Polymyositis (PM) and Dermatomyositis (DM) are two similar and rare chronic (long-term) diseases that affect the muscles. They cause inflammation in the muscles that leads to weakness but not usually pain or swelling.

The most significant difference between Polymyositis and Dermatomyositis in terms of symptoms is that a rash also occurs with Dermatomyositis that does not occur with Polymyositis.

When polymyositis and dermatomyositis are severe, they can cause problems with things like swallowing. This is serious because it can cause a person to choke or breathe things into their lungs. In the rarest and worst of cases, it can even impact the muscle of the heart and the muscles used for breathing.

Polymyositis and Dermatomyositis are types of inflammatory myopathies, which refer to diseases that cause inflammation of the muscles.

While anyone can get Polymyositis or Dermatomyositis, they most often affect two age groups: children aged 10 to 15 and adults aged 45 to 60. Myositis is very rare: it affects only about 10 people in every million.

Who Gets Polymyositis and Dermatomyositis

Women are about 2 times more likely to get myositis than men.

Autoimmune Disease

Polymyositis and Dermatomyositis are autoimmune diseases, meaning that they occur when the body’s immune system attacks its own healthy cells and tissues. The reason why it does this is not well understood. When the body’s immune system is “activated” in this way, it can make a person feel very tired, similar to when they have the flu.

Types of Myositis

For simplicity, this article will refer to polymyositis and dermatomyositis as myositis. Note that there are other types of myositis that are not related to the autoimmune disorders Polymyositis and Dermatomyositis discussed in this article.

Myositis can exist on its own, or it can sometimes be seen along with other rheumatic diseases. Muscle inflammation can be seen with Sjogren’s syndrome, lupus, and systemic sclerosis (scleroderma). In a small number of people, myositis can be accompanied by a cancer that somehow triggers the immune system and causes the condition. Everybody who gets myositis should be screened for cancer.

Understanding Polymyositis and Dermatomyositis



One of the first things people with myositis notice is fatigue. Muscle weakness comes on slowly over time.

Muscle Weakness

It can take a while before a person notices the muscle weakness since it happens gradually, usually over several months. People with myositis might start to have trouble getting up from a chair, climbing the stairs, getting up off the floor, or lifting things above their head.

When myositis is more severe, it can cause problems with things like swallowing. This is serious because it can cause a person to choke or breathe things into their lungs.

In the rarest and worst of cases, myositis can cause weakness in the heart muscle and the muscles that enable breathing.

Arthritis Symptoms

In some people with myositis, inflammation can be seen in other places. One of the most common places it can affect after the muscles are the joints, leading to pain and stiffness (arthritis). The stiffness is usually worse in the morning or after a period of rest. In some people, it gets better once the joints have had a chance to get “worked out.”

Weight Loss

Myositis can cause weight loss. People with the disease can lose weight because of a loss of appetite or from the disease itself.

Lung Inflammation

If myositis causes inflammation in the lungs it can lead to shortness of breath. It can also rarely cause pain with a deep breath.

Rash (Dermatomyositis Only)

People who have dermatomyositis get a rash. In some cases, the rash happens up to 1 year before the muscle weakness comes on. The rash can occur in several different spots, but the most common ones are:

  • Backs of the hands or over the knuckles
  • Chest (the rash often forms a “V” shape)
  • Shoulders (like a shawl)
  • Face and forehead

Eye Swelling

Some people with myositis may also develop swelling around the eyes.

Raynaud’s Phenomenon

They can also develop a condition called Raynaud’s phenomenon. This causes the fingers and toes to turn white and discolour after exposure to cold.


Myositis is best diagnosed by someone who is very familiar with the condition. This is usually a rheumatologist, a type of doctor that is an expert in autoimmune disease and arthritis, or a neurologist, a type of doctor that is an expert in the nervous system.

The doctor will take a thorough history and will then perform a complete physical examination. Following this various tests will be arranged to confirm Polymyositis and Dermatomyositis and to rule out other possible conditions.

Other causes of inflammation in the muscles that are not related to myositis include: certain medications and drugs, hormone imbalances (thyroid, cortisone), infections, and rare genetic conditions.

If a diagnosis of Polymyositis and Dermatomyositis is confirmed in a patient, they should be screened for cancer as well.

Common Tests to Diagnose Myositis

Blood Tests

Looking for high levels of muscle enzymes: The first type of blood tests look for high levels of muscle enzymes in the blood. When the muscles are inflamed they become damaged and break down. The enzymes inside the muscles then “leak out” into the blood. The levels of these enzymes can be measured with a blood test.

The most common enzyme measured is called Creatine Kinase (CK). Creatine Kinase is often very elevated in people with myositis, ranging from the 1000’s to 10,000’s. In 5% of patients the Creatine Kinase level can actually be normal.

Other muscle enzymes including Aspartate Aminotransferase, Alanine Aminotransferase, Lactate Dehydrogenase, and aldolase can also be measured. These are also often elevated.

Looking for antibodies against muscles: The second type of blood tests look to see if the body is producing antibodies against the muscles. Common antibodies tested for include Anti-Nuclear Antibody (ANA) Test and the Extractable Nuclear Antigen (ENA) Panel.

Electromyography (EMG) Test

Looking for muscle inflammation: An Electromyography (EMG) test can determine whether the muscles are inflamed (which could be polymyositis or dermatomyositis) or if there is something else causing them to break down and weaken. During an Electromyography test, small needles are inserted to test the muscles.

Muscle Biopsy (Tissue Sample)

Looking for findings (or signs) of Polymyositis or Dermatomyositis: The best test to ultimately confirm a diagnosis is a muscle biopsy. This is a type of operation where a surgeon removes a small piece of muscle tissue to be examined in a lab. If the patient has Polymyositis or Dermatomyositis, common findings, or signs, consistent with these diseases will be seen in the tissue sample.

Common Tests to Screen for Cancer

Everyone with myositis should be screened for cancer because in a small number of patients, their myositis is accompanied by cancer. Patients should tell their doctor if they have a first degree relative (e.g. mom, dad, brother, or sister) that has ever had cancer. Common tests for cancer include:

  • CT scan of Chest, Abdomen, and Pelvis
  • Mammogram for women
  • Prostate exam and PSA blood test for men
  • Pap smear for women
  • Ca 125 (cancer antigen 25) blood testing for women
  • Stool testing to look for blood and a colonoscopy
  • Thyroid ultrasound and thyroid testing


Scientists don’t know exactly what causes the immune system to start attacking the muscles in polymyositis or dermatomyositis, or why some people develop these diseases but others don’t.

In rarer cases, Polymyositis or Dermatomyositis may be accompanied by cancer. The cancer somehow triggers the immune system in a way that causes the myositis.


Treatment of myositis usually involves both medications and physical therapy. It is very important to treat myositis as early and aggressively as possible to prevent severe muscle damage and ensure the best possible treatment outcome.

In rarer cases where myositis is accompanied by cancer, treating the cancer often helps the myositis. It is very important to treat the myositis as well as the cancer.


Prednisone is the main medication used to treat myositis. It is a powerful medication that helps control the immune system and its attack on the muscles. It is usually started at high doses that will usually be tapered (lowered over time).

Disease Modifying Anti-Rheumatic Drugs (DMARDs)

The Disease Modifying Anti-Rheumatic Drugs that treat other types of arthritis can be effective in treating myositis. Methotrexate and Azathioprine (Imuran) are common choices that are often combined with prednisone to get the disease under control quickly. These medications can sometimes take a few months before patients start to notice improvements.

Intravenous Immune Globulin (IVIG)

Intravenous Immune Globulin is sometimes used in people with severe myositis. This drug doesn’t work in everyone, but when it does work for someone, it can work extremely well.

Physical Therapy and Exercise

Physical therapy and exercise are important treatments to rebuild and to regain strength, flexibility, and range of motion. The right stretching and exercises can also reduce fatigue and emotional distress caused by myositis, and are most effective when done daily.

A trained arthritis physiotherapist can help design an exercise program that helps return muscles to their former strength. The earlier patients start this type of therapy, the better the results.

Cancer Screening

Everyone with myositis should be screened for cancer because for a small number of patients, their myositis can be related to cancer.

In these cases, cancer somehow turns on the immune system and causes the myositis. When this happens, treating the cancer often helps the myositis.

Patients with myositis should tell their doctor if they have a first degree relative (mom, dad, brother, or sister) that has had cancer.