Prospective Analysis of Muscle Adiposity in Children With X-linked Hypophosphatemic Rickets vs Control Children

CONTEXT: Children with X-linked hypophosphatemic (XLH) rickets have muscle weakness that severely impairs their function. Intermuscular and intramuscular adipose tissue (IMAT and intraMAT, respectively) may contribute to this muscle weakness. OBJECTIVE: This work aimed to compare IMAT and intraMAT in XLH children vs typically developing (TD) children. METHODS: A prospective, monocentric cohort study was conducted of XLH (n = 11; aged 10.3 years [6-17]) and TD children (n = 22; aged 10.2 years [5-15.5]). All children underwent magnetic resonance imaging of the lower limbs; IMAT and intraMAT percentages were calculated after manual contouring of each muscle of the thigh and the deep fascia at mid-thigh level. RESULTS: XLH children were comparable in age but shorter and heavier than TD children (P = .001 and P = .03, respectively). They had smaller muscle length and volume than TD children (P < .001) but there was no statistically significant difference in muscle cross-sectional area between the groups (P = .833). The total percentage of IMAT was higher in XLH children (8.66% vs 3.60% in TD children; P < .0001). In addition, though the total percentage of intraMAT did not differ significantly (12.58% and 10.85% in XLH and TD children, respectively; P = .143) intraMAT was statistically significantly higher in XLH children than TD children in 4 of the 13 muscles studied. CONCLUSION: Our results show that IMAT is higher in young children with XLH, independently of obesity and overweight. Further, these results will facilitate both the early prevention of functional and metabolic consequences of the increase in adipose tissue in XLH children.