Polymyositis (PM) is thought to be an autoimmune disease where inflammation resulting in cell damage is created when the immune system attacks healthy muscle tissue.
Muscle weakness can develop over a period of days, weeks, or months, is typically symmetrical (both sides of the body), and affects proximal muscles (muscles which are closest to the body’s core), such as the quadriceps, hip flexors, neck, and deltoid muscles. It affects and weakens the skeletal muscles, those involved with voluntary movement. This weakness often leads to difficulty climbing stairs, rising from a chair or the floor, turning in bed, lifting, styling hair, or reaching over the head for something like an item on a shelf. Some people also have problems swallowing, a disorder called “Dysphagia.” Muscle pain (myalgia) and joint pain, with or without true arthritis and joint inflammation, can also be a part of PM.
PM is one of several diseases in the Idiopathic Inflammatory or Acquired Myopathy category. The term “idiopathic” means no known cause, however some cases are known to have been caused by medications, viruses, bacteria, trauma, toxins, cancer, or other illness.
It is called an “acquired” myopathy because it is not an inherited disorder. However it is thought that people with PM may have a genetic predisposition and some factor, such as those mentioned above, triggers the immune system, which goes haywire and attacks healthy tissues and cells.
Polymyositis is a chronic illness with no cure, however many respond well to certain treatments or a combination of treatments with improvement in symptoms or even complete remission of symptoms.
PM is a rare disease. Approximately 5 to 10 people per million in the U.S. are diagnosed with a form of Myositis each year.
As indicated, each case of PM may be quite different from others. There have been cases where people originally diagnosed with PM to later find they actually have Inclusion Body Myositis (sIBM) or a non-inflammatory Myopathy such as a form of Muscular Dystrophy.
Some PM patients, although not common, may experience weakness in distal muscles, those farther away from the body’s core such as hands and fingers.
Myositis Specific and Myositis Associated antibodies can be checked through simple blood tests and may offer physicians the best chance of treating the disease successfully as well as offering “predictors” of possible disease progression and complications.
Symptoms of Polymyositis
It is important to note that PM symptoms vary from patient to patient. Not all patients display all of these symptoms.
Complications of Polymyositis
PM complications do not occur in all patients. Some patients may develop one or two complications, but rarely all of them.
Polymyositis in images
Polymyositis and Cancer
Cancer can be an underlying cause of Polymyositis, but to a lesser extent than with Dermatomyositis. Cancer may precede the onset and if the cancer precipitated PM, removal or treatment of the cancer may result in the remission of Polymyositis.
Polymyositis in adults has also been linked to an increased likelihood of developing cancer post diagnosis, particularly cancers of the cervix, lungs, pancreas, colorectal system, breasts, ovaries, and gastrointestinal tract, as well as non-Hodgkin’s lymphoma. The risk of cancer appears to level off about three years after a diagnosis of PM. The incidence of cancer is higher in Dermatomyositis than in Polymyositis.
It is strongly suggested that appropriate cancer screenings be conducted upon diagnosis and in the years after a Myositis diagnosis.
Muscle biopsy comparison of normal muscle tissue versus muscle tissues affected with polymyositis.
Image courtesy of
The Muscular Dystrophy Association
“Simply Put” is a service of Myositis Support and Understanding, to provide overviews of Myositis-related medical and scientific information in understandable language.
MSU volunteers, who have no medical background, read and analyze often-complicated medical information and present it in more simplified terms so that readers have a starting point for further investigation and consultation with healthcare providers. The information provided is not meant to be medical advice of any type.
Join our email updates and newsletter
Subscribe to our mailing list and stay updated about all things myositis related.