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Diseases > Polymyositis (PM) > What is it?

What is Myositis?

When you break it down, the word “myositis” makes sense.  “Myo” means muscle, and “itis” means inflammation. Putting it all together, myositis means inflammation in the muscles. Likewise, the word “dermato” means skin. So dermatomyositis means inflammation of muscles and the skin. To make it easier we’ll call both polymyositis and dermatomyositis simply myositis.

Myositis is a chronic (long-term) disease that leads to muscle weakness, but not usually pain or swelling. People with myositis develop weakness in the muscles around the shoulders and hips. However, other muscles in the body can become weak.

What causes Myositis?

At this time, we don’t know exactly what causes myositis or why some people develop the condition but others don’t. Myositis is caused by the immune system. For some reason, the immune system gets confused and starts to attack the muscle. The reason for this is not understood. The attack on the muscle leads to weakness, but not usually pain or swelling.

The immune system attack can also cause fatigue. You can think of it like this: your immune system is “turned on” or activated. When the immune system is activated it makes you feel very tired. It’s like when your immune system is turned on when you’re fighting the flu. Except with myositis, you’re not fighting the flu.

Who gets Myositis?

Myositis typically affects two age groups. Children between the ages of 10-15 and adults between the ages of 45-60. However, anyone can get myositis. It just occurs less often in other age groups. Myositis is very uncommon: it affects only about 10 people in every million.

Women are about 2 times more likely to get myositis than men. That means for every 3 people with myositis, 2 will probably be women and 1 will be a man.

How is Myositis diagnosed?

Myositis is best diagnosed by someone who is very familiar with the condition. This is usually a rheumatologist or a neurologist. The doctor will take a thorough history and then perform a complete physical examination. Following this various tests will be arranged (see below).

It’s important to realize that other things can cause inflammation in your muscles. Some examples include:

  • Certain medications and drugs
  • Hormone imbalances (thyroid, cortisone)
  • Infections
  • Other rarer genetic conditions

What tests are done to diagnose Myositis?

Special tests can help to confirm if you have myositis.

Blood tests for myositis

The first test looks 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 Creatinine Kinase (CK).  CK is often very elevated in myositis ranging from the 1000’s to 10,000’s. In 5% of patients the CK can actually be normal.

Other muscle enzymes including AST, ALT, LDH, and aldolase can also be measured. These are also often elevated.

The second type of blood tests look to see if there are antibodies against the muscles. Common antibodies tested for include Anti-Nuclear Antibodies (ANA) and ENA (Extractable Nuclear Antibodies).

Electromyography (EMG) for Myositis

The next step is to determine if the muscles are inflamed or is there something else causing them to breakdown? The first step is usually and electromyogram or “EMG”. During an EMG test, small needles are inserted to test the muscles.

Muscle biopsy for Myositis

The best test to ultimately make the diagnosis is a muscle biopsy. This is an operation. A surgeon removes a small piece of muscle tissue. The tissue is examined to look for the common findings of DM or PM.

Read more – What is it going to do to me?