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What is polymyositis?

Polymyositis is a type of myopathy—or muscle disease. To break the word down, poly means many, myo refers to muscle, and itis means inflammation. So, polymyositis is an inflammatory muscle disease that affects many muscles. It is a rare disease involving the skeletal muscles—the muscles necessary for movement. Dermatomyositis is a closely related disease. Derma refers to the skin. So, in polymyositis vs. dermatomyositis, the latter also involves a skin rash.

Polymyositis causes muscle weakness, typically in the muscles closest to the trunk. This includes the neck, shoulders, upper arms, back, hips and thighs on both sides of the body. Some people with the disease also experience weakness in muscles farther away from the trunk, such as the hands. The weakness can start over just a few days or take a few months to develop. Sometimes, other symptoms occur, such as pain or tenderness. In some cases, the disease can affect muscles of the digestive tract, heart or lungs.

Doctors are unsure of the exact cause of polymyositis. The main theory is that it is a type of autoimmune disorder. It shares many characteristics as other autoimmune diseases. Women are twice as likely as men to get the disease and usually it begins in middle age. It is rare in young adults and the elderly. Because the polymyositis cause is unknown, another name for it is idiopathic inflammatory myopathy.

Currently, there is no known cure for polymyositis. However, medications can relieve symptoms, improve strength, and prevent complications. Physical therapy can also be helpful. Generally, the earlier you start treatment, the better the polymyositis prognosis.

Polymyositis is not a medical emergency, but it can eventually lead to complications. And muscle weakness can be a symptom of several, more common medical conditions. See your doctor promptly if you notice muscle weakness that persists or worsens.

What are the symptoms of polymyositis?

The main symptom of polymyositis is muscle weakness on both sides of the body. The weakness primarily affects skeletal muscles closest to the trunk. The most common sites are the neck, back, shoulders, upper arms, hips and thighs. However, sometimes weakness affects muscles that are more distant, such as the hands or feet.

The weakness makes it difficult to walk, climb stairs, stand up from a seated position, and lift things. Muscle pain and stiffness, joint pain and stiffness, fatigue, poor appetite, and weight loss can also occur. Typically, the weakness and other symptoms develop over weeks to months.

In some cases, polymyositis affects smooth muscle and cardiac muscle. Smooth muscle is present in various body systems, such as the digestive tract and respiratory system. So, other symptoms can occur, such as trouble swallowing, shortness of breath, and abnormal heart rhythms.

See your doctor if you notice muscle weakness developing. There are more common conditions that may be causing it. If you end up with a polymyositis diagnosis, starting treatment early often means it will be more effective.

What causes polymyositis?

The cause of polymyositis is unclear. Most experts believe it is a form of autoimmune disorder. These disorders occur when the immune system mistakenly attacks the body’s own tissues as if they were foreign invaders. In polymyositis, the muscle fibers are the focus of the abnormal attack. This causes damage, weakness, and other symptoms, such as pain and tenderness.

Like other autoimmune diseases, the trigger is likely a combination of environmental and genetic factors. While no specific genetic link has been found, a person’s genetic makeup may make them more prone to inflammatory disorders. In polymyositis, researchers have also linked certain viral illnesses, drug exposures, and vaccines as potential triggers.

What are the risk factors for polymyositis?

Several factors increase the risk of developing polymyositis including:

  • African American race
  • Age 30 to 60 years
  • Exposure to certain drugs and vaccines, including collagen injections, interferon alpha, growth hormone, cimetidine, phenytoin, and hepatitis B vaccine
  • Female sex

Reducing your risk of polymyositis

Unfortunately, there is no known way to reduce the risk of developing polymyositis. As research continues, doctors may eventually understand more about the cause of the disease. This could provide insight into what specifically triggers the disease and how to avoid it. In the meantime, regular medical care can help doctors catch potential problems early, when treatment is most likely to be effective.

How is polymyositis treated?

While there is no cure for polymyositis, effective treatments are available. Treating the disease early allows many people to recover from the disease, regain muscle strength, and lead a normal life. Treatments include:

  • Corticosteroids, which are powerful anti-inflammatory medicines that usually control the symptoms within 4 to 6 weeks
  • Immunosuppressants, including azathioprine (Azasan, Imuran), methotrexate (Rheumatrex, Trexall), mycophenolate mofetil (CellCept), cyclosporine (Gengraf, Neoral, Sandimmune), and tacrolimus (Prograf). Corticosteroid-sparing agents is another name for this category of drugs since they allow doctors to decrease the dose of corticosteroids and reduce side effects.
  • Biologics, including rituximab (Rituxan), and IVIG (intravenous immunoglobulin) which may be useful when the disease remains active despite corticosteroid treatment
  • Physical therapy to strengthen and maintain muscle function

Other treatments, such as speech or swallowing therapy, may be necessary if the disease progresses.

What are the potential complications of polymyositis?

The polymyositis prognosis varies from person to person. The disease generally progresses slowly and for most people, it isn’t life threatening. People who recover with treatment are at risk of a recurrence because there is no cure. Continuing medical treatment can help prevent future problems and slow progression.

Without treatment, or if treatment doesn’t work, the disease can progress and cause complications and disability. Complications can include:

  • Cancer, which along with lung disease is a major cause of death in polymyositis
  • Difficulty swallowing, which can lead to weight loss and malnutrition. Eventually, aspiration pneumonia becomes a risk when swallowing is severely impaired. It happens when you inhale food or liquid into your lungs when you aren’t able to swallow properly.
  • Severe muscle weakness
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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Dec 24
THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
  1. Polymyositis. Genetics and Rare Diseases Information Center. https://rarediseases.info.nih.gov/diseases/7425/polymyositis
  2. Polymyositis. Johns Hopkins University. https://www.hopkinsmedicine.org/health/conditions-and-diseases/polymyositis
  3. Polymyositis. Mayo Foundation for Medical Education and Research. https://www.mayoclinic.org/diseases-conditions/polymyositis/symptoms-causes/syc-20353208
  4. Polymyositis. Myositis Association. https://www.myositis.org/about-myositis/types-of-myositis/polymyositis/
  5. Polymyositis – Adult. MedlinePlus, U.S. National Library of Medicine. https://medlineplus.gov/ency/article/000428.htm
  6. Polymyositis and Necrotizing Myopathy. National Organization for Rare Disorders. https://rarediseases.org/rare-diseases/polymyositis/
  7. Polymyositis (PM). Muscular Dystrophy Association. https://www.mda.org/disease/polymyositis
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