Immune-mediated Necrotizing Myopathy in a Patient with Microscopic Polyangiitis

We herein report a case of immune-mediated necrotizing myopathy (IMNM) in a patient with microscopic polyangiitis (MPA). A 77-year-old Japanese woman presented with a 2-day history of proximal muscle weakness and myalgia, with elevated serum creatinine kinase (CK) levels. Findings of a muscle biopsy were compatible with IMNM; however, anti-signal recognition particle (SRP) and anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) antibodies were negative. She also had peripheral neuropathy with elevated serum myeloperoxidase anti-neutrophil cytoplasmic antibody titers, leading to a diagnosis of MPA. IMNM can be a pathological result of MPA muscle involvement.