To determine the muscle MRI T1 fat fraction at baseline and at 12 months in patients with facioscapulohumeral dystrophy (FSHD) and assess 1) radiographic factors associated with fatty infiltrative progression, and 2) correlation with functional outcome measures and quantitative strength measurements.

FSHD, the third most common muscular dystrophy, is heterogeneous in its presentation and progression of muscle weakness.  The disease manifestation is variable but mostly slow in progression necessitating the identification and validation of measures that can inform clinical trial efficacy in a time efficient manner.  

A prospective, longitudinal, observational study assessing MRI, quantitative strength measurements, and ambulatory functional outcome measures in 32 FSHD patients. 

While there was no significant decline in functional outcome measures or quantitative tibialis anterior (TA) strength, there was a small overall shift to higher fat fractions. The greatest increase in fat fraction were observed in muscles having intermediate fat fraction; with minimally (baseline fat fraction < 0.10) or severely (> 0.70) affected muscles less likely to progress.  Large increases (> 0.20) in fat fraction were not seen in muscles with >0.50 fat fraction at baseline.  A higher STIR hyperintensity at baseline was associated with larger fat fraction increase at follow-up. A pre-specified Muscle Fat Fraction Burden Index (summated from 16 lower-extremity muscles) correlated with the ambulatory functional outcome measures.  

Over a 12-month period, leg muscles show a small but significant increase in fat fraction without associated measurable decline in motor function or strength.  The general magnitude of change is consistent with other reports, with a few muscles with moderate fatty infiltration and STIR positivity demonstrating more dramatic changes. Fat fraction was most correlated with functional status when modeled as compartments. Further work is needed to refine the model and integrate specific strength measures across longer-intervals to better characterize functional progression.