Internalized nuclei were seen in 13% ± 14% [95%CI 8-18%] of fibers. 70% ± 29% [95%CI 49-90%] of fibers in the TA muscle and 56% ± 18% [95%CI 47-65%] in the QF were classified as type 1. Patients in whom both QF and TA specimens were available, the difference of the percentage of fiber type I was significant (32% ± 33% [95%CI 4-60%]). In both QF and TA CD68+ cells were abundant, often in clusters, with an average of 0.6 ± 0.7 [95%CI 0.4-1.0] CD68+ elements/fiber. We found 0.2 ± 0.3 [95%CI 0.05-0.3] CD3+ elements, 0.2 ± 0.3 [95%CI 0.06-0.4] CD8+ elements and 0.06 ± 0.1 [95%CI 0.01-0.1] CD20+ elements/fiber. A significantly higher number of CD4+ elements per fiber in FSHD1, 0.3 ± 0.3 [95%CI 0.2-0.4], compared to controls, 0.09 ± 0.05 [95%CI 0.04-0.1] was found. 7 biopsies showed MAC deposits in fibers and/or endomysium. MHC I was variably expressed, ranging from light sarcolemmal expression to dense sarcolemmal and sarcoplasmic expression.

We confirm the diverse morphology in FSHD1, including the inflammatory markers and fiber type predominance. This study gives insights into the complicated myopathology of FSHD1 to reach trial readiness.