A case report of autosomal dominant hypophosphatemic rickets due to a mutation in the FGF23 gene in an adult: diagnostic difficulties

Osteomalacia is a systemic disease of the skeleton, accompanied by the formation of an unmineralized or poorly mineralized osteoid instead of full-fledged bone tissue. The most common cause is severe vitamin D and calcium deficiency, phosphorus deficiency (kidney pathology, mesenchymal tumors secreting an excess of FGF23, genetic diseases). Among inherited pathologies, X-linked dominant hypophosphatemic rickets (XLHR, gene PHEX , OMIM: 307800) is the most frequent form, while autosomal dominant hypophosphatemic rickets (ADHR, gene FGF23 , OMIM: 193100) and autosomal recessive hypophosphatemic rickets 1,2 (ARHR1-2, genes DMP1 , ENPP1 , FAM20C , OMIM: 241520, OMIM: 613312) are much less common. ADHR is an extremely rare form of genetic rickets caused by mutations in the FGF23 gene. It can manifest at any age. About 50 cases of this disease have been reported in the literature. This article presents the first clinical case of ADHR in an adult in the Russian Federation. Severe vitamin D deficiency, renal tubular disorders and tumor-induced osteomalacia were excluded in differential diagnosis. The patient underwent a genetic test, which revealed a mutation in the FGF23 gene and confirmed the diagnosis of ADHR. Therapy with an active vitamin D analog and phosphate supplement was initiated, after which the patient noticed decreased pain when walking and increased muscle strength. Difficulties in diagnosing osteomalacia are due to the lack of routine determination of serum phosphate and low awareness of doctors about this disease. In some cases, genetic tests make it possible to confirm hereditary forms, which prevents unnecessary surgical treatment, ensures timely prescription of therapy and significantly improves the quality of patients’ lives.