Myositis is commonly associated with several connective tissue diseases and shows a relatively frequent association with mixed connective tissue disease, systemic sclerosis, systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and Sjogren syndrome (Troyanov, 2005). The relative contribution of myositis and other connective tissue diseases to the severity of clinical symptoms and differences in laboratory data varies significantly across patients. Many of these patients predominantly present with symptoms of collagen disease; however, others primarily show symptoms of myositis.
Histopathological findings in muscle tissue in the former case are often limited to moderate perimysial cell infiltration in areas with minimal muscle fiber degeneration, whereas in the latter case, histopathological findings often resemble those of myopathy with evident inflammation. Infiltrating cells are usually CD4+ cells and macrophages. However, in some cases with Sjogren syndrome, CD20+B and plasma cells may be noted. Vasculitic changes are present in patients with polyarteritis nodosa (PN) and microscopic PN as described later. RA and SLE can also present with vasculitis. Perifascicular atrophy is relatively infrequent and if present, it suggests a possible association with DM.
