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Last Updated: Mar 1, 2019


Myositis literally means inflammation (swelling, soreness) of the muscles. General muscle soreness is common and has many causes, including infection, injury, overuse and medication side effects. However, the term “myositis” most commonly refers to a group of diseases also known as idiopathic inflammatory myopathies (IIMs).

In people with IIM, the body’s immune system attacks its own muscle tissue. The resulting inflammation causes pain, swelling and weakness. In addition to the muscles, IIM may affect the skin, joints, lungs, heart and digestive system.

There are many kinds of IIM. The most common are:

  • Polymyositis (PM) affects the muscles nearest the trunk.
  • Dermatomyositis (DM) causes both muscle inflammation and skin rash.
  • Inclusion body myositis (IBM) appears to overlap with degenerative muscle disease and mostly affects people over 50.
  • Juvenile myositis begins in childhood or adolescence.
  • Some IIMs overlap with other health conditions like arthritis or cancer.

Most cases of IIM are chronic, meaning symptoms come and go over a long period of time. Symptoms during flares (periods of active disease) may range from mild to severe and disabling.

IIM is quite rare, affecting only about 20,000 people in the United States. The condition is two to three times more common in females and presents most often in childhood (ages 5-15) and midlife (ages 30-50). An exception is IBM, which is most common in men over 50.

Causes and Risk Factors

Most experts consider IIM a form of autoimmune disease, which occurs when the body’s own immune system malfunctions and attacks healthy cells.

Normally, when muscle tissue is subjected to infection, injury or irritants, specialized cells within that tissue release substances that open the blood vessels and increase blood flow to the affected area. Special “attack” cells move in to engulf harmful germs and cell debris. This response, called inflammation, is beneficial because it flushes away harmful stimuli and promotes healing of the affected tissues.

Shoulder muscles

In a person with autoimmune disease, the immune system sometimes misreads the body’s own cells as dangerous and initiates an inflammatory response against them. It’s not clear what causes autoimmunity, but certain people appear to have a genetic predisposition to it. Environmental triggers may also play a role.


IIM symptoms vary depending on the specific disease and the body systems involved. The following symptoms are most common:

  • Muscle pain and tenderness, often appearing first in the hips and shoulders
  • Muscle weakness
  • Difficulty with every day tasks such as climbing stairs or rising from a chair
  • General fatigue (tiredness)
  • Difficulty speaking or swallowing
  • Shortness of breath or coughing
  • Skin rash
  • Joint pain and swelling
  • Weight loss
  • Sensitivity to cold in the extremities (Raynaud’s phenomenon)
  • Fever
  • Atrophy (loss of muscle mass).

Dermatomyositis and polymyositis symptoms tend to come on gradually over a few weeks or months, whereas IBM may take several years to develop.

Diagnosis and Treatment

IIM is very difficult to diagnose because the symptoms are nonspecific, meaning they’re common to many other diseases and conditions.

A diagnosis of IIM usually involves:

  • A thorough patient history and physical exam, including hands-on tests of muscle strength.
  • Blood tests for elevated levels of muscle enzymes and/or chemical inflammation markers.
  • Electro-diagnostic testing, which uses needles and electrodes to measure muscle and motor nerve activity.
  • Muscle or skin biopsy, in which a tissue sample is examined under a microscope.
  • Magnetic resonance imaging (MRI) to pinpoint areas of muscle weakness.
  • Other tests to rule out diseases with similar symptoms.

Medications are the first line of treatment for polymyositis and dermatomyositis. These may include:

  • Corticosteroids to help suppress the immune system and fight inflammation.
  • Immunosuppressive drugs to slow down autoimmune reactions.
  • Intravenous immune globulin (a blood product made from human plasma), which can “confuse” the recipient’s immune system and reduce inflammatory activity.
  • Biologics (substances derived from the human body) can sometimes relieve symptoms when other therapies fail.

To date, no medications have proved effective in the treatment of IBM.

In addition to medication, people with IIM often benefit from physical therapy to help maintain strength and range of motion. Regular low-impact exercise like aqua therapy may also be helpful.

People with IIM may need to adjust their schedules to allow extra time for sleep as well as periods of rest throughout the day.

Canes, walkers and wheelchairs may help patients stay mobile during flare-ups or in severe cases of the disease.

The prognosis (outlook) for people with IIM varies greatly. Some patients, especially children, recover completely. Others need long-term treatment with medication to reduce flares and manage symptoms. Even when effective, medication therapy may take 2-3 years to achieve beneficial results.

The degree to which IIM impacts quality of life varies greatly. However, most people with the condition can expect to live regular lifespans.


At present, there is no known way to prevent IIM, though research continues in this area.


  • “Facts About Inflammatory Myopathies (Myositis).” Muscular Dystrophy Association, 2011. Accessed April 25, 2014.
  • “Myositis.” American Academy of Orthopaedic Surgeons, 2007. Accessed April 25, 2014.
  • “Myositis.” The Arthritis Foundation. Accessed April 25, 2014.
  • “Myositis 101 (Slideshow).” Chester V. Oddis, University of Pittsburgh Medical Center, presented at The Myositis Association’s Annual Patient Conference, 2013. Accessed April 25, 2014.

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Tina Shahian, PhD

Tina is a writer for Innerbody Research, where she has written a large body of informative guides about health conditions.


A communication specialist in life science and biotech subjects, Tina’s successful career is rooted in her ability to convey complex scientific topics to diverse audiences. Tina earned her PhD in Biochemistry from the University of California, San Francisco and her BS degree in Cell Biology from U.C. Davis. Tina Shahian’s Linkedin profile.


In her spare time, Tina enjoys drawing science-related cartoons.