What Is Polymyositis?

Overview

An idiopathic inflammatory myopathy is a disease of an unknown cause (idiopathic) that is characterised by chronic inflammation of the skeletal muscles. Your skeletal muscles include all the muscles you use to produce voluntary movements of your body. Myositis is a rare type of idiopathic inflammatory myopathy that generally presents with muscle fatigue, weakness and pain. You might also hear it being described as a rheumatic disease, a term that describes diseases affecting muscles, tendons, bones, or joints. 

So, what is polymyositis? Polymyositis is a form of myositis that affects many muscles at once. It predominantly occurs in people assigned female at birth (AFAB) who are between the ages of 30 and 60.¹ The prognosis for polymyositis, like most chronic diseases, is best when it has been diagnosed and treated early. 

This article will outline the causes, symptoms, diagnosis, and treatment of polymyositis to help give you a better understanding of this rare disease.  

Causes of polymyositis

The causes of polymyositis are complex and not fully understood. Research has been slow due to the small number of cases and the difficulty in identifying these. Nevertheless, polymyositis is generally thought to be an autoimmune disorder.² 

The body’s immune system is normally activated when foreign material, like bacteria or viruses, enters the body, and immune cells specifically attack these disease-causing substances. However, if there is a defect in initiating the immune response, the immune system can become overactive and attack your body’s own tissues, including skeletal muscle. This causes autoimmune diseases, such as polymyositis. 

Research has suggested that the primary cause of muscle damage in polymyositis is overactive white blood cells in the immune system, specifically the overactivity of cytotoxic T cells.¹ This muscle damage causes an inflammatory response, attracting more white blood cells to the site of injury and causing chronic inflammation. This is characterised by elevated levels of pro-inflammatory mediators of the immune system, including antibodies, chemical messengers called cytokines and muscle enzymes.^2,3

The abnormal immune activity in polymyositis is thought to be caused by a range of factors. Some people may be more likely to develop an autoimmune disease due to their genes and certain environmental triggers. An infection with a specific virus or UV damage caused by sunburn are examples of such environmental triggers. 

Additionally, polymyositis may co-occur with other autoimmune diseases, including systemic lupus erythematosus (lupus) and rheumatoid arthritis.⁴

Signs and symptoms of polymyositis

Polymyositis symptoms result from muscle damage caused by the immune system. The characteristic sign of any myositis condition is muscle weakness. In polymyositis, this occurs progressively over a period of months and affects muscles on both sides of the body.⁴ Often, large muscle groups are first to be affected, such as the upper arm, shoulder, thigh, back and hip muscles. 

Polymyositis can also lead to muscle pain, persistent tiredness and even depression. It can also affect muscle groups that regulate vital functions, like the muscles of the throat, which can have more serious effects. 

You might become aware of these effects if you experience the following:

• Muscle tenderness
• Pain in your joints
• Difficulty lifting your arms
• Difficulty sitting down or standing up
• Feeling tired when walking, even short distances
• Difficulty swallowing

Some individuals can experience heightened sensitivity to cold temperatures, which is called Raynaud’s phenomenon.⁴ This is caused by restricted blood flow to the extremities, mainly the fingers and toes, which results in a loss of feeling and colour. 

Polymyositis also often presents in conjunction with other conditions, including interstitial lung disease and cardiovascular disease.³

The signs and symptoms of polymyositis are very similar to another form of myositis, dermatomyositis. The key distinction is that if you are suffering from dermatomyositis, you will also develop a red/purple rash that usually appears on the face and hands. 

Management and treatment for polymyositis

Unfortunately, there is no cure for polymyositis. However, if diagnosed and treated early, you can live a relatively normal life without any significant disability.⁵ There are several forms of treatment and management that are outlined below. 

Steroids 

Steroids, also called corticosteroids, are often the first line of treatment as they have a rapid effect and are generally effective for most patients.² Initially, individuals will be prescribed a high dosage, which will gradually be decreased over time. These can be administered as tablets, an injection or even as an infusion.

Unfortunately, they can have severe side-effects that you need to be mindful of if you are taking steroids regularly, including: 

• Weight gain
• Diabetes
• Cataracts
• High blood pressure
• Weak bones (osteoporosis) 

Immunosuppressants

Often, to help reduce the side-effects of steroid use, immunosuppressant drugs are also given to patients. These take a longer time to have any effect on the body but can be a more efficient treatment in the long run. As the name suggests, they suppress the immune system and can be particularly effective for dealing with any inflammation or pain that might return as steroid doses are reduced. 

Immunoglobulins (antibodies)

In more serious cases of polymyositis, or in cases where steroids and immunosuppressants have failed, immunoglobulin treatments can be used. This involves the intravenous administration of antibodies from healthy donors to reduce the body’s overactive immune response and inflammation. Unlike steroids, there are very few side effects but this treatment option may require frequent visits to the doctor in order to complete it.  

Exercise 

Exercise and physiotherapy can be an excellent way of managing polymyositis by helping to strengthen your muscles and reverse some of the damage. Additionally, exercise is a great way to support your overall health, which can help reduce the likelihood of you developing other chronic conditions associated with polymyositis.

Exercise programmes should be approved by your doctor or physiotherapist if you have been diagnosed with any form of myositis. This is to prevent any further damage to your muscles. Exercise should also be avoided if you are experiencing severe symptoms. 

Healthy diet 

A balanced diet can help to support your overall health and can potentially have an anti-inflammatory effect. You can find out more about an anti-inflammatory diet here. 

If you are diagnosed with polymyositis, it is important to tell your doctor if you experience any side effects from the medications. 

FAQs

How is polymyositis diagnosed?

Diagnosis involves several steps⁴:

1. Reviewing the medical history of the patient and their symptoms
2. Performing blood tests to check the levels of immune cells and muscle enzymes
3. Electromyography (EMG) to check the electrical activity of the muscles, which is often abnormal in polymyositis
4. Muscle biopsy to check for characteristic signs of inflammation

How can I prevent polymyositis?

While you can’t prevent polymyositis, eating a healthy balanced diet and exercising regularly can help to maintain your health and support muscle health. 

Who is at risk of polymyositis?

Polymyositis largely affects middle-aged adults (older than 20 years), with people AFAB being nearly twice as likely to develop it than people assigned male at birth (AMAB).

How common is polymyositis?

Polymyositis is considered to be a rare disease that is estimated to affect 6-8 out of every 100,000 people.

When should I see a doctor?

If you experience any abnormal muscle weakness, fatigue, or other symptoms of polymyositis, you should talk to your doctor.

Summary

Polymyositis is a rare autoimmune disease that affects the skeletal muscles in the body. It is important to remember that while it is incurable, noticing symptoms like muscle weakness will increase your chances of getting an early diagnosis and preventing significant muscle damage. 

If you are worried about developing this disease, the best course of action is to maintain a healthy diet and exercise regularly to maintain your muscle strength. It is important to speak to your doctor if you experience any signs or symptoms of polymyositis.