Autoimmune myositis causes inflammation and weakness in the muscles (polymyositis) or in the skin and muscles (dermatomyositis).
Muscle damage may cause muscle pain and muscle weakness may cause 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, immunosuppressive drugs, immune globulin, or a combination is usually helpful.
(See also Overview of Autoimmune Disorders of Connective Tissue.)
These disorders 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.
Autoimmune myositis usually occurs in adults aged 40 to 60 or in children aged 5 to 15 years. Women are twice as likely as men to develop it. Black people are 3 to 4 times more likely to develop it than White people. In adults, these disorders may occur alone or as part of other connective tissue disorders, such as mixed connective tissue disease, systemic lupus erythematosus, or systemic sclerosis.
The cause of autoimmune myositis is unknown. Viruses may play a role, and cancer may occasionally trigger this disorder. The disorder can run in families.
There are four types of autoimmune myositis:
Polymyositis
Dermatomyositis
Necrotizing immune-mediated myopathies
Inclusion body myositis
Dermatomyositis usually causes skin changes that do not occur in polymyositis, which helps doctors differentiate the two disorders. Biopsy samples of muscle also look different under the microscope.
Necrotizing immune-mediated myopathies are disorders that kill muscle cells (myocytes) and do not damage tissue other than the muscles.
Inclusion body myositis is a separate disorder that can cause muscle weakness and wasting away of muscle, most commonly in the legs, hands, and feet. However, this disorder develops in older people, progresses slower, and does not generally respond to therapy. Also, the muscle tissue has a different appearance under the microscope.
Symptoms of Autoimmune Myositis
The symptoms of autoimmune myositis 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, hips, and thighs), joint pain (but often little muscle pain), difficulty in swallowing, 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 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, 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, and face, however, are not usually affected.
Joint aches and inflammation occur in about 30% of people.
Internal organs are usually not affected except for the throat and esophagus. However, the lungs and heart may be affected, causing abnormal heart rhythms (arrhythmias), shortness of breath, and a cough. Gastrointestinal symptoms, which can occur in children but typically not in adults, are 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.
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Skin changes occur in people who have dermatomyositis. Rashes tend to appear at the same time as muscle weakness and other symptoms. A dusky or purplish rash (called a heliotrope rash) can appear on the face with reddish purple swelling around the eyes. The rash may also be raised and scaly and may appear almost anywhere on the body but is especially common on the knuckles, elbows, knees, outer part of upper thighs, and parts of the hands and feet. The area around the nails may redden or thicken. When the rash fades, brownish pigmentation, scarring, shriveling, or pale depigmented patches may develop on the skin. The rash on the scalp may look like psoriasis and be intensely itchy. Sun sensitivity and sores on the skin occur as well. Bumps composed of calcium may develop under the skin or in muscle, particularly in children. Raised, reddish bumps may appear on the large knuckles (called Gottron papules) and sometimes on the small knuckles.
Sometimes these characteristic skin changes occur in people who do not have muscle weakness and inflammation. In this case, the disorder is called amyopathic dermatomyositis.
Diagnosis of Autoimmune Myositis
Established criteria
Muscle biopsy
Doctors use the following criteria to make the diagnosis of autoimmune myositis:
Muscle weakness at the shoulders or hips and thighs
Sometimes a characteristic rash
Increased levels of certain muscle enzymes (especially creatine kinase) in the blood, indicating muscle damage
Abnormalities in electrical activity of muscles as measured by electromyography or in appearance of muscles on a magnetic resonance imaging (MRI) scan
Characteristic changes in muscle tissue obtained by biopsy and observed under a microscope (the most conclusive evidence)
The diagnosis of autoimmune myositis is based on all of the information doctors gather, including symptoms, physical examination results, and all test results.
Muscle biopsy is often done and is the most conclusive way to diagnose autoimmune myositis, especially when the diagnosis is not clear. Muscle biopsy is not usually necessary when people have characteristic skin changes of dermatomyositis.
Other laboratory tests cannot specifically identify autoimmune myositis, but they can help doctors rule out other disorders, detect the risk of complications, and determine severity.
Blood tests are done to detect levels of antinuclear antibodies (ANA) and other antibodies, which are present in most people who have autoimmune myositis. Although blood test results can help doctors diagnose autoimmune myositis, they alone cannot confirm a definite diagnosis of autoimmune myositis because sometimes the abnormalities they detect are present in healthy people or in people who have other disorders.
MRI may also help the doctor select a site for biopsy. Special tests done on muscle tissue samples may be needed to rule out other muscle disorders.
Doctors often suggest cancer screening for people age 40 or older who have dermatomyositis or for people age 60 or older who have polymyositis because these people may have unsuspected cancers.
Prognosis for Autoimmune Myositis
Up to 50% of people (especially children) who have received treatment within 5 years of diagnosis often have a long symptom-free period (remission), and some may even appear to recover completely. However, the disorder may still return (relapse) at any time. About 75% of people survive at least 5 years after the diagnosis is made. This percentage is even higher among children.
Adults are at risk of death resulting from severe and progressive muscle weakness, difficulty swallowing, undernutrition, inhalation of food that causes pneumonia (aspiration pneumonia), or respiratory failure, which often occurs at the same time as pneumonia.
Children who have the juvenile form of dermatomyositis may develop severe inflammation of the blood vessels (vasculitis) that supply the bowel, which can ultimately lead to bowel perforation if untreated.
Polymyositis tends to be more severe and resistant to treatment in people whose heart or lungs are affected. Polymyositis and particularly dermatomyositis have been linked to an increased risk of cancer. In people who have cancer, it is the cancer rather than the autoimmune myositis that is the cause of death.
Treatment of Autoimmune Myositis
Corticosteroids
Immunosuppressive drugs
Sometimes immune globulin
Modest restriction of activities when the muscle inflammation is most intense often helps.
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, immunosuppressive drugs, and immune globulin.
People who take corticosteroids are at risk of fractures related to osteoporosis. To prevent osteoporosis, these people are given the drugs used to treat osteoporosis, such as bisphosphonates and .
People who are receiving immunosuppressants are also given drugs to prevent infections such as by the fungus Pneumocystis jirovecii (see prevention of pneumonia in immunocompromised people) and vaccines against common infections such as pneumonia, influenza, and COVID-19.
People who have autoimmune myositis are at increased risk of atherosclerosis and are closely monitored by doctors.
