Chronic Myopathy Associated with Anti-Signal Recognition Particle Antibodies Can Be Misdiagnosed As Facioscapulohumeral Muscular Dystrophy

Objectives: To report cases of chronic autoimmune necrotizing myopathy with anti-signal recognition particle antibodies (anti-SRP myopathy) initially misdiagnosed as muscular dystrophy, in particular, facioscapulohumeral muscular dystrophy (FSHD). Methods: Medical records of patients with anti-SRP myopathy in our institution were retrospectively reviewed. Results: All 6 patients were initially diagnosed with muscular dystrophy because of the long-term clinical course and lack of inflammation on biopsy; 5 were diagnosed with FSHD based on a winged scapula. However, the following features suggested an alternative diagnosis, leading to anti-SRP antibody measurement: (1) lack of family history, (2) lack of facial involvement and asymmetry, (3) prominent dysphagia, and (4) profuse spontaneous activities on needle electromyography. All patients showed improvement with immunomodulating therapy. Conclusions: Anti-SRP antibody measurement should be considered in patients diagnosed with FSHD if they present with diagnostic hallmarks of anti-SRP myopathy listed above, to avoid oversight of this potentially treatable disorder.