Idiopathic inflammatory myopathies: diagnostic criteria, classification and epidemiological features.

Idiopathic inflammatory myopathies (IIM) are a group of rare autoimmune disorders characterized by muscle inflammation and progressive weakness. The cause of IIM is unclear but it is believed that disease expression may be triggered by unknown factors in genetically predisposed individuals. Diagnosis is based on a combination of clinical, laboratory and electromyography findings. Muscle biopsy is the definitive diagnostic test. Research into IIM has been limited by the rarity of the disease, a somewhat insidious onset, difficulties with classification and diagnostic methods and heterogeneous study populations making cross-study evaluations difficult. This paper reviews the diagnostic and classification criteria of the IIM and examines epidemiological studies that have been performed, focusing on demographics.