Dramatic Transformation After Burosumab in a Young Boy With X-linked Hypophosphatemia: A Life-Changing Saga.

X-linked hypophosphatemia (XLH), also referred to as vitamin D-resistant rickets or X-linked dominant hypophosphatemic rickets, is a very rare metabolic disorder. Despite its rarity, it is the most common form of genetic rickets. XLH is caused by loss of function mutation in the phosphate-regulating endopeptidase homolog X-linked (PHEX) gene, resulting in excessive fibroblast growth factor 23 (FGF23) activity. The end result is renal phosphate wasting leading to hypophosphatemia. It is frequently misdiagnosed as nutritional rickets as it mimics clinical manifestations of vitamin D-deficient rickets; however, it remains refractory to vitamin D repletion. The clinical expression can be variable from progressive bowing to severe skeletal and dental abnormalities. Treatment was limited to calcitriol and phosphate supplementation until the emergence of burosumab, a humanized monoclonal antibody against FGF23. We here share our first-hand experience of the use of burosumab in a 14-year-old boy with XLH and how it dramatically improved his quality of life along with the review of the literature regarding XLH and burosumab.