Polymyositis

Other targets for auto-immune muscle diseases include the skin.  Rashes are quite common, particularly in dermatomyositis. These rashes can look like unusual sunburn on the chest and a purplish sort of area on the face, with red thick raised areas on the skin sometimes appearing over the knuckles or on the person's hands. Sometimes the lungs are also targeted. This is called interstitial lung disease and is quite a serious complication.

What causes it?

It's not certain what causes polymyositis, but many of the conditions are associated with the presence of autoantibodies and some are triggered by chemicals. One such chemical trigger is the group of drugs called statins (often prescribed to lower cholesterol levels). Very rarely, statins can cause a condition called Necrotising Autoimmune Myopathy (NAM). It is uncommon, but approximately one in every 100,000 patients on a statin might experience this condition.

There are many autoantibodies associated with myositis and these can be investigated through tests that your Rheumatologist can organise for you. Some of these autoantibodies target specific enzymes within the DNA and RNA of the person's cells. These are sometimes referred to as antisynthetase antibodies. There are many other autoantibodies as well, which will sometimes give a clue to the existence of an autoimmune muscle disease like myosotis.

How is it diagnosed?

Polymyositis and the autoimmune muscle diseases can be hard to diagnose initially. Apart from the symptoms of weakness and tiredness, the test that will most likely suggest there is a muscle disease problem is an elevated level of the enzyme called the creatine kinase (CK) in the blood.

This is an enzyme produced by normal muscle function in a range of about 50 to 200 units. However, when the muscles are being attacked by the immune system, the CK level might rise up to 5,000 or 10,000 units. Finding evidence of this very high level of CK will sometimes be the clue that your doctor needs to start the process of further investigating the presence of myositis and formulating a treatment plan for you.

Other conditions can also give you a high CK, including things like thyroid disease; toxins like alcohol and other drugs; acute dehydration; acute fever and illness. With many of these however, the elevated CK level will return to normal once the cause is removed. But with an autoimmune muscle disease like polymyositis, the CK level stays elevated unless it is actually treated.

Other blood tests will sometimes provide clues to the existence of myositis, including tests for autoantibodies and muscle-specific antibodies. The most well-known ones being the antisynthetase antibodies.

An electrical test called an EMG can sometimes show abnormalities of the muscles. And an MRI is often done on arms or legs to investigate muscle injury or muscle oedema (or swelling). An abnormal MRI will identify any area of muscle that is affected. This affected area can then be targeted by a surgeon to get an accurate muscle biopsy. So a muscle biopsy is probably the definitive test for diagnosing polymyositis. Although, if the characteristic rash, symptoms, elevated CK and auto-antibodies are all present and confirming an autoimmune muscle disease, a muscle biopsy is not always required.