Laboratory Evaluation of Inflammatory Myopathies

Myositis may be idiopathic or caused by conditions such as microbial infections, drugs, or injury leading to inflammation and damage in the muscles.

Idiopathic inflammatory myopathies (IIM) are a rare and heterogeneous group of autoimmune diseases characterized by acute, subacute, or chronic muscle weakness. IIM may broadly be categorized into dermatomyositis (DM), polymyositis (PM), inclusion body myositis (IBM), necrotizing autoimmune myopathy (NAM), and overlap syndromes.


Myositis is listed as a "rare disease" by the Office of Rare Diseases (ORD) of the National Institutes of Health (NIH). This means that myositis, or a subtype of myositis, affects less than 200,000 people in the US population. The incidence of idiopathic inflammatory myopathy is approximately 2 to 8 cases per million people each year. For unknown reasons, polymyositis and dermatomyositis are about twice as common in women as in men, while sporadic inclusion body myositis is more common in men.

Disease Presentation

The symptoms of myositis can include muscle weakness, muscle pain and muscle tenderness. Muscle weakness may not be observed in the early stages of disease as in some patients with DM. In general patients with myositis may present with one or more of these symptoms at disease onset:

  • Difficulty standing up from a seated position
  • Difficulty lifting the arms
  • Fatigue after standing or walking a long time
  • Trouble swallowing or breathing
  • Muscle pain that does not subside within a few weeks
  • A red or purple colored rash on the eyelids, elbows, knees and knuckles