Sporadic Inclusion Body Myositis

Sporadic inclusion body myositis (sIBM) is currently considered one of the five main phenotypes of the inflammatory myopathies, besides polymyositis, dermatomyositis, immune-mediated necrotizing myopathy, and the antisynthetase syndrome. It is the most common acquired muscle disease in elderly individuals, particularly men. Its prevalence varies among ethnic groups but is estimated to range between 1 to 182 per million among those aged 50 and older, this wide range depending on the variability of the diagnostic criteria used. Genetic as well as environmental factors and autoimmune processes might both have a role in its pathogenesis. Unlike other inflammatory myopathies, sIBM is a slowly progressive disease, having a distinctive pattern of muscle involvement and different forms of clinical presentation. In some cases, a primary autoimmune disease coexists. Diagnosis is suspected on clinical grounds and is established by typical muscle pathology. However, current criteria allow for a diagnosis on clinical grounds in absence of some characteristic pathological features. As a rule, sIBM is refractory to conventional forms of immunotherapy.