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Anti‐SRP associated necrotizing autoimmune myopathy presenting with asymptomatically elevated creatine kinase

Necrotizing autoimmune myopathy (NAM) typically manifests with painful subacute symmetrical proximal muscle weakness accompanied by persistent and significantly elevated creatine kinase (CK) levels, often more than 10 times the upper limit of normal (ULN). A chronic slowly progressive form, similar to muscular dystrophies, and a rapidly progressive form with CKs resembling those seen in rhabdomyolysis, are also reported. The diagnosis of NAM relies on a combination of clinical and pathologic features. Muscle biopsy often reveals no or minimal inflammation but prominent myofiber necrosis and regeneration. Focal up-regulation of major histocompatibility complex class 1 (MHC1) and C5b9 deposits on the sarcolemma of nonnecrotic myofibers are also encountered. Anti-signal recognition particle (SRP) antibody is identified in approximately 18% of cases, and is associated with a more severe clinical course than other NAM associated antibodies. We describe two patients presenting asymptomatically with significantly elevated CK levels who were subsequently diagnosed with anti-SRP-associated NAM.