HMGCR and polymyositis: Although it is often difficult at first glance to distinguish the symptoms of SINAM from other forms of inflammatory myopathies, such as dermatomyositis or polymyositis, SINAM is characterized by its rare extramuscular manifestations, the presence of positive anti-HMGCR antibodies and specific pathological changes, including complement deposition on capillaries and/or the sarcolemma.