Endocrine Abstracts

Background: X-linked hypophosphatemia (XLH) is a rare, hereditary, progressive musculoskeletal disorder. Prompt diagnosis and treatment in childhood ensures adequate bone matrix mineralization and skeletal growth. There is no consensus on indications for treatment in adult patients.

Case Presentation: A 25 year old female was referred to endocrinology with a right ankle fragility fracture. She was known to have XLH, diagnosed in Poland at the age of 1 year. Her father and grandparents were also affected. She underwent multiple osteotomies and corrective surgeries for lower limb deformities and had recurrent dental abscesses. She was previously treated with Vitamin D supplements and phosphate salts; old case notes were not available. On examination, her height was 148cm (0.4^th centile). Investigations showed mild hypophosphatemia with normal levels of corrected calcium, alkaline phosphatase, magnesium, 25-OH vitamin D and parathyroid hormone with no phosphaturia or hypercalciuria and no evidence of nephrocalcinosis on sonography. DEXA scan was normal. Genetic testing confirmed heterozygous PHEX mutation. She was started on Phosphate-Sandoz and Rocaltrol. She was non-compliant and stopped treatment herself, but remained asymptomatic with stable biochemistry. Five years later, she presented with severe bone pain affecting shoulders, spine and legs, and occasional difficulty in walking without support. Repeat biochemistry was stable. She was keen to commence Burosumab, a monoclonal FGF23 antibody, and was referred to a tertiary Metabolic Bone Unit.

Conclusion: Conventional treatment with active Vitamin D and oral phosphate salts for clinical improvement should be considered in symptomatic adults with XLH. In UK, Burosumab is approved for children and adolescents, but not adults due to lack of evidence on the long-term metabolic consequences. This case highlights management challenges for rare conditions and need for further studies to provide evidence based treatments.