MADD – A myositis mimic

<h3></h3> Myopathies commonly present with proximal muscle weakness and raised serum muscle enzymes. The differential diagnosis is wide- it is important to consider metabolic myopathies. A sixty-three-year-old lady experienced proximal leg pain and weakness followed by a fall prompting hospital admission. Serum total creatine kinase(CK) was 1183IU/L. Myositis autoantibodies including anti-HMGCR were negative. EMG confirmed myopathic features. A provisional diagnosis of polymyositis was made. Prednisolone treatment was started and CK normalised before discharge. Her weakness progressed over two years despite the addition of Methotrexate. As part of further investigations, a muscle biopsy was performed which showed an excess of lipid droplets suggesting a lipid storage myopathy. Plasma carnitine levels demonstrated increased medium and long chain acylcarnitines indicating a diagnosis of Multiple Acyl-CoA Dehydrogenase Deficiency(MADD). MADD is a fatty acid oxidation disorder caused by defects in an intramitochondrial electron transfer flavoprotein causing impaired beta-oxidation of fatty acids of all lengths. Elevated plasma acylcarnitine or urine organic acid levels are suggestive of MADD, and genetic confirmation is diagnostic. Polymyositis is now considered a rare entity. Clinicians should be vigilant of non- inflammatory conditions which mimic this presentation. This case highlights the usefulness of muscle biopsy alongside several red flags for suspecting myositis mimics: seronegative status, lack of other connective tissue disease features and lack of response to immunosuppression.