 1 (n = 8) than in group 2 (n = 11) at treatment onset [-0.4 SD score (SDS) vs. -1.7 SDS; P = 0.001], at the end of the first treatment year (-0.7 SDS vs. -1.8 SDS; P = 0.009), throughout childhood (P > 0.05) and until predicted adult height (-0.2 SDS vs. -1.2 SDS; P = 0.06). The degree of hypophosphatemia was similar in both groups, but serum alkaline phosphatase remained higher in group 2 throughout childhood. Radiographic signs of rickets were more marked in group 2, but even patients with early treatment developed significant skeletal changes of rickets. These data suggest that treatment commenced in early infancy results in improved outcome in patients with XLH, but does not completely normalize skeletal development.

This work was supported by the Foundation for Pediatric Research, the Paulo Foundation, the Jalmari and Rauha Ahokas Foundation, the Maritza and Reino Salonen Foundation, and an ESPE Research Fellowship, sponsored by Novo Nordisk A/S.

Abbreviations: ALP, Alkaline phosphatase; 1,25-(OH)₂D₃, 1,25-dihydroxyvitamin D₃; PAH, predicted adult height; Pi, phosphate; S-, serum level; SDS, SD score; XLH, X-linked hypophosphatemic rickets.

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