Inflammatory myopathy  is disease featuring weakness and inflammation of muscles and (in some types) muscle pain. The cause of much inflammatory myopathy is unknown (idiopathic), and such cases are classified according to their symptoms and signs and electromyography, MRI and laboratory findings. It can also be associated with underlying cancer. The main classes of idiopathic inflammatory myopathy are polymyositis (PM), dermatomyositis (DM), and inclusion-body myositis (IBM). Diagnosis
There are a number of known causes of myopathy, and it is only once these have been ruled out that a clinician will assign an idiopathic inflammatory myopathy (IIM) syndrome to a case. The usual criteria for a diagnosis of PM are weakness in muscles of the head, neck, trunk, upper arms or upper legs; raised blood serum concentrations of some muscle enzymes such as creatine kinase; unhealthy muscle changes on electromyography; and biopsy findings of (i) muscle cell degeneration and regeneration and (ii) chronic inflammatory infiltrates in muscle cells. If heliotrope (purple) rash or Gottrons papules are also present, then the diagnosis is DM. In DM, myositis may not be clinically apparent but detectable via biopsy or MRI. If the criteria for PM are met but muscle weakness also affects the hands and feet or is not accompanied by pain IBM should be suspected, and confirmed when muscle cell biopsy reveals (i) cytoplasmic vacuoles fringed by basophilic granules and (ii) inflammatory infiltrate comprising mostly CD8 T lymphocytes and macrophages; and electron microscopy reveals filamentous inclusions in both cytoplasm and nucleus.
PM is an adult disease, usually emerging after the age of twenty. PM and DM are more common in females, more common in Caucasians, and least common in Asians. At any given time, about 35.5 people per million have IBM; it emerges after the age of 30 (usually after 50), and may be more common in males. References


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