ESPE Abstracts

Background: X-linked Hypophosphatemia (XLH), the most common heritable form of rickets, is a disorder of renal phosphate wasting caused by high circulating levels of fibroblast growth factor 23 (FGF23) that impairs normal phosphate reabsorption in the kidney and production of the active form of vitamin D. Affected children present with hypophosphatemia resulting in rickets, bowing of the legs and short stature. Limited information is available about the disease burden in children with XLH.

Objective and hypotheses: The objective was to characterize the clinical condition of children with XLH and assess the impact on function and quality of life.

Method: An IRB-approved, web-based questionnaire was completed by parents on behalf of children with XLH. English and French versions were available.

Results: 70 paediatric surveys were completed for children from 1–17 years of age with a median age of 8 years. The median age at diagnosis was 2 years. Nearly all children were being treated with a standard of care (SOC) regimen that includes multiple daily doses of oral phosphate and active vitamin D (69/70 (99%)). Most children were diagnosed prior to 3 years of age (53/70 (76%)). Reported skeletal abnormalities and resulting complications included bowing of the femur, tibia/fibula (61/70 (87%)), gait disturbance (60/70 (86%)), joint pain (45/70 (64%)), bone pain (41/70 (59%)) and restricted range of motion (29/70 (41%)). Over 30% of responders had undergone at least one surgery to correct a skeletal defect. Diminished height was reported for (57/70 (81%)) of children with the majority of boys under the 25^th percentile and the majority of girls under the 50^th percentile for height. Scores on two PROs, the PODCI and the SF-10 were >1 SD below the US general population norms indicating significant issues with pain, mobility and quality of life.

Conclusion: Children with XLH experience significant complications despite SOC treatment highlighting the need for more efficacious therapies.

Funding: This work was supported by Ultragenyx Pharmaceutical Inc, the study sponsor.