Approximately 1 per 100,000 individuals is affected by PM in the general population. PM affects females twice more than males. It is also more common in the African-American population compared to the Caucasian population.PM rarely occurs in children with the onset of disease typically being between the ages of 40 and 50.

The triggering factors of PM are unknown. There is evidence suggesting that individuals who are genetically predisposed may develop the disorder in response to environmental stimuli. It is possible that an injury to small blood vessels might trigger the appearance of molecules known as muscle autoantigens. An autoantigen is a normal protein that is detected by the immune system, causing the immune system to attack self-cells instead of foreign invaders. The immune system is normally very effective in distinguishing foreign molecules from self-molecules. However, the immune system sometimes mistakens self-molecules as foreign invaders and attacks the body itself, also known as an autoimmune attack.

Although the exact mechanism is very unclear, there are different hypotheses and suggested mechanisms that may contribute to the development of PM. The immune system has two arms called the innate and adaptive immune system. The innate immune system is the first line of defense which acts similar against all foreign molecules. Inflammation is an innate response. The adaptive immune system acts when the innate immune system fails to contain the invasion and is specific to each disease-causing organism or molecule. It includes two major types of cells called B-cells and T-cells. These cells have to undergo a maturation process for optimal effectiveness and this process usually happens in specific structures such as an organ called the spleen. There is evidence suggesting that in PM, B-cell maturation may occur in the muscle where the muscle abnormally provides an environment that allows for this maturation. These B-cells will then attack the autoantigens present in muscle tissue with higher effectiveness.

T-cells also play a central role in the autoimmune attack in PM. T-cells attack autoantigens in muscles and release substances that induce cell death.

Another group of molecules that play a crucial role in inflammatory reactions are chemokines. Inflammation is a complex process involving various types of immune cells, signaling molecules, and increasing blood flow to the site of injury or invasion. Chemokines are an example of signaling molecules that affect the behavior of other cells. Chemokines are molecules that attract immune cells to the inflamed area. Chemokines have been found to be elevated in the muscle tissue of individuals affected by PM. There is data suggesting that in affected individuals, muscle tissue may produce chemokines which are not produced in healthy muscle tissues. This attracts more immune cells to the area which maintains the autoimmune activity.

The most common symptom of PM is the weakness in muscles around the hip and lower extremity muscles close to the midline of the body which can manifest as difficulty walking, standing up after sitting in a chair, or climbing stairs. The muscle weakness is typically painless and could vary in severity from mild to extreme. Other muscles such as the neck and shoulder girdle muscles may become involved in later stages. Muscle weakness is symmetric, meaning that it affects both sides of the body equally and develops over weeks to months. Other possible symptoms are shortness of breath, fatigue, joint pain, difficulty speaking, difficulty swallowing, morning stiffness, weight loss, and fever.

PM should be suspected in individuals with symmetric, progressive weakness in muscles closest to the midline of the body, where no loss of sensation is observed. Deep tendon reflexes are muscle movements in response to stretch in structures that attach bone to muscle. This is typically what the physician is testing when they tap on your joints with a reflex hammer and it is used to evaluate the integrity of the nervous system. In PM, deep muscle reflexes are preserved.

A widely used criterion for the diagnosis of polymyositis proposes that PM must include symmetric, proximal muscle weakness, elevated serum enzymes, and changes in electromyographic (EMG) findings associated with muscle dysfunction. EMG is a diagnostic technique which records the electrical activity produced by voluntary muscles, recorded as a series of defined waves. The characteristic findings in EMG are increased spontaneous electrical activity with sharp positive waves, polyphasic motor unit action potentials (MUAPs), and early recruiting MUAPs. Polyphasic action potentials are an abnormal electrical configuration that indicates injury to the nerve terminal (axon). MUAPs are spikes of electrical activity that reflect the number of motor neurons (nerve cells associated with movement) and the muscle they transmit signals to.

Elevated levels of creatine kinase (CK) is observed in PM. CK is a muscle enzyme that tends to be elevated in presence of muscle damage. Other muscle enzymes such as myoglobin, aldolase, lactate dehydrogenase, aspartate aminotransferase, and alanine aminotransferase may also be elevated.

Other clinical findings may include, clicking noises heard during lung examination as the affected individuals inhale. This noise is known as crackles. Difficulty speaking or dysphonia with a nasal speech quality may also be present.

Muscle biopsy is the procedure that can provide a definitive diagnosis of PM. However, other tests can indicate the areas of potentially highest yield for biopsy. In a muscle biopsy, a piece of muscle tissue is removed and examined microscopically to find indications of muscle damage. The sample is commonly taken by inserting a biopsy needle. If a larger sample is required, your physician may need to make a small incision. Then the sample is then examined for signs of inflammation, tissue death (necrosis), or tissue regeneration. Other findings include the presence of particular immune cells (monocytes) within a thin layer that wraps around each muscle cell, destruction of small vessels (capillaries) within vessels, and damage to cells the line the interior surface of vessels (endothelial cells).

A blood test may also be performed when PM is suspected to check blood levels of muscle enzymes. Creatine kinase is the most reliable enzyme to measure and is expected to be elevated in individuals affected by PM. An MRI may also be obtained to localize the most abnormal areas which are likely to have the most accurate biopsy results. MRI can detect small amounts of inflammation in the muscle which may be present in earlier stages of PM. Finally, electromyography (EMG) can aid in diagnosis in presence of abnormal EMG findings as explained in the “Diagnosis” section.

Treatment in PM aims to improve muscle weakness and avoid the development of extra-muscular disease of vital organs. In general, oral steroids are used as the first-line treatment and other immunosuppressive drugs are used second-line. Oral steroids decrease inflammation and reduce tissue damage as a result of prolonged inflammation. It is important to supplement the treatment plan with non-pharmacological therapies such as exercise and physical therapy. Prednisone is the most common steroid used in PM. It is best and safest to use the lowest effective dose for steroids. .

Some affected individuals may also require steroid-sparing agents, which are immunosuppressive medications that reduce the need for steroids. Commonly used steroid-sparing agents include azathioprine (AZA) and methotrexate (MTX). Steroids can either be initiated first and steroid-sparing agents are added on later or both medications can be initiated together from the beginning; depending on the needs of the affected individuals.

Some affected individuals develop refractory PM. Refractory PM are those that do not respond to the standard treatment or those that initially responded to the standard treatment but no longer do. Immunosuppressive drugs suppress the immune system and have shown some efficacy in the treatment of refractory PM. These are immunosuppressive drugs, meaning that they suppress the immune system. Some examples of immunosuppressive drugs are cyclosporine, tacrolimus, and mycophenolate.  

Some affected individuals have other underlying conditions that can be worsened by the use of steroids, such as high blood pressure, diabetes, osteoporosis, and obesity. For these affected individuals, steroid-sparing agents are preferred to be initiated early in the course of treatment. Other individuals with respiratory muscle failure and organ damage should use the steroid-sparing agents as well.

In determining the treatment plan, the progression of PM is vital. The more progressive the PM, the less responsive the affected individuals will be. Early diagnosis and treatment are recommended.

Overall, the prognosis for individuals affected by PM varies. Treatment is usually effective, allowing most affected individuals to regain muscle weakness or decrease disease progression.  Affected individuals may or may not have to continue long-term treatment. In affected individuals who respond well to treatment, the quality of life is expected to return close to normal.

However, some affected individuals may not respond to treatment where significant and prolonged disability and muscle weakness may occur. In severe cases, respiratory failure or pneumonia may occur. Weight loss may be observed due to difficulty swallowing.

Bronner IM, van der Meulen MF, de Visser M, et al. Long-term outcome in polymyositis and dermatomyositis. Ann Rheum Dis. 2006;65(11):1456-61. doi:10.1136/ard.2005.045690

Findlay A, Goyal N, Mozaffar T. An overview of polymyositis and dermatomyositis. Muscle and Nerve. 2015; 51(5):638-656. https://doi.org/10.1002/mus.24566

Genetic and Rare Disease Information Center. Polymyositis. 2018. Available from https://rarediseases.info.nih.gov/diseases/7425/polymyositis.

Hunter K, Lyon MG. Evaluation and management of polymyositis. Indian J Dermatol. 2012;57(5):371-4. doi:10.4103/0019-5154.100479.

Mantegazza R, Bernasconi P. Inflammatory Myopathies: Dermatomyositis, Polymyositis and Inclusion Body Myositis. In: Madame Curie Bioscience Database [Internet]. Austin (TX): Landes Bioscience; 2000-2013. Available from: https://www.ncbi.nlm.nih.gov/books/NBK6196/

Raychaudhuri SP, Mitra A. Polymyositis and dermatomyositis: Disease spectrum and classification. Indian J Dermatol. 2012;57(5):366-70. Doi:10.4103/0019-5154.100477.