Spontaneous Growth and Effect of Early Therapy with Calcitriol and Phosphate in X-linked Hypophosphatemic Rickets

Whereas nutritional vitamin D deficient rickets affects many people world-wide, X-linked hypophosphatemic rickets (XLH, MIM 307800) has a prevalence of only 1:25.000. Like other rare diseases burden of disease in XLH and the effect of the current standard of care are inadequately described. Only few height data of untreated patients with XLH have been published. Here we report on height before start of therapy of 127 patients with XLH from 49 centres. One investigator collected all data from patient files documented at regular visits by treating physicians. Height standard deviation score (HSDS) was calculated and the geometrical mean was analysed. At birth all patients had a documented height within the healthy reference population. In this cross-sectional analysis of documented height at time of diagnosis decelerates until a mean age of 4.3 years to a nadir, i.e. lowest HSDS of -3.2 HSDS. Afterwards a spontaneous catch-up growth of +1.3 HSDS occurs until start of puberty. To assess the impact of calcitriol and phosphate supplementation on growth we analysed from a cohort of 18 patients treated at the Dept. of Paediatrics at O.-v.-Guericke-University Magdeburg. In this subgroup, size at birth and all time lowest HSDS (r=0.56 p=0.002) are correlated as well as all time low HSDS and last height during puberty (r=0.62 p=0.001). 10 of 18 patients were treated before age 18 months. Within this group the mean HSDS decelerates to -2.2 SDS at age 4.4 y. and increased to -1.4 SDS at age 9.9 years. Adult height, i.e. mean age 17.6 years was -2.4 HSDS. In conclusion, untreated children with XLR are characterized by normal length at birth, diminished growth rate compared to reference children until 4.3 years and spontaneous catch-up growth of 1.3 HSDS until start of puberty. Improved growth rate in XLR children occured by combined phosphate and calcitriol treatment before 18 months.