An Overview of Polymyositis

Polymyositis is a systemic, inflammatory muscle disease, primarily characterized by muscle weakness. It is part of a disease category referred to as myositis, which means inflammation of muscle. Polymyositis typically affects muscles that are closest to the body's trunk, but over time other muscles may become involved. Typically, polymyositis develops gradually, and while it can develop in anyone, including children, it does not usually affect people under 18 years of age. Most commonly, it affects people between 40 and 60 years old. Polymyositis affects women more commonly than men, by a two to one ratio.

Dermatomyositis is a similar inflammatory condition to polymyositis, the difference being that dermatomyositis also affects the skin. Polymyositis can occur in combination with certain cancers, including lymphoma, breast cancer, lung cancer, ovarian cancer, and colon cancer. Polymyositis can occur with other rheumatic diseases, such as systemic sclerosis (scleroderma), mixed connective tissue disease, rheumatoid arthritis, systemic lupus erythematosus, and sarcoidosis.

Causes

While the cause of polymyositis is said to be unknown, it does appear that hereditary factors are at play. Researchers believe that an autoimmune reaction to muscle occurs in people who have a genetic predisposition. HLA subtypes -DR3, -DR52, and -DR6 appear to be linked to the predisposition. There may also be a triggering event, possibly viral myositis or pre-existing cancer.

Symptoms

Muscle weakness, as mentioned above, is the most common symptom. Muscle weakness associated with polymyositis may progress over weeks or months. According to Merck Manual, destruction of 50% of muscle fibers causes symptomatic weakness, meaning by that time, myositis is quite advanced.

Common functional difficulties that occur with polymyositis include rising from a chair, climbing steps, and raising arms. Weakness of pelvic and shoulder girdle muscles may result in being bed-bound or wheelchair-bound. If neck muscles are involved, it may be difficult to raise your head from a pillow. Involvement of pharyngeal and esophageal muscles may affect swallowing. Interestingly, muscles of the hands, feet, and face are not involved in polymyositis.

There may be joint involvement which appears as a mild polyarthralgia or polyarthritis. This tends to develop in a subset of polymyositis patients who have Jo-1 or other antisynthetase antibodies.

Other symptoms associated with polymyositis may includeG

• Generalized fatigue
• Shortness of breath
• Voice abnormalities due to affected throat muscles
• Fever
• Loss of appetite

Diagnosis

As with any disease or condition, your healthcare provider will consider your medical history and perform a complete physical examination as part of the diagnostic process. Blood tests would likely be ordered to look for the presence of specific autoimmune antibodies and to detect non-specific inflammation. Electromyography and nerve conduction tests can provide the healthcare provider with useful diagnostic information, too.

MRI of the affected muscles is typically ordered. Additionally, a urine test can check for myoglobin, a protein in muscle cells that is released into the bloodstream and cleared by the kidneys when muscle is damaged. Blood tests to check the level of serum muscle enzymes, such as CK and aldolase, can be ordered and performed. With muscle damage, the levels of muscle enzymes are usually elevated. Another blood test, the ANA (antinuclear antibody test), is positive in up to 80% of people with polymyositis.

Ultimately, a muscle biopsy can be performed in order to confirm the diagnosis of polymyositis. A biopsy is recommended before treatment is started so that other muscle diseases can be ruled out.

Treatment

Corticosteroids at high doses are the first line of treatment for polymyositis to decrease inflammation in the muscles. If that alone is insufficient, immunosuppressants may be added to the treatment regimen. Immunosuppressants that may be considered include methotrexate (Rheumatrex), azathioprine (Imuran), mycophenolate (CellCept), cyclophosphamide (Cytoxan), rituximab (Rituxan), cyclosporine (Sandimmune), and IV immunoglobulin (IVIG).

Polymyositis associated with cancer is often less responsive to corticosteroids. Tumor removal, if possible, may be most helpful in cancer-linked myositis cases.

A Word From Verywell

With early treatment for polymyositis, remission is possible. The 5-year survival rate for adults with polymyositis, according to Merck Manual, is 75 to 80%. Death may result from consequences of severe and progressive muscle weakness. People who have cardiac or pulmonary involvement seem to have a worse prognosis. That can also be said of polymyositis patients with cancer.

Cancer screenings are recommended for polymyositis patients who are 60 years and older. Be mindful of the need for routine cancer screenings. Finding undetected cancer may be the key to changing your prognosis with polymyositis.