Abnormalities in muscle density and muscle function in hypophosphatemic rickets.

CONTEXT: Animal studies suggest that hypophosphatemic rickets (HPR) is associated with muscle function deficits, but it is unknown whether humans with HPR have a muscle disorder.
OBJECTIVE: Our objective was to assess calf muscle size and density (an indicator of muscle quality) and lower extremity muscle function in patients with HPR.
SETTING: The study was carried out in the outpatient department of a pediatric orthopedic hospital. PATIENTS AND OTHER PARTICIPANTS: Participants included 34 individuals with HPR (6-60 yr; nine males) and 34 age- and gender-matched controls. MAIN OUTCOME MEASURES: Calf muscle parameters (muscle cross-sectional area and density) were measured by peripheral quantitative computed tomography. Lower extremity muscle function (peak force per body weight and peak power per body mass) was measured by jumping mechanography through five tests with different levels of difficulty: multiple two-legged hopping, multiple one-legged hopping, single two-legged jump, chair-rise test, and heel-rise test.
RESULTS: Compared with age- and gender-matched controls, patients with HPR had normal muscle size (P = 0.58) but lower muscle density (P = 0.008) and lower peak muscle force and power (P < 0.001 in each test). Muscle function tests were also lower in the subgroup of patients with straight legs (n = 13) than in controls, even though patients with straight legs had higher muscle function test results than patients with severe leg deformities.
CONCLUSIONS: The present study suggests that muscle weakness is a clinical feature of HPR. Lower muscle quality and limb deformities contribute to this functional deficit.