Idiopathic inflammatory myopathies are a group of autoimmune diseases that cause inflammation and weakness in the muscles and can also affect the skin and other organs. "Idiopathic" means "unknown," and "myopathy" means "abnormality or disease of muscle tissue."
Muscle damage may cause muscle weakness that leads to difficulty lifting the arms above the shoulders, climbing stairs, or arising from a sitting position.
Doctors check muscle enzymes in the blood and may test electrical activity of muscles, do magnetic resonance imaging on muscles, and examine a piece of muscle tissue.
Corticosteroids and other immunosuppressants, immune globulin, or a combination is usually helpful.
(See also Overview of Systemic Rheumatic Diseases.)
There are several types of idiopathic inflammatory myopathy:
Dermatomyositis
Antisynthetase syndrome
Immune-mediated necrotizing myopathy
Inclusion body myositis
Polymyositis
Overlap myositis
These diseases result in muscle inflammation (myositis), disabling muscle weakness, and occasionally tenderness. The weakness typically occurs in the shoulders and hips but can affect muscles symmetrically throughout the body.
Idiopathic inflammatory myopathies usually occur in adults aged 40 to 60 or in children aged 5 to 15. Women are more likely than men to develop a type.
In adults, idiopathic inflammatory myopathies may occur alone or as part of other systemic rheumatic diseases, such as mixed connective tissue disease, systemic lupus erythematosus, or systemic sclerosis.
The cause of idiopathic inflammatory myopathy is unknown, but an autoimmune reaction to muscle tissue in people who have certain genes seems to be a trigger, and these diseases can run in families. Other triggers include viral infections, certain medications, and cancer.
Types of Idiopathic Inflammatory Myopathies
Dermatomyositis usually causes skin changes that do not occur in other inflammatory myopathies, which helps doctors differentiate between the different types. In children (and occassionally in adults), dermatomyositis causes calcium to collect in or under the skin and in muscles or tendons. This complication is called calcinosis.
Antisynthetase syndrome can also cause various changes and symptoms, including inflammatory arthritis, fever, interstitial lung disease, thick and rough patches of skin on the fingers (mechanic's hands), and Raynaud syndrome.
Immune-mediated necrotizing myopathies can cause a weakness that is more severe and rapidly progressive than the other inflammatory myopathies and is more likely to cause difficulty swallowing.
Inclusion body myositis causes muscle weakness and wasting away of muscle, most commonly in the legs, hands, and feet. This disease develops in older people, progresses slower, and does not usually go away when people take immunosuppressants. Also, the muscle tissue has a different appearance under the microscope.
Polymyositis also causes weakness, but people do not have skin changes.
Overlap myositis is when an idiopathic inflammatory myopathy develops in a person who has another systemic rheumatic disease such as systemic lupus erythematosus or systemic sclerosis. People who have a systemic rheumatic disease have symptoms of that disease in addition to the myopathy.
Symptoms of Idiopathic Inflammatory Myopathies
The symptoms of an idiopathic inflammatory myopathy are similar for people of all ages, but the muscle inflammation often appears to develop more abruptly in children than in adults. Symptoms, which may begin during or just after an infection, include symmetrical muscle weakness (particularly in the upper arms and thighs), joint pain (but often little muscle pain), difficulty in swallowing, cough, fever, fatigue, and weight loss. Raynaud syndrome (in which the fingers suddenly become very pale and tingle or become numb in response to cold or emotional upset) may also occur.
Muscle weakness
Muscle weakness may start slowly or suddenly and may worsen for weeks or months.
Because muscles close to the center of the body are affected most, tasks such as lifting the arms above the shoulders (to brush the hair), climbing stairs, and getting out of a chair or off of a toilet seat can become very difficult. If the neck muscles are affected, even raising the head from a pillow may be impossible.
People who have weakness in the shoulders or hips may need to use a wheelchair or may become bedridden.
Muscle damage in the upper part of the esophagus can cause swallowing difficulties and regurgitation of food.
The muscles of the hands, feet, eyes, and face, however, are not usually affected, except in people who have inclusion body myositis.
Joint aches and inflammation
Joint aches and inflammation occur in some people.
Internal organ problems
Internal organs are usually not affected except for the throat and esophagus.
However, the lungs may be affected, causing interstitial lung disease, shortness of breath, and a cough. When the heart is affected, problems include pericarditis and cardiomyopathy.
Gastrointestinal symptoms are more common among people who have an overlap syndrome with systemic sclerosis. In children who have dermatomyositis, gastrointestinal symptoms may be caused by inflammation of the blood vessels (vasculitis) and may include vomiting of blood, black and tarry stools, and severe abdominal pain, sometimes with a hole (perforation) in the lining of the bowel.
Skin changes
Skin changes occur in people who have dermatomyositis.
Rashes tend to appear at the same time as muscle weakness and other symptoms. A reddish or purplish rash (called a heliotrope rash) can appear on the face with reddish purple swelling around the eyes. The rash on the scalp may look like psoriasis and be intensely itchy. The rash may also be raised and scaly and may appear almost anywhere on the body but is especially common on the knuckles, V of the neck and shoulders, chest and back, forearms and lower legs, elbows, knees, outer part of upper thighs, and parts of the hands and feet. Raised, reddish bumps may appear on the large knuckles (called Gottron papules) and sometimes on the small knuckles. The area around the nails may redden or thicken, and cuticles tend to overgrow. Sun sensitivity can occur as well.
When the rash fades, brownish pigmentation, scarring, shriveling, or pale depigmented patches may develop on the skin.
Bumps composed of calcium may develop under the skin or in muscle (calcinosis), particularly in children.
This photo shows a heliotrope (reddish/purplish) rash across the forehead, cheeks, eyes, and bridge of the nose of a person with dermatomyositis.
RICHARD USATINE MD / SCIENCE PHOTO LIBRARY
This photo shows Gottron papules (on the large knuckles), bumps composed of calcium under the skin (on the large and small knuckles), and redness and thickening around the nails.
© Springer Science+Business Media
This photo shows a heliotrope (reddish/purplish) rash across the forehead, cheeks, eyes, and bridge of the nose of a person with dermatomyositis.
RICHARD USATINE MD / SCIENCE PHOTO LIBRARY
This photo shows Gottron papules (on the large knuckles), bumps composed of calcium under the skin (on the large and small knuckles), and redness and thickening around the nails.
© Springer Science+Business Media
Sometimes these characteristic skin changes occur in people who do not have muscle weakness and inflammation. In this case, the disease is called amyopathic dermatomyositis.
Diagnosis of Idiopathic Inflammatory Myopathies
A doctor's evaluation
Blood tests, electromyography, magnetic resonance imaging (MRI), and sometimes muscle biopsy
The diagnosis of idiopathic inflammatory myopathy is based on all of the information doctors gather, including symptoms, physical examination results, and all test results.
Blood tests
Blood tests help doctors rule out other disorders, detect the risk of complications, determine severity, and which organs may be affected. For example, doctors test for increased levels of certain muscle enzymes (especially creatine kinase) in the blood, which indicate muscle inflammation, and for the presence of antinuclear antibodies (ANA) and other antibodies, which are often present in people who have idiopathic inflammatory myopathy.
Although blood test results can help doctors diagnose idiopathic inflammatory myopathy, they alone cannot confirm a definite diagnosis because sometimes the abnormalities they detect are present in healthy people or in people who have other disorders.
Electromyography and imaging tests
Electromyography (EMG) is used to detect abnormalities in the electrical activity of muscles. This test may be done in people who have muscle weakness and elevated levels of muscle enzymes in their blood. EMG may be done with or without a magnetic resonance imaging (MRI) scan, which shows abnormalities in the muscles. MRI may also help the doctor select a site for biopsy.
X-rays and computed tomography (CT) of the chest are typically done in people who might have interstitial lung disease. Tests of the lungs and muscles needed for breathing are also often done.
A barium x-ray is done in people who have swallowing difficulty.
Biopsy
A muscle biopsy can help doctors distinguish between different types of idiopathic inflammatory myopathy and rule out other causes of muscle weakness.
Muscle biopsy is not usually necessary when people have the characteristic skin changes of dermatomyositis.
Other tests
Doctors may screen people for cancer. Screening may include blood and imaging tests.
People age 40 or older may need additional cancer screening because there may be an increased risk of cancer with some of the inflammatory myopathies.
Treatment of Idiopathic Inflammatory Myopathies
Corticosteroids
Other immunosuppressants
Sometimes immune globulin
Modest restriction of activities often helps when muscle inflammation is most intense.
prednisone
To monitor how the disorder is responding to corticosteroid treatment, doctors periodically do a blood test to measure the levels of muscle enzymes. The levels usually fall to normal or near normal during treatment and muscle strength returns after approximately 6 to 12 weeks. MRI may also show areas of inflammation and help doctors determine whether treatment is working. Once enzyme levels have returned to normal, prednisone can be gradually reduced. If muscle enzyme levels rise again, the dose is increased.
Another possible treatment is immune globulin (a substance that contains large quantities of many antibodies) given by vein (intravenously). Some people receive a combination of corticosteroids, immunosuppressants, and immune globulin.
When the idiopathic inflammatory myopathy is related to a cancer, it is usually more difficult to treat with immunosuppressants. However, if the cancer is successfully treated, the myopathy becomes easier to treat.
People who take corticosteroids are at risk of fractures related to osteoporosis. To prevent osteoporosis, these people are given and sometimes medications to treat osteoporosis.
People who are receiving immunosuppressants are also given medications to prevent infections such as by the fungus Pneumocystis jirovecii (see prevention of pneumonia in people with a weakened immune system) and vaccines against common infections such as pneumonia, influenza, and COVID-19.
People who have idiopathic inflammatory myopathy are at increased risk of atherosclerosis and are closely monitored by doctors.
Prognosis for Idiopathic Inflammatory Myopathies
People who have idiopathic inflammatory myopathy and who are treated survive for many years after diagnosis.
Children who have dermatomyositis may develop severe inflammation of the blood vessels (vasculitis) that supply the bowel, which can ultimately lead to bowel perforation and death if untreated.
