Electrical Impedance Myography Detects Disease Progression over 12 to 24 Months in Facioscapulohumeral Muscular Dystrophy.

Abstract

Targeted therapies for facioscapulohumeral muscular dystrophy (FSHD) are progressing through clinical trials. Electrical impedance myography (EIM) provides a noninvasive biomarker of muscle composition that may be valuable especially in early phase trials. This study evaluated EIM data from a multicenter FSHD cohort over 24 months. Adult patients with FSHD at 8 sites underwent EIM in 6 muscles bilaterally (deltoid, biceps, triceps, vastus lateralis, tibialis anterior, and medial gastrocnemius). EIM outcomes phase and reactance (50 and 100 kHz [kilohertz] frequencies) and 50 of 211 kHz phase ratio were evaluated for reliability, correlations with clinical measures, and sensitivity to change. One hundred fifty-seven patients (53% male patients) were included. Test-retest reliability was excellent for all EIM outcomes (intraclass correlation coefficient [ICC] ≥0.94). Phase outcomes strongly correlated with the FSHD-composite outcome measure (FSHD-COM; r ≤ -0.69) and Motor Function Measure Domain 1 (MFM1; r ≥ 0.75); reactance outcomes exhibited moderate correlations with the FSHD-COM (r ≥ -0.41) and MFM1 (r ≤ 0.44). Mean declines in phase and phase ratio were apparent at 12 months (eg, -0.25, 95% confidence interval [CI] = -0.45 to -0.05 at 50 kHz), and further progressed through 24 months (-0.66, 95% CI = -0.92 to -0.40] at 50 kHz and -0.65 [95% CI = -0.87 to -0.44] at 100 kHz; both p < 0.0001). Reactance changes were smaller and not significant: -0.21 (95% CI = -0.44 to 0.02) at 50 kHz and -0.13 (95% CI = -0.35 to 0.10) at 100 kHz. EIM phase outcomes are reliable, valid, and sensitive to change over 12 to 24 months, supporting their potential utility as biomarkers in FSHD clinical trials. ANN NEUROL 2026;99:1545-1554.