High prevalence of facioscapulohumeral muscular dystrophy (FSHD) and inflammatory myopathies association: Is there an interplay?

Introduction: Certain types of muscular dystrophy (MD), notably facioscapulohumeral muscular dystrophy (FSHD), exhibit muscle fiber necrosis with regeneration and a nonspecific inflammatory process. Although rare, the coexistence of MDs and autoimmune myositis has been observed. We hypothesized that, in some circum- stances, FSHD may predispose individuals to myositis through muscle damage-induced autoantigen over- expression, contributing to an autoimmune response. Methods: We conducted a retrospective analysis of patient data from neuromuscular disease centers in France and Italy between September 2012 and May 2024. Clinical, immunological, and myopathological features of 1750 myositis patients were comprehensively reviewed. Results: Five patients were identified with both FSHD and IIM. Two patients were first diagnosed with FSHD and later developed IIM, while two others initially had IIM followed by an FSHD diagnosis. The fifth patient received simultaneous diagnoses of both conditions. The prevalence of FSHD in the IIM cohort was 1/350, and the prevalence of IIM in the FSHD cohort was 1/40 (p < 0.0001). Discussion: Our study showed a high prevalence of FSHD and IIM association compared to the general population, with underlying mechanisms that remain unclear. This association might be more frequent than previously re- ported, indicating a need for increased clinical awareness. Understanding the interplay between FSHD and autoimmune myositis could reveal insights into the immunopathological processes of these diseases and improve diagnostic and therapeutic approaches.