Polymyositis (PM) is an inflammatory muscle disease with an unknown cause. The disease has a gradual onset and generally begins in the second decade of life and, thus, it rarely affects persons under the age of 18. It causes muscles to exhibit varying degrees of decreased strength, usually affecting those muscles that are closest to the trunk of the body. Trouble with swallowing (dysphagia) may occur with polymyositis.
In polymyositis, muscles exhibit varying degrees of weakness, evolving gradually over weeks to months. It is known that PM begins when white blood cells, the immune cells of inflammation, spontaneously invade muscles, and is thus termed an autoimmune disease. In PM, muscle fibers are found to be in varying stages of necrosis (tissue death) and regeneration. The muscles affected are typically those closest to the trunk or torso, resulting in weakness that can be severe. Eventually, patients have difficulty rising from a sitting position, climbing stairs, lifting objects, or reaching overhead. In some cases, distal muscles (those not close to the trunk of the body) may also be affected later in the course of the disease. Polymyositis is a chronic illness with periods of increased symptoms, called flares or relapses, and decreased symptoms, known as remissions.
Polymyositis mimics many other muscle disorders and remains a diagnosis of exclusion. It should be viewed as a syndrome of diverse causes that occurs separately or in association with other autoimmune disorders or viral infections. A similar inflammatory myopathy is often associated with skin rash and is referred to as dermatomyositis.
Polymyositis in the United States is most common among African Americans. The disorder is most prevalent in women in a male/female ratio of 1:2. In the United States, its incidence is one per 100,000 persons per year; internationally, a lower incidence among the Japanese has been observed. The age of onset is normally above the second decade of life and it is rare or nonexistent for persons under the age of 20.
Causes and symptoms
To date, no cause of polymyositis has been isolated by scientific researchers. While the initial inciting agent remains unknown, possibilities include infection with certain viruses or muscle trauma. There are many infectious agents that are thought to trigger the disease, mainly Coxsackie virus B1, HIV, human T-lymphotropic virus 1 (HTLV-1), hepatitis B and C, influenza, echovirus, and adenovirus. Certain drugs are also thought to be potential triggers, including D-penicillamine, hydralazine, procainamide, and phenytoin.
There are indicators of heredity (genetic) susceptibility that can be found in some patients, mainly the HLA (human leukocyte antigen) genes, which are responsible for encoding some proteins that can activate the immune system.
The muscle weakness affecting mainly the proximal (closest to the trunk of the body) muscles is the first sign of PM. The onset can be gradual or rapid, but normally progresses over weeks or months. This results in varying degrees of loss of muscle strength and atrophy (tissue degeneration). The loss of strength can be noticed as difficulty getting up from chairs, climbing stairs, or lifting above the shoulders. Trouble with swallowing (dysphagia) and weakness lifting the head from the pillow may occur. Occasionally, the muscles ache at rest or with use, and are tender to the touch (occurs in about 25% patients). Persons with polymyositis can also feel fatigue, a general feeling of discomfort, and have weight loss, and/or low-grade fever. Heart and lung involvement can lead to irregular heart rhythm and shortness of breath.
Persons with polymyositis generally seek initial medical help due to weakness. A physician typically reviews the condition of other body systems, including the skin, heart, lungs, and joints. Blood tests are helpful to reveal abnormal high levels of muscle enzymes in the serum of PM patients, mainly creatinine phosphokinase (CPK) and aldolase. In PM, muscle damage causes the muscular cells to break open and spill their content into the bloodstream. Since most of CPK and aldolase exist in muscles, an increase in the amount of these enzymes in the blood indicates that muscle damage has occurred, or is occurring. Blood tests can also point to active inflammation.
The muscle biopsy is one of the best ways to diagnose myositis and other muscle disorders. A muscle biopsy is used to confirm the presence of muscle inflammation typical only of polymyositis. This is a surgical procedure whereby muscle tissue is removed for analysis by a pathologist, a specialist in examining tissue under a microscope. Muscles often used for biopsy include the quadriceps muscle of the front of the thigh, the biceps muscle of the arm, and the deltoid muscle of the shoulder. The results can show conditions such as inflammation, or
Imaging of the muscles using radiology tests such as magnetic resonance imaging (MRI) can show areas of inflammation of muscle, swelling, or scarring. This sometimes can be used to determine muscle biopsy sites. MRIs show signal intensity abnormalities of muscle due to inflammation.
Another test, an electromyogram (EMG), is used to measure the activity of muscles and to provide clues to the cause of muscle weakness or paralysis, muscle problems such as muscle twitching, numbness, tingling, or pain, and nerve damage or injury. EMG is useful in the diagnosis of PM and to exclude other nerve-muscle diseases. Although EMG and MRI imaging are helpful in many cases, the diagnosis of PM is definite when a patient has subacute elevated levels of serum creatine kinase and characteristic findings on muscle biopsy.
In PM, high-dose corticosteroids constitute the first line of treatment, and are effective in more than 70% of patients. Alternatives include immunosuppressant medications, notably azathioprine, methotrexate, and intravenous immunoglobulins (IVIg).
Recovery and rehabilitation
Before the era of corticosteroids, PM was a particularly severe disease with a spontaneous survival rate of less than 40%. Polymyositis in adults now has a relatively favorable prognosis, with a five-year survival rate of around 90%. Only 30–50% of persons with polymyositis achieve complete recovery; the majority of patients have persistent functional problems. However, patients can ultimately do well, especially with early medical treatment of disease and early recognition of disease flares. The disease frequently becomes inactive, and rehabilitation of atrophied (withered) muscles becomes a long-term project.
The National Institute of Environmental Health Sciences (NIEHS) is recruiting patients for a study entitled "Myositis in Children." The aim of the study is to learn more about the immune system changes and medical problems associated with myositis. The National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) is examining whether infliximab (Remicade[r]) is safe for treatment of PM. Updated information is available at The National Institutes of Health website for clinical trials at <http://www.clinicaltrials.gov>.
The prognosis for PM and the response to therapy vary from very good to satisfactory. Most patients respond well to treatment, although residual weakness is common. Osteoporosis, a common complication of chronic corticosteroid therapy, may be significant. For African Americans, older people, females, people with interstitial lung disease and associated malignancies, those who delay treatment, and those with trouble swallowing or heart involvement, the prognosis is much less favorable.
Exercise is generally beneficial, and helps to get the most out of diseased muscles. Falls and injuries, however, can cause substantial disability. People with PM, therefore, have the difficult task of undertaking regular exercise within their capability, but avoiding injury through accident. Because weakened muscles cannot carry an excess load, keeping to an ideal weight is critical. Although this may seem obvious, weight control is more difficult when exercise is limited.
Staff. The Official Patient's Sourcebook on Polymyositis: A Revised and Updated Directory for the Internet Age. San Diego: Icon Health International, 2004.
Mastaglia, F. L., M. J. Garllep, B. A. Phillips, and P. J. Zilko. "Inflammatory Myopathies: Clinical, Diagnostic and
"NINDS Polymyositis Information Page." National Institute of Neurological Disorders and Stroke. May 2, 2004 (June 2, 2004). <http://www.ninds.nih.gov/health_and_medical/disorders/polymyos_doc.htm>.
"Myositis." Medline Plus. May 1, 2004 (June 2, 2004). <http://www.nlm.nih.gov/medlineplus/myositis.html>.
Myositis Association of America. 755 Cantrell Ave., Suite C, Harrisonburg, VA 22801. (540) 433-7686; Fax: (540) 432-0206. email@example.com. <http://www.myositis.org>.
Marcos do Carmo Oyama
Iuri Drumond Louro, MD, PhD