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Basil T Darras, MD
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Ira N Targoff, MD
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Paul L Romain, MD


Last literature review version 19.3: Fri Sep 30 00:00:00 GMT 2011 | This topic last updated: Thu Aug 26 00:00:00 GMT 2010 (More)

MYOSITIS OVERVIEW — Myositis is the word used to describe a group of muscle diseases caused by inflammation in the muscle tissues.

Normally, electrical signals travel from the brain through the spinal cord and into nerves. From there, the nerve signals are transmitted into muscles, where the signals stimulate the muscle tissue to contract or relax. Along the way, the nerves must be in good working order and the muscles must be able to accept the impulses and generate a response.

Problems can occur anywhere along this route. If the brain, spinal cord, or nerves are damaged or diseased, the electrical signal may not be generated or it may not reach the muscle. If muscles are inflamed or abnormal, they may not be able to respond properly to a nerve impulse.

In people with muscle disease, nerve impulses are usually able to reach the muscles without a problem, but the muscles are unable to respond in a normal way. Many things can cause muscle disease, a few of which include:

  • Inherited conditions, especially muscular dystrophies
  • Problems with certain hormones that affect muscle function
  • Chemical imbalances in the body
  • Drugs and toxic substances
  • Infections
  • Inflammation of the muscles

This topic review will focus on inflammatory forms of muscle disease or myositis. Dermatomyositis (DM), polymyositis (PM), and inclusion body myositis (IBM) are three types of myositis; each disorder has unique characteristics.

DERMATOMYOSITIS AND POLYMYOSITIS — Dermatomyositis and polymyositis cause inflammation of the muscles. These are rare disorders, together affecting only about one in 100,000 people per year. More women than men are affected. Although the peak age of onset is in the 50s, the disorders can occur at any age. Dermatomyositis and polymyositis have similar symptoms, although there are different mechanisms that cause the muscle inflammation.

Dermatomyositis and polymyositis symptoms — The most common symptom of dermatomyositis and polymyositis is muscle weakness. People with dermatomyositis also have a skin rash, although it can sometimes be very mild. More detailed information about dermatomyositis and polymyositis symptoms is available separately. (See “Clinical manifestations and diagnosis of adult dermatomyositis and polymyositis”.)

Muscle weakness — A person with dermatomyositis or polymyositis may notice muscle weakness that worsens over several months, although in some cases symptoms develop more rapidly. The affected muscles are typically close to the trunk (as opposed to the wrists or feet), involving the hip, shoulder, or neck muscles. Muscles on both sides of the body are equally affected. In some cases, muscles are sore or tender. There may be a loss of muscle in more advanced disease.

Some people with dermatomyositis or polymyositis have involvement of the muscles of the pharynx (throat) or the esophagus (the tube leading from the throat to the stomach) (figure 1), causing problems with swallowing. In some cases, the muscle weakness allows food to be misdirected to the lungs, which can lead to pneumonia.

Skin changes — People with dermatomyositis often develop a rash or other changes in the skin. Sometimes the rash develops before muscle problems occur. In some cases, the rash of dermatomyositis appears but myopathy never develops. Several types of rash may occur:

  • Gottron’s sign — Gottron’s sign is a reddened, often scaly rash that develops on the finger joints, although it can spread from there (picture 1A-B). Similar lesions can be seen over the front of the knees or outer arms, similar to psoriasis.
  • Shawl sign — The shawl sign is a widespread, flat reddened area that is seen over the upper back, shoulders, and back of the neck. This can be worsened by exposure to ultraviolet light.
  • V sign — The V sign has an appearance similar to that of the shawl sign, but occurs over the front of the chest in the area of skin exposed by a V-necked sweater.
  • Heliotrope rash — The heliotrope rash is located on the upper eyelids, and is often accompanied by eyelid swelling (picture 2). The heliotrope sign is named for the heliotrope flower, which is violet colored.
  • Nail abnormalities — The nailfolds (the skin around the fingernails) may become reddened (picture 3) and develop changes in the blood vessels (picture 4).
  • Mechanic’s hands — People with dermatomyositis or polymyositis may develop “mechanic’s hands,” a roughening and cracking of the skin of the tips and sides of the fingers, resulting in irregular, dirty-appearing lines that resemble those of a manual laborer (picture 5).
  • Scalp — Changes in the scalp resembling psoriasis often occur in people with dermatomyositis.
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Dermatomyositis and polymyositis diagnosis — In addition to a careful history and physical exam, a number of tests are useful in diagnosing dermatomyositis and polymyositis. Blood tests typically show evidence of muscle damage and/or characteristic antibodies. An electromyogram (EMG) often shows abnormal muscle electrical activity.

Muscle biopsy is the most accurate test to definitively diagnose dermatomyositis or polymyositis. Careful analysis of the muscle cells with a microscope will reveal the typical inflammation seen in each disorder. However, in patients with typical dermatomyositis, EMG and muscle biopsy are not always required to make the diagnosis. (See “Clinical manifestations and diagnosis of adult dermatomyositis and polymyositis”.)

Electromyography — Electromyography (EMG) is a test that evaluates the health of muscles and the nerves associated with the muscles by measuring the electrical activity. It is used to aid in the diagnosis of most types of myositis.

A small needle is inserted through the skin into a muscle in several locations (usually the arms and legs). The needle is connected to a recording device that displays the muscle’s electrical activity at rest and in response to contraction. The electrical activity may also be heard as a static-type noise through a speaker.

The size and pattern of electrical activity recorded from different muscles is analyzed to determine whether the muscles and nerves are affected. You may feel some discomfort as the needle is inserted, and the muscle may feel sore or bruised for several days. The test generally takes about 30 minutes.

Muscle biopsy — A muscle biopsy is a procedure that removes one or more small piece(s) of muscle in order for a pathologist to examine it with a microscope.

Before the biopsy, the patient is given local anesthesia (numbing medicine) to prevent pain. The clinician then makes a small incision, usually in the thigh or shoulder, and removes a piece of muscle. The procedure usually takes between 15 and 30 minutes. The area will feel sore for several days after the biopsy. The biopsy results are usually available within two to three weeks.

MRI — Magnetic resonance imaging (MRI) can be useful in evaluating patients with dermatomyositis or polymyositis because it can assess large areas of muscle and is not invasive (like biopsy and EMG). MRI cannot replace muscle biopsy to confirm the diagnosis, but it may be used to evaluate changes in the muscles over time.

Dermatomyositis and polymyositis treatment — Treatments for dermatomyositis and polymyositis are intended to suppress the immune system, which is thought to be the cause of muscle inflammation. However, dermatomyositis and polymyositis are rare disorders and the optimal treatment regimen is not known. Until more data are available, treatment is guided by the severity of an individual’s symptoms and their complications. (See “Initial treatment of dermatomyositis and polymyositis in adults”.)

Glucocorticoids (steroids) — Glucocorticoids such as prednisone are typically used for treating people with dermatomyositis or polymyositis. Prednisone is usually started at a high dose, which is often continued for a number of weeks or sometimes longer before decreasing slowly to the lowest dose that is effective. The duration of therapy depends upon the person’s response and other factors.

Side effects of prednisone are common, and can include skin changes (thinning, development of small red spots), cataracts, heart disease, gastrointestinal problems, and fluid retention, among others. The risk and severity of side effects are decreased by using the lowest necessary dose.

Some people begin to see an improvement in muscle strength within several weeks of starting prednisone while others require up to three months to see improvement. To monitor improvement, the clinician will see the person periodically to measure strength. Blood testing may also be recommended, and the clinician will monitor for medication side effects or complications.

If improvement is seen, the prednisone dose may be slowly decreased. If improvement is not seen, a second medication, such as methotrexate or azathioprine, may be added. Most people with inflammatory myopathies can improve with steroids alone. However, polymyositis may be more difficult to treat in some cases.

SEE MORE:  Myositis

Methotrexate or azathioprine — Some people are given a second medication, such as azathioprine or methotrexate, to slow or stop the progression of their disease. Use of a second medication can help to reduce the dose of prednisone that is required to control muscle damage, which can reduce the risk of prednisone side effects and complications. These medications are usually well-tolerated, but side effects may sometimes occur:

  • Side effects of azathioprine — Flu-like reactions, including fever, develop in a small percentage of patients treated with azathioprine. This type of reaction requires that you stop azathioprine. Other side effects can include nausea and vomiting, low blood count, inflammation of the pancreas (pancreatitis), and liver problems. Long-term side effects may include an increased risk of infection or cancer.
  • Side effects of methotrexate — Common side effects of methotrexate include upset stomach and a sore mouth. Methotrexate can interfere with the bone marrow’s production of blood cells; folic acid or folinic acid can reduce the risk of some side effects. Low blood cell counts can cause fever, infections, swollen lymph nodes, and easy bruisability and bleeding. Liver or lung damage can occur as a result of methotrexate, even with low doses; monitoring can reduce the risk of these problems.

Tapering treatment — If dermatomyositis or polymyositis is well controlled for a period of time, your clinician may consider slowly decreasing and then stopping your medication. You and your healthcare provider must watch closely for any signs of worsening muscle weakness during this time; if weakness develops or worsens, consult with your clinician to determine if additional evaluation or treatment is needed.

Treatment of resistant disease — If your dermatomyositis or polymyositis does not improve with glucocorticoids and a medicine such as methotrexate or azathioprine, you may be need to try other options, including intravenous immune globulin or other medications.

Preventive measures — In addition to medications, people with dermatomyositis or polymyositis should take precautions to prevent complications related to the disease and its treatments. These precautions include:

  • Osteoporosis prevention — A calcium supplement with vitamin D, and a prescription medication to prevent osteoporosis, such as one of the bisphosphonates, are often recommended. The potential benefits and possible side effects of such medications should be discussed with a healthcare provider. (See “Patient information: Calcium and vitamin D for bone health” and “Patient information: Osteoporosis prevention and treatment”.)
  • Exercise — Physical therapy and rehabilitation should begin soon after the diagnosis of dermatomyositis or polymyositis to prevent contractures (shortening of the muscle that can limit joint movement).
  • Avoidance of sunlight – People with dermatomyositis and rash should protect themselves from the sun with sunscreen or by sitting in a shaded area. (See “Patient information: Sunburn prevention”.).
  • Preventing aspiration — If you have difficulty swallowing, you must take care to avoid inhaling (aspirating) foods and drinks. This may be done by elevating your head, using thickened drinks, or in severe cases, by having a feeding tube placed in the stomach.
  • Pneumonia prevention — The medications used to treat dermatomyositis and polymyositis (glucocorticoids, azathioprine, and methotrexate) weaken the immune system. This can increase the risk of certain infections, including a type of pneumonia called pneumocystis. To decrease this risk, an antibiotic (eg trimethoprim-sulfamethoxazole) may be recommended.

Pregnancy and myositis — Little information is available regarding the impact of pregnancy on myositis or the impact of myositis on pregnancy. The data that are available indicate that complications of pregnancy are less likely in women who have inactive disease. Complications may include a smaller than normal infant, stillbirth, or premature birth.

Women with polymyositis or dermatomyositis who want to become pregnant should discuss their condition and current medications with a rheumatologist or maternal fetal medicine specialist before trying to conceive. Some medications, particularly methotrexate, are not safe for use during pregnancy due to a seriously increased risk of miscarriage and birth defects.

Dermatomyositis and polymyositis prognosis — The severity of disease in a person with dermatomyositis or polymyositis is highly variable, ranging from mild weakness that responds well to treatment to a rapid progression of symptoms that are unresponsive to all treatments. Less commonly, people with these conditions improve spontaneously without any treatment.

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People with dermatomyositis or polymyositis tend to have a better outcome if they are treated promptly, have mild muscle weakness, have no difficulty swallowing, and have no signs of disease in other organ systems such as the heart and lungs.

INCLUSION BODY MYOSITIS — Inclusion body myositis (IBM) differs in a number of ways from polymyositis and dermatomyositis. It affects more men than women, and the age of onset is almost always after age 60, although it can affect persons between 30 and 90 years.

Inclusion body myositis symptoms — Typically, inclusion body myositis symptoms come on very slowly, over an average period of six years. The first area to be affected by weakness may be the hips and upper legs, but some people with inclusion body myositis have weakness in the hands or feet. Weakness is accompanied by muscle pain in 40 percent of patients. In 10 to 15 percent of cases, one side of the body is affected more than the other. Facial muscles may be involved, although the eye and mouth muscles are usually not affected. (See “Clinical manifestations and diagnosis of inclusion body myositis”.)

Muscle atrophy often progresses along with the weakness. Some people have profound thinning of the muscles in the upper arms or upper legs. Between one-third and one-half of patients have difficulty swallowing due to weakness in the throat muscles.

Inclusion body myositis diagnosis — Diagnosis of inclusion body myositis is usually based upon signs and symptoms of progressive muscle weakness and results of a muscle biopsy.

A muscle biopsy is done to determine if abnormal structures are present in the muscle cells. These features are seen in over 90 percent of people with inclusion body myositis (see ‘Muscle biopsy’ above).

Blood tests may be done to look for signs of muscle damage, although the results may be normal or only mildly elevated. Electromyography (EMG) usually shows abnormal electrical activity, and MRI may reveal abnormalities in some cases (see ‘Electromyography’ above).

Inclusion body myositis treatment — Unlike dermatomyositis and polymyositis, inclusion body myositis often does not improve with treatments that suppress the immune system. Muscle strength usually improves minimally, if at all, with glucocorticoids and other treatments. Some of the same medications used for dermatomyositis or polymyositis may be tried. (See “Course and therapy of inclusion body myositis”.)

Preventive treatments are also important for people with inclusion body myositis (see ‘Preventive measures’ above).

Inclusion body myositis prognosis — People with inclusion body myositis may become progressively weaker if not treated or if treatment is not successful. Progression is usually more rapid in older patients.

Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.

Patient Level Information:

Patient information: Calcium and vitamin D for bone health
Patient information: Sunburn prevention

Professional Level Information:

Clinical manifestations and diagnosis of adult dermatomyositis and polymyositis
Clinical manifestations and diagnosis of inclusion body myositis
Clinical manifestations of mixed connective tissue disease
Course and therapy of inclusion body myositis
Diagnosis of juvenile dermatomyositis and polymyositis
Drug-induced myopathies
Initial treatment of dermatomyositis and polymyositis in adults
Interstitial lung disease in dermatomyositis and polymyositis: Clinical manifestations and diagnosis
Interstitial lung disease in dermatomyositis and polymyositis: Treatment
Malignancy in dermatomyositis and polymyositis
Pathogenesis and clinical manifestations of juvenile dermatomyositis and polymyositis
Treatment and prognosis of juvenile dermatomyositis and polymyositis
Treatment of recurrent and resistant dermatomyositis and polymyositis in adults
Viral myositis

The following organizations also provide reliable health information.

  • National Library of Medicine


  • American Academy of Family Physicians


  • National Institute of Arthritis and Musculoskeletal and Skin Diseases


  • American College of Rheumatology



  1. Dalakas MC, Hohlfeld R. Polymyositis and dermatomyositis. Lancet 2003; 362:971.
  2. Plotz PH, Dalakas M, Leff RL, et al. Current concepts in the idiopathic inflammatory myopathies: polymyositis, dermatomyositis, and related disorders. Ann Intern Med 1989; 111:143.
  3. Bohan A, Peter JB. Polymyositis and dermatomyositis (second of two parts). N Engl J Med 1975; 292:403.
  4. O’Connell MJ, Powell T, Brennan D, et al. Whole-body MR imaging in the diagnosis of polymyositis. AJR Am J Roentgenol 2002; 179:967.
  5. Lundberg IE. Idiopathic inflammatory myopathies: why do the muscles become weak? Curr Opin Rheumatol 2001; 13:457.