2020 Oct 24. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan–.
Idiopathic inflammatory myopathies involve four major subtypes that include polymyositis, dermatomyositis, inclusion body myositis, and necrotizing myopathy. Bohan and Peter categorized myopathies into 7 classes. Polymyositis, an autoimmune and chronic inflammatory myopathy, is characterized by symmetrical proximal muscle weakness due to the involvement of endomysial layers of skeletal muscles versus dermatomyositis, which involves the perimysial layers of muscles along with dermatological presentations.
Polymyositis develops over the months as compared to inclusion body myositis (IBM), which is a slowly progressive chronic myopathy developing in older individuals over a period of months to years with more severe symptoms. IBM develops secondary to either an autoimmune reaction or due to a degenerative process as a result of a persistent retroviral infection such as human T-cell leukemia virus type 1 (HTLV-1). Being an autoimmune disorder, polymyositis, a rheumatological disease, requires long-term treatment with steroids or immunomodulators along with the treatment of the underlying etiological factors. Although this is a rare disorder, polymyositis should be a part of the differential diagnosis of patients experiencing unexplained muscle weakness as the failure to make a diagnosis can have a significant impact on the patient's quality of life due to severe complications.