NORD gratefully acknowledges Kristina Bundra, Pharm. D, NORD Editorial Intern, and Chester V. Oddis, MD, Professor of Medicine, University of Pittsburgh School of Medicine, for assistance in the preparation of this report.
Polymyositis (PM) is a type of inflammatory myopathy characterized by inflammatory and degenerative changes in the muscles, leading to symmetric weakness and some degree of muscle wasting (atrophy). The muscles that are principally affected include those closest to and within the trunk of the body such as the hip, shoulders, arms, pharynx and neck. Polymyositis occurs most often in women over 20 years of age, but men can also be affected. Muscle weakness usually happens over days, weeks or months. Some affected people have muscle pain, breathing problems, and trouble swallowing.
The inflammatory myopathies are a group of diseases that involve chronic muscle inflammation and weakness. They are thought to be autoimmune diseases, meaning the body’s natural defenses (antibodies, lymphocytes, etc.) against invading organisms begin to attack perfectly healthy tissue for unknown reasons, leading to inflammation or swelling.
The symptoms of PM may start gradually or suddenly and often wax and wane for no apparent reason.
The major symptom of the disorder is muscle weakness, most often in the hip and shoulder areas, eventually making it difficult for patients to lift their arms, get out of a chair or to climb steps. Other muscles which may be affected are the neck and throat muscles, which may result in difficulty swallowing and cause changes in the voice due to muscle weakness. Rarely, chest muscles are affected.
The muscle weakness may appear suddenly and progress over weeks to months. Additionally, the degree of muscle weakness may fluctuate over time as well. The muscles of the hands, feet and face are rarely involved in polymyositis, but generalized muscle pain may occur. Atrophy (loss of bulk) of muscle and contractures of the muscle around larger joints may develop late in the chronic stage.
Other symptoms of PM may include fever, weight loss, fatigue, joint pain and Raynaud phenomenon which is a sensitivity to the cold that is most often felt in the fingers. Raynaud phenomenon is caused by constriction (narrowing) of the blood vessels in the fingers upon exposure to the cold. (For more information, choose “Raynaud” as your search term in the Rare Disease Database.) Cracking of the fingers, termed “mechanic’s hands” may also develop.
The joint pain seen in polymyositis (polyarthralgia) may be accompanied by swelling and mimic the findings seen in patients with rheumatoid arthritis but generally these rheumatic complaints are mild and respond well to glucocorticoids (i.e. prednisone). Gastrointestinal involvement can lead to difficulty swallowing as the muscles involved in swallowing can become inflamed and weak, and the esophagus can be affected leading to heartburn but lower GI involvement is uncommon. Involvement of the heart, detected chiefly by irregularities in the electrocardiogram (ECG), has been reported but inflammation and weakening of the heart muscle (myocarditis) is rare.
Interstitial lung disease is a serious complication seen in PM and leads to cough with shortness of breath as inflammation occurs in the lung tissue. This may progress to scarring (fibrosis) of the lung tissue making the lungs stiff and inelastic. In some instances lung disease may precede myositis and dominate the clinical picture.
Symptoms may worsen during pregnancy in women whose disease is active and active PM can increase the risk of premature birth or stillbirth.
Myositis can occur in overlap with other autoimmune or rheumatic diseases, including Sjogren’s syndrome (For more information, choose “Sjogren” as your search term in the Rare Disease Database.) or scleroderma (systemic sclerosis) (For more information, choose “Scleroderma” as your search term in the Rare Disease Database). In addition, malignancy may complicate the course and presentation of PM but is more common in dermatomyositis. This is more common in males and females over the age of 50.
Abdominal symptoms, more common in children with juvenile dermatomyositis (JDM), may be associated with the passage of dark stools or the vomiting of blood from gastrointestinal ulcerations that may progress to perforation and require surgical intervention. Calcinosis (the deposition of calcium deposits in the skin and soft tissue or muscle) is also more frequent in JDM.
The exact cause of PM is unknown. Like other autoimmune diseases, the body’s natural immune defense mechanisms attack its own tissue in polymyositis. This can occur through a T-cell mediated destructive process directed against unidentified muscle antigens, but the finding of myositis-specific autoantibodies in many PM patients also supports disorders associated with B cells.
Exposure to certain drugs has been shown to increase the likelihood of developing myopathy. A common drug known to induce a necrotizing form of myositis are the statin agents and a specific autoantibody (anti-HMGCR) has been found to be associated with statin-induced myopathy.
The injection of collagen for cosmetic use has been implicated in the onset of a polymyositis-like syndrome. Injectable bovine collagen has been used to reduce wrinkles and facial scars, usually in women. The onset of PM in such patients is felt to result from an autoimmune reaction to the collagen.
Polymyositis may appear at any time from infancy through the age of 80 years, but most commonly occurs in adults greater than 20 years of age, especially those aged 45 to 60 years. Juvenile PM is very rare (much less common than JDM) and the symptoms usually appear between the ages of five to 15 years. Females are affected twice as often as males and PM is more common in African Americans than in Caucasians.
The diagnosis of PM is often delayed due to the lack of physical findings before the onset of muscle disease. Both family history and medication history are important in excluding other causes of myopathy. Additionally, various tests may be performed to establish a diagnosis. Tests may include: (a) electromyography done by a specialist in neuromuscular diseases which detects characteristic electrical patterns in muscle tissue and are abnormal in almost all patients with polymyositis; (b) muscle biopsy which reveals inflammation in the muscle tissue; (c) magnetic resonance imaging (MRI) of the affected muscle(s) which demonstrates inflammation within the muscle tissue. Blood tests can be performed to detect elevated levels of muscle enzymes, predominantly creatine kinase (CK) and aldolase among others, which are indicative of muscle damage. Autoantibodies have been identified in many PM patients consistent with an autoimmune cause as discussed earlier.
Polymyositis generally responds well to treatment in most patients, although, residual weakness may occur in approximately 30% of patients.
Glucocorticoids (i.e. steroids or prednisone) are widely used as the initial form of treatment in PM. They control the inflammation, lessen pain, and increase muscle strength through their ability to suppress the body’s immune system. The specific dose is individualized; however, the starting dose of prednisone is generally high at about 1mg/kg of body weight (often equating to about 60mg/day). They are then tapered depending on the response of the patient and associated side effects. Measurement of the serum CK is used to gauge the effectiveness of therapy and reduction of the muscle enzymes to normal values is noted in a majority of patients with this disorder within 4 to 8 weeks after treatment is started. This is followed by an improvement in muscle strength. As the steroids are reduced other medications that also suppress the immune system may be added so that the glucocorticoids can be tapered. This is necessary to reduce the side effects (i.e. osteoporosis, weight gain, change in mental status, increased blood sugar, cataracts, stomach upset, etc.) that the steroids cause. In many cases of adult polymyositis prolonged maintenance therapy with prednisone or other glucocorticoids may be necessary indefinitely. A major concern with these agents is osteoporosis, which may cause significant morbidity. In this instance, additional calcium and vitamin D supplementation may be beneficial.
Immunosuppressive drugs such as methotrexate, azathioprine, mycophenolate mofetil, tacrolimus, cyclosporine cyclophosphamide, and others have been beneficial to patients who fail to respond to steroids alone and can be used as second-line therapies or in combination with prednisone. Cyclophosphamide and tacrolimus are used more frequently in patients with interstitial lung disease.
Acthar is a synthetic adrenocorticotropic hormone (ACTH) that is FDA-approved for the treatment of polymyositis and dermatomyositis but the exact mechanism of action is unknown and this agent acts through melanocortin receptors. Currently, there is limited data on its effectiveness. Intravenous immunoglobulin (IVIG) is a blood product that enhances the body’s immune system response. IVIG is usually used in patients who do not respond to other therapies. Biologic agents provide a more targeted therapy and are often monoclonal antibodies or proteins targeting various immune system mediators. Anti-tumor necrosis factor (anti-TNF) agents such as etanercept, adalimumab, and infliximab have potential for use in polymyositis as they suppress tumor-necrosis factor proteins that are associated with inflammation but data supporting their effectiveness are limited. Rituximab is another biologic monoclonal antibody that targets B cells which play a role in inflammation. This agent has been studied in treating polymyositis and may provide some benefit to these patients; however, more studies are required.
Various vitamins and supplements, although not approved specifically for use in polymyositis, have been useful to patients. Examples include: coenzyme Q10 (CoQ10), creatine, fish oil, and others.
Physical therapy and a regular exercise routine are often recommended even when the myositis is active as the beneficial effects of exercise are being increasingly reported. Patients also benefit from heat therapy, passive range-of-motion exercises, and splints to avoid contractures. Speech therapy may be useful if the swallowing muscles become weakened. Additionally, a registered dietician can be recommended if chewing and swallowing become problematic.
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