Concurrent positive anti-3-hydroxy-3-methylglutaryl-coenzyme a reductase antibody with reducing body myopathy: Possible double trouble

•We report anti-HMGCR antibody positivity with reducing body myopathy in a 6-year-old boy.•Unusual clinical and pathological features could be a mixed result of two entities.•Comprehensive workup lead to the "possible double-trouble” diagnosis. Anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase myopathy is less common in children but has been associated with more favorable prognosis than adult patients after immunotherapies. We report anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase antibody positivity in a 6-year-old boy with progressive muscle weakness, scoliosis, spinal rigidity, multiple joint contractures, mild left ventricular hypertrophy, and elevated serum creatine kinase. In contrast to most of previously reported pediatric anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase myopathy, he showed little response to immunotherapies. Muscle biopsy contained changes suggestive of myofiber necrosis and regeneration and reducing bodies. The diagnosis of reducing body myopathy was later confirmed by reported c.368A>G (p.His123Arg) mutation in the FHL1 gene. Although the level of association between these two conditions is still inconclusive, this is the first report of concurrent positive anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase antibody with reducing body myopathy emphasizing the possibility of co-occurrence of immune mediated necrotizing myopathy and muscular dystrophy and importance of comprehensive diagnostic investigations in unusual cases.