400 Section - Multidimensional Functional Assessment in Pediatric Patients With X-linked Hypophosphatemic Rickets Treated With Burosumab: An Observational Study

X-linked hypophosphatemic rickets (RC0170) is a rare genetic disorder (≈1/47,000) caused by PHEX mutations, leading to increased serum FGF23, renal phosphate wasting, and impaired vitamin D activation. Chronic hypophosphatemia results in defective mineralization, short stature, skeletal deformities, dental dysplasia, muskoloskeletal pain, and reduced mobility. Burosumab, an anti-FGF23 antibody was approved in Italy by National Regulatory Agency (AIFA) in 2019 and resulted more effective and safe than conventional therapy. Given limited normative data on musculoskeletal function in pediatric XLH patients treated with Burosumab, this study aimed to characterize this cohort using a standardized physiatric protocol to support rehabilitation and monitor therapeutic response.

Design:

Pediatric XLH patients on Burosumab underwent multidimensional evaluation: isometric grip strength, upper limb dexterity (Nine Hole Peg Test), gait (10-m walk), lower limb power (Jump Test) with , static balance (IPODO platform, PBS), body composition (DEXA, BIA), fatigue (PedsQL Multidimensional Fatigue Scale).

Results:

Twelve patients (3 males, 9 females; mean age 7.6 ± 3.8 years; BMI 18.1 ± 2.3 Kg/m2) were assessed. Compared with normative values, 83% of patient were within height limits but below expected means. Hand Grip strength adjusted for sex/height was normal in 80% patients; manual dexterity was normal in 63%; static balance preserved in 91%. DEXA showed ASMM/h² below expected mean in 77%  but within normal ranges; BIA revealed reduced lean mass in 70% of patient. Parent-reported fatigue was clinically significant in 18% (general) and 27% (sleep/rest), while children reported milder fatigue.

Conclusion:

Functional assessment in pediatric XLH patients treated with Burosumab showed preserved muscle strength, balance, and dexerity in most of cases, but muscle mass was significantly reduced. Fatigue perception differed between parents and children. Findings highlight the need for multidimensional evaluation to guide individualized rehabilitation, and monitor treatment efficacy.