Cystinosis Is Associated with Deficits in Muscle Mass, Strength, and Hip Bone Density in Children and Young Adults

Cystinosis is associated with deficits in muscle mass and strength beyond what is noted in CKD. Cystinosis is associated with low proximal femur bone mineral density.

Cystinosis is associated with growth failure, myopathy, and multiple risk factors for impaired bone development. The objectives of this study were to quantify muscle mass, muscle strength, and bone mineral density (BMD) in children and adults with cystinosis.

This cross-sectional study assessed dual-energy X-ray absorptiometry regional lean mass, spine and hip BMD, handgrip, and leg strength in 38 participants with cystinosis (ages 5-37 years) and 289 healthy controls. All BMD, muscle mass, and muscle strength measures were expressed as sex-specific Z -scores relative to age and adjusted for height Z -score or limb length. Linear regression models were used to assess muscle strength relative to muscle mass (muscle specific force) and determine the effect of adjusting BMD results for muscle status.

Among adults, arm and leg lean mass ( P < 0.001), handgrip strength ( P <0.001), proximal and distal leg strength ( P < 0.001), muscle specific force ( P < 0.01), and femoral neck and total hip BMD ( P < 0.001) were markedly low, compared with controls. On average, muscle strength was more than two SD below normal due to both low muscle mass and poor muscle quality. Among the children and adolescents, upper extremity lean mass and grip strength were preserved. However, leg lean mass ( P < 0.01), strength ( P < 0.001), muscle specific force ( P < 0.001), and femoral neck and total hip BMD ( P < 0.01) were reduced, compared with controls, approaching deficits seen in adults. Adjustment for lean mass and muscle strength markedly attenuated the BMD deficits.

Cystinosis is associated with deficits in muscle mass and strength that far exceed those observed in CKD alone and is associated with low proximal femur BMD. Future studies are needed to determine if physical activity or other interventions to address sarcopenia in cystinosis will improve physical function and bone strength.