Anti-HMGCR Antibody-Related Necrotizing Autoimmune Myopathy Mimicking Muscular Dystrophy

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IntroductionNecrotizing autoimmune myopathies (NAMs) are acquired myopathies with myofibrillar necrosis and weak or absent inflammatory component, sometimes associated with anti-signal recognition particle (SRP) or 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) antibodies.ObservationThe patient, a girl now aged 20 years, was first assessed at the age of 5 years for abnormal gait revealing frank pelvic deficit. Creatine kinase (CK) levels were as high as 7,500 IU/L. Subsequent muscle biopsy showed some necrosis, fiber regeneration, and fibrosis consistent with muscular dystrophy (MD). Protein immunohistochemistry was normal. The disease course was progressive until wheelchair use at the age of 9 years. At 12 years of age, a second muscle biopsy found an advanced MD with some perivascular inflammatory mononuclear cells. All molecular analyses done through 14 years of follow-up were negative till anti-HMGCR antibodies were detected at a significant amount when she was 19 years old.DiscussionNAMs begin at a pediatric age and may have a chronic course mimicking MDs. Muscular biopsy can be misleading with a predominantly dystrophic pattern without inflammation.ConclusionThis observation should prompt the assessment of NAMs in all MDs, even pediatric, without molecular solutions.

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