Medically reviewed by: Stephanie Curreli, MD, PhD
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:

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:

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:

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

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.


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Tina Shahian, PhD

Tina is a Life Science Writer for a number of online publications, including Her expertise is in conveying 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. In her spare time, she enjoys drawing science-related cartoons.