Eosinophilic myositis: an updated review

Eosinophilia-associated myopathies are clinically and pathologically heterogeneous conditions characterized by the presence of peripheral and/or muscle eosinophilia. There are at least three distinct subtypes: focal eosinophilic myositis, eosinophilic polymyositis, and eosinophilic perimyositis. Infiltrating eosinophils are not always identified in conventional muscle histologic examination, but the eosinophil major basic protein, whose extracellular diffusion is considered a hallmark of eosinophilic cytotoxicity, is usually detected by immunostaining in muscle biopsy. Whereas focal eosinophilic myositis seems to be a benign and isolated condition, and perimyositis is usually related with the inflammatory infiltrate due to fasciitis, eosinophilic polymyositis can be associated with muscular dystrophy or be a feature of multiorgan hypereosinophilic syndrome. Muscle biopsy remains the cornerstone for the diagnosis. Parasitic infections, connective tissue disorders, hematologic and non-hematologic malignancies, drugs, and toxic substances are the main etiologic agents of eosinophilia-associated myopathy. However, in some cases, no known etiologic factor is identified, and these are considered idiopathic. Glucocorticoids are the mainstay therapy in idiopathic forms. Imatinib and mepolizumab, a humanized anti-interleukin 5 monoclonal antibody, may be useful in patients with eosinophilic myositis as part of a hypereosinophilic syndrome.