Anthropometric characteristics at birth and growth outcome in patients with X-linked hypophosphatemia treated with oral phosphate and active vitamin D

BACKGROUND: X-linked hypophosphatemia (XLH) is the most common form of inherited rickets, resulting in short stature despite treatment with oral phosphate and active vitamin D. Detailed data of anthropometric parameters at birth, during infancy, and on the influence of an affected parent on outcome are lacking. METHODS: In this prospective multicenter observational study, conducted from 1998 to 2023 in Germany, Austria, and Switzerland, body length, body weight, and head circumference were investigated in 198 children with XLH from birth until the age of 18 years, all being only on supplementation therapy. RESULTS: XLH newborns presented with disproportionate body shape characterized by decreased birth length relative to weight and head circumference with a 2.4-fold increased risk to be born small for gestational age (SGA). A positive family history for XLH was associated with lower anthropometric characteristics at birth. Body disproportion increased during 0-2 years old, resulting in significantly increased head circumference (+ 0.82 SD) and reduced body length (-2.00 SD) compared to healthy 2-year-old patients. Supplementation therapy failed to prevent progressive growth failure and reduced final height, regardless of whether treatment was started early due to an affected family member, which was associated with overall poor control of metabolic bone disease indicated by persistent hypophosphatemia and rising alkaline phosphatase z-score. CONCLUSIONS: XLH is associated with an increased risk for SGA and progressive disproportional body growth during infancy. Routine medical check-ups may soon use this unique growth pattern to identify children with XLH. Supplemental therapy fails to prevent progressive growth failure.