What Is Myositis?

Myositis is the general inflammation of muscle that then leads to diffuse muscle weakness. It is a rare autoimmune disease with no known cure. If left untreated, it could lead to severe disability or death.

However, this disease is treatable, and the outcomes are generally favorable. Like most autoimmune conditions, myositis can be chronic. It can require frequent follow-up appointments with a rheumatologist and other specialists.

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Idiopathic Inflammatory Myopathy

When people refer to myositis, they often mean a disease involving chronic inflammation of the muscles known as idiopathic inflammatory myopathy (IIM). IIM affects some 50,000 to 75,000 people in the U.S., but the cause is unknown.

In general, patients may have a genetic predisposition to develop an autoimmune problem. An additional “trigger” could set the process off.

Some possible triggers include:

  • Infections
  • Toxins
  • Viruses

When someone has an autoimmune disease, the body’s immune system attacks its own healthy tissue – leading to chronic inflammation in the muscle. Unfortunately, this autoimmune attack can affect other organs or tissues, such as the skin, joints, and lungs. Muscles involved with swallowing also can be affected.

Forms of Myositis

Myositis comes in many forms that share similar symptoms while also affecting the body differently.

The forms include:


Polymyositis (PM) typically affects middle-aged and elderly individuals. It is characterized by muscle weakness generally affecting the muscles closest to the torso, including:

  • The neck.
  • The back.
  • The upper arms.
  • The thighs.

Lung involvement and arthritis may occur, and patients often are fatigued. There may be difficulty swallowing if these muscles also are affected.


Dermatomyositis (DM) is similar to PM but is distinguished by the presence of characteristic skin rashes on the eyelids, face, scalp, back, and upper chest. The rashes also may appear over the knuckles, knees, and elbows.

Other findings in DM include a higher likelihood of certain cancers (particularly in older individuals).

Necrotizing myopathy

Necrotizing myopathy (NM) is another form of myositis. It is characterized by very severe muscle weakness in a similar pattern to that found in PM and DM, including problems with swallowing. However, the skin, lungs, and joints are rarely involved.

These patients often have very high levels of muscle enzymes that a blood test can detect. NM can occur in some patients taking cholesterol-lowering drugs, which are very commonly prescribed. However, this complication is extremely rare.

Sporadic inclusion body myositis

Sporadic inclusion body myositis (sIBM) is most common in patients over age 50 and is more common in men.

This form of myositis progresses more slowly, and the pattern of muscle weakness is different from that seen in PM and DM. People may have sIBM for many years without knowing before getting a diagnosis.

Patients with sIBM experience difficulty:

  • Flexing the fingers, writing, and gripping.
  • Using keys or hand tools.
  • Walking. They may have falling episodes.

sIBM patients generally do not respond to the medications that are effective for PM and DM. That’s because the cause of weakness is likely not autoimmune.

Juvenile myositis

Juvenile myositis is the rarest form of myositis. It affects only two to four children per million.

The most common form of juvenile myositis is juvenile DM. It causes a similar pattern of muscle weakness as the DM found in adults, including the same types of skin rashes. However, children with DM do not have an increased risk for cancer.

Learn more about myositis at the UPMC Myositis Center in the Division of Rheumatology and Clinical Immunology. Or, call 1-800-533-8762 to schedule an appointment.

About UPMC

Headquartered in Pittsburgh, UPMC is a world-renowned health care provider and insurer. We operate 40 hospitals and 800 doctors’ offices and outpatient centers, with locations throughout Pennsylvania, Maryland, New York, West Virginia, and internationally. We employ 4,900 physicians, and we are leaders in clinical care, groundbreaking research, and treatment breakthroughs. U.S. News & World Report consistently ranks UPMC Presbyterian Shadyside as one of the nation’s best hospitals in many specialties and ranks UPMC Children’s Hospital of Pittsburgh on its Honor Roll of America’s Best Children’s Hospitals. We are dedicated to providing Life Changing Medicine to our communities.