What is Polymyositis (PM)?

Simply Put

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.

Muscles typically affected by PM/DM
Muscles typically affected by PM/DM. Image courtesy of the Muscular Dystrophy Association.

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.

MSU Observation: Overlap Syndrome, having more than one autoimmune disease, is somewhat common in patients with Dermatomyositis and Polymyositis. These can include Lupus, Scleroderma, Raynaud’s phenomenon, and Sjögren’s syndrome.

Symptoms of Polymyositis

It is important to note that PM symptoms vary from patient to patient. Not all patients display all of these symptoms.

  • Proximal muscle weakness (muscles closest to the trunk, such as shoulders, thighs, and hips)
  • Symmetrical muscle weakness (the same muscles on both sides of the body)
  • Muscle pain (“Myalgia”)
  • Difficulty swallowing (“Dysphagia”)
  • Fatigue (can be extreme and life-altering)
  • Low-grade fever
  • Labored breathing (“Dyspnea”)
  • Vocal changes which can produce a hoarse or changing voice  (“Dysphonia”)
  • Difficulty maintaining enough air to finish a sentence
  • Difficulty turning in bed, rising from a seated position, and raising arms over the head

Complications of Polymyositis

PM complications do not occur in all patients. Some patients may develop one or two complications, but rarely all of them.

  • Aspiration pneumonia
  • Increased risk of Infection
  • Increased risk of falls
  • Interstitial lung disease (ILD)
  • Inflammation of the myocardium or inflammatory cardiomyopathy
  • Irregular heartbeat (“Arrhythmia”)
  • Raynaud’s phenomenon (numbness and cold in areas of the body, often in fingers and toes)
  • Cardiovascular disease
  • Other connective tissue diseases
  • Complications and side effects related to Myositis treatments
  • Increased risk of having certain types of cancer (see below)

According to some physicians that are experienced in diagnosing and treating Polymyositis, each case can be drastically different from each other and they are finding that the old diagnostic criteria may no longer apply to Polymyositis patients.

Polymyositis in images

inflammatory myopathies
The Inflammatory Myopathies, an overview
Polymyositis flare -extreme weight loss
Muscle biopsy showing Polymyositis
Muscle biopsy showing Polymyositis

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.

Polymyositis vs. Normal muscle biopsy

Muscle biopsy comparison of normal muscle tissue versus muscle tissues affected with polymyositis.

Image courtesy of

The Muscular Dystrophy Association

Simply Put

“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.

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