Inflammatory Diseases of the Muscle: Polymyositis and Dermatomyositis

Polymyositis and dermatomyositis are autoimmune diseases that cause inflammation of the muscles, most commonly the upper arms and thighs, resulting in weakness. Polymyositis affects mainly the muscles while dermatomyositis can cause skin lesions in addition to muscle inflammation. Because these conditions are systemic disorders, they can affect other organs as well, such as the lungs, the skin and the esophagus.

Causes and Risk Factors

The exact causes of polymyositis and dermatomyositis are not known, though they most often occur in people aged 31 to 60 and rarely occur in people younger than 18.


Symptoms of inflammatory diseases of the muscle most often include:

  • Weakness in the upper arms leading to difficulty raising the arms over their head
  • Difficulty getting out of a chair
  • Skin rashes over the face arm, chest and hands, especially in sun exposed areas
  • Difficulty swallowing due to muscle weakness of the esophagus
  • Difficulty breathing related to lung inflammation (interstitial lung disease)


Diagnosis of the inflammatory muscle disease is usually done with a combination of physical examination and lab tests that measure muscle inflammation. Other tests that may be needed include:

  • Biopsy of the affected muscle to show inflammation
  • MRI, which can show inflammation in the muscle
  • Electromyography (EMG), which can show which muscles are involved

Especially when EMG or muscle biopsy is being considered, rheumatologists will consult with a neurologist to determine the best diagnostic plan to guide treatment. It there is a suspicion of inflammation of the lung, specific breathing tests that measure lung function (pulmonary function tests) and sometimes a CT scan of the chest may be needed.


Treatment is centered around the use of medication that will limit and reverse inflammation, allow the muscle to heal and return the muscle to normal function again. This approach applies to inflammation in the lung as well. Potential treatment options include:

  • Corticosteroids
  • Additional medications such methotrexate, azathioprine or mycophenolate mofetil
  • Intravenous immunoglobulin is used if muscle or skin inflammation is very active

Working closely with other specialists such as neurologists, dermatologists and pulmonologists, your rheumatologist can develop a diagnostic and treatment plan to improve muscle strength and function and enhance quality of life.

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To learn more about our services or to make an appointment with a Brigham and Women’s Hospital rheumatologist, contact one of our trained coordinators at 1-800-294-9999 to get connected with the best doctor for your needs.

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