OBJECTIVE: Unlike most muscular dystrophies that progress symmetrically at a constant rate, facioscapulohumeral muscular dystrophy (FSHD) is characterized by stepwise, asymmetric progression of muscle wasting, and weakness. Muscle tissue is progressively replaced by fat; however, its relation to preceding inflammation is unclear. In this longitudinal study of FSHD, we assessed muscle inflammation and fat replacement and their relation quantitatively. We also investigated whether fat replacement in muscle varies along its length.

METHODS: Forty-five patients with FSHD were evaluated twice, 14 months apart. Using MRI sequences with short TI inversion recovery (STIR), we quantified the degree of STIR hyperintensity in muscles (≥ 2 SD above control intensity). STIR hyperintensities (STIR+) suggest edema or inflammation. We used Dixon MRI to quantify fat content.

RESULTS: Of 370 thigh muscles, 83 were STIR+ at baseline and 103 at follow-up. The highest frequency of STIR+ was seen in muscles with inter-mediate fat content (40-60% fat). The progression of fat replacement was higher in STIR+ muscles (5.0 ± 4.0%) vs. STIR- muscles [2.3 ± 3.3% (P < 0.0001)]. In addition, muscles with severe STIR+ at baseline had a higher fat replacement progression than muscles with milder STIR+ (R = 0.39, P = 0.001). The fat content was higher in the distal part vs. proximal part of most muscles (P < 0.05). However, the progression of the fat replacement was uniform along the length of all the muscles.

CONCLUSION: Muscles with STIR+, indicating inflammation, have a faster progression of fat replacement than STIR- muscles, and the fat replacement progression correlated with the severity of STIR+.