VP.44 Juvenile Anti-PM Scl75 myositis with necrosis, phagocytosis, and endomysial fibrosis

Abstract

The most common myositis are usually divided in two groups: the autoimmune myositis or idiopathic inflammatory myopathies (dermatomyositis, immune-mediated necrotizing myopathy, antisynthetase syndrome, and inclusion body myositis), and the overlap myositis, manifesting with inflammatory myositis with extramuscular symptoms. Most children present dermatomyositis but other types of juvenile myositis may be a diagnostic challenge. A 3 years and 11 months old girl presented subacute muscle weakness with profound hypotonia since 2 years and 6 months of age. No Gottron´s papules or heliotrope rash were observed. Magnetic resonance imaging demonstrated muscle STIR hyperintensity. Her muscle biopsy showed necrosis, phagocytosis, endomysial fibrosis, lymphohistiocytic endomysial infiltrate, strong membrane attack complex (MAC/C5b9) deposition in the sarcolemma, and weak MHC-I (HLA-ABC) sarcolemmal expression. Electron microscopy revealed some fibres with numerous autophagic vacuoles, and endothelial cells presented no tubuloreticular inclusions. On her last follow-up at the age of 10 years old, she presented good response to immunosuppressive treatment with stable muscle strength. Her serological profile, performed after treatment, revealed Anti-PM Scl75 antibodies. The appropriate diagnosis and treatment was of utmost importance for muscle function preservation.