Immunoglobulin (Ig)G-4 related myositis – A new entity?

Abstract Immunoglobulin (Ig)G4-related disease is an uncommon systemic autoimmune disorder characterized by infiltration of IgG4 + plasma cells in different organs and elevated levels of IgG4 in peripheral blood. So far, only one case of myositis with abundant IgG4 + plasma cells has been reported and classified as ‘polymyositis’. We present an unusual case of chronic inflammatory myopathy in a context of rheumatoid arthritis. Severe granulomatous myositis, featuring abundant IgG4 + plasma cells was identified in two skeletal muscle biopsies within a five-year-interval. We suggest this entity to be a new subtype of immunoglobulin G4-related disease: IgG4-related myositis, while there were no diagnostic criteria fulfilled for the known idiopathic inflammatory myopathies.