Diagnostic Utility of Muscle Ultrasound for Sarcopenia in Prader-Willi Syndrome: A Cross-Sectional Study

Prader-Willi syndrome (PWS) is characterized by sarcopenic obesity; however, validated screening tools for muscle mass in clinical settings are lacking. This study aimed to evaluate the diagnostic utility of muscle ultrasound (US) for detecting low muscle mass in individuals with PWS.

This observational, cross-sectional study recruited 48 individuals with genetically confirmed PWS (International Classification of Diseases, 10th Revision Code Q87.1) from a specialized clinic (October 2022-June 2024). Appendicular skeletal muscle index (ASMI) via dual-energy x-ray absorptiometry (DXA) served as the primary outcome. Sarcopenia was defined based on Asian Working Group for Sarcopenia (AWGS) 2019 criteria. US predictors included muscle thickness (MT) of the rectus femoris (RF), vastus lateralis and gastrocnemius medialis (GM); pennation angle; and RF cross-sectional area (CSA). Analysis included Spearman's correlation (ρ), multivariable linear regression (B) and receiver operating characteristic (ROC) curves.

The cohort (n = 48; 47.9% women; median age 19.5 years, interquartile range [IQR] 12.3-26.8) had a 100% prevalence of obesity (median BMI 27.1 kg/m², IQR 22.2-31.4). All participants (100%) had a history of growth hormone treatment; 62.5% exhibited the deletion subtype. Low muscle mass was observed in 60.4% (n = 29), confirmed sarcopenia in 52.1% (n = 25) and severe sarcopenia (including low physical performance) in 20.8% (n = 10) participants. GM MT showed the strongest correlation with ASMI (ρ = 0.689, p < 0.001, 95% confidence interval [CI] 0.50-0.81), followed by RF CSA (ρ = 0.587, p < 0.001). Multivariable regression identified GM MT as a significant independent predictor of ASMI (B = 0.957, p = 0.011) after adjusting for BMI (B = 0.128, p < 0.001), age (B = 0.020, p = 0.044) and sex (B = -0.376, p = 0.029; R² = 0.67). ROC analysis for detecting low muscle mass yielded an area under the curve for GM MT of 0.759 (95% CI 0.616-0.901, p = 0.003), with an optimal cut-off of 1.69 cm (sensitivity 86.2%, specificity 63.2%).

Sarcopenia affects 52.1% of patients with PWS, with 20.8% meeting criteria for severe sarcopenia. Ultrasound of the gastrocnemius medialis is a valid, radiation-free predictor of skeletal muscle mass and serves as a practical diagnostic tool for sarcopenic obesity in PWS.