51VPMitochondrial pathology in Mi-2 beta dermatomyositis with lymphocyte fibre invasion

Dermatomyositis with mitochondrial myopathologic abnormalities outside areas of perifascicular atrophy has rarely been described. Here we report a patient with Mi-2 beta dermatomyositis with mitochondrial morphologic abnormalities outside perifascicular areas. A 49-year-old man, was admitted with difficulties walking and raising the arms, with slight symptoms improvement in use of prednisone. He was born of nonconsanguineous parents. Physical examination demonstrated proximal lower limbs weakness, decreased reflexes, and the characteristic dermatomyositis upper chest sun exposed area V-sign. Electromyogram demonstrated asymmetric myopathic muscle action potentials with muscle membrane hyperexcitability. Laboratorial investigation demonstrated total creatine kinase of 498 IU/L (39-308)(1.6x), and aldolase 10.3 U/L (< 7.6)(1.4x). Speckled nuclear Antinuclear antibodies (ANAs) pattern until 1:1280 dilution (reference below 1:80), and Anti-Mi-2 beta antibodies on Immunoblot. Magnetic resonance imaging demonstrated STIR-weighted hyperintensities in posterior distal muscles. Right vastus lateralis muscle biopsy demonstrated endomysial inflammatory infiltrate surrounding and infiltrating non necrotic muscle fibers, regeneration, ragged red fibres, COX negative fibres, major histocompatibility complex (MHC-I/ HLA-ABC) membrane positivity in non-necrotic fibres, membrane attack complex (MAC/ C5b9) deposition in capillaries and sarcolemmal membrane, isolated fibres with fine diffuse p62 positive deposits, and no MxA sarcoplasmic reaction. Transmission Electron Microscopy demonstrated accumulation of dense mitochondriae with abnormal cristae, and paracrystalline inclusions. Later studies will be necessary to confirm if Mi-2 dermatomyositis with mitochondrial abnormalities represents a subset with poor treatment response.