Polymyositis (PM)

Polymyositis (PM)

What is polymyositis (PM)?

Polymyositis (PM) is one of the idiopathic inflammatory myopathies (IIMs), a group of muscle diseases that involves inflammation of the muscles or associated tissues. A myopathy is a muscle disease, and inflammation is response to cell damage. Another word for inflammatory myopathy is myositis. The myo root means muscle, and the itis root means inflammation; so a myositis is an inflammatory muscle disease.

In PM, inflammatory cells of the immune system directly attack muscle fibers, leading to weakness in the skeletal muscles (muscles that connect to bone and allow movement). PM presents with some overlapping symptoms to other IIMs and is sometimes misdiagnosed as one of the other conditions.

What are the symptoms of PM?

The muscles of the shoulders, upper arms, hips, thighs and neck display the most weakness in PM. There also can be pain or tenderness in the affected areas, as well as swallowing problems and inflammation of the heart and lung muscle tissues. For more, see Signs and Symptoms.

What causes PM?

In most cases, the cause of an IIM is unclear. For some reason, the body’s immune system turns against its own muscles and damages muscle tissue in an autoimmune process. In PM, the inflammatory cells of the immune system directly attack muscle fibers.

PM is not a genetic disorder, although there may be genetic factors that make it more or less likely that an inflammatory myopathy will develop. For more, see Causes/Inheritance.

What is the progression of PM?

The symptoms of PM usually begin in adulthood, after age 20, and the disease progresses gradually. Over a period of weeks to months, people with PM will begin to experience slowly worsening weakness in the skeletal muscles.

Although IIMs like PM can lead to great discomfort for at least a period of time, effective treatments are available, and for the most part these conditions aren’t life-threatening. Many people recover partially or completely from PM.

What is the status of research on PM?

Research studies to increase the understanding of PM and clinical trials of medications to treat IIMs are underway. For a list of polymyositis clinical trials, see MDA's Clinical Trials Finder Tool.

References

1. Findlay AR, Goyal NA, Mozaffar T. An overview of polymyositis and dermatomyositis. Muscle Nerve. 2015;51(5):638-656. doi:10.1002/MUS.24566
2. Baig S, Paik JJ. Inflammatory muscle disease – An update. Best Pract Res Clin Rheumatol. 2020;34(1):101484. doi:10.1016/j.berh.2019.101484

Last update: Feb 2023
Reviewed by Julie Paik, MD, MHS; Johns Hopkins University