What are polymyositis and dermatomyositis?

Myositis means inflammation of the muscles (myo = muscle, itis = inflammation).

Polymyositis affects many areas (poly = many), mainly the larger muscles like those around your shoulders, hips and thighs. When polymyositis develops alongside a skin rash, the condition is called dermatomyositis (derm = skin).

Both conditions are autoimmune diseases, which means your immune system attacks your body’s own tissues.

Who gets polymyositis and dermatomyositis?

Polymyositis and dermatomyositis are rare diseases, affecting only 6–8 people out of 100,000. They mostly affect adults, although children can be affected by a type of dermatomyositis (juvenile dermatomyositis).

Symptoms

Most people will only have mild and short-lived symptoms. These can include:

  • weak and tired muscles – making normally easy tasks very tiring
  • inflamed muscles – causing pain (known as myalgia) and feeling tender to the touch
  • generally feeling unwell (malaise)
  • weight loss
  • night sweats

If you have dermatomyositis, you may get some of the above symptoms as well as:

  • a red/pink rash on your upper eyelids, face and neck, and on the backs of your hands and fingers
  • swelling of the affected skin, causing a characteristic puffiness and colouring around your eyes.

Other conditions can sometimes mimic polymyositis. These include:

  • side-effects of some medications (for example, steroids or statins)
  • toxic effects of long-term alcohol excess
  • hormonal conditions such as under or over-active thyroid
    low vitamin D levels or abnormal calcium or magnesium levels
  • infections
  • rarer nerve–muscle diseases such as muscular dystrophies.

Causes

Polymyositis and dermatomyositis are autoimmune diseases where your immune system, which normally protects your body against infections, attacks your body’s own tissues. This causes inflammation in your muscles and skin. We don’t yet know why this happens.

How will polymyositis and dermatomyositis affect me?

The outlook is good for cases of polymyositis and dermatomyositis. Symptoms aren’t usually severe and in most cases can be eased fairly quickly with drug treatment. Even severe cases respond to treatment. The condition frequently becomes inactive.

Muscle strength will take longer to recover, but once the condition is controlled you’ll be given exercises to counter any wasting and to improve the strength of the recovering muscle.

Diagnosis

Your doctor will talk to you about your symptoms and examine you. Because the symptoms of polymyositis and dermatomyositis are similar to many other conditions you’ll probably need some tests.

Tests

Blood tests

Creatine phosphokinase

Creatine phosphokinase is an enzyme that leaks out of damaged muscle cells. It’s likely that you’ll have a high level of this in your blood if you have polymyositis or dermatomyositis, but the level should fall as the condition is treated and brought under control.

You’ll need to have repeat tests to tell your doctor how well your condition is responding to treatment.

Erythrocyte sedimentation rate (ESR)

ESR detects and measures inflammation by assessing how quickly blood cells settle at the bottom of a test tube. Inflammation occurs in other conditions, so a high ESR alone won’t confirm polymyositis or dermatomyositis.

Autoantibodies

Antibodies are part of the immune system and usually help us to fight off infections by recognising and attacking viruses or bacteria. Some people’s immune systems develop autoantibodies, antibodies that attack their body’s own tissues. This can contribute to causing autoimmune diseases like polymyositis and dermatomyositis.

An anti-nuclear antibody test (ANA) tests whether your blood contains antibodies to the nucleus (the central part) of cells. About 80–90% of people with polymyositis and dermatomyositis test positive for ANA, but people without either condition can also test positive so it’s only helpful alongside all the other information about your symptoms.

Other autoantibody tests include ENA (extractable nuclear antigens) and the anti-Jo-1 test. These tests are sometimes positive in patients with polymyositis and dermatomyositis.

Other tests

These tests may not be able to diagnose polymyositis or dermatomyositis, but they can help to rule out other conditions.

Electromyography (EMG)

A thin electrode is inserted into your muscle to record electrical discharges from nerve endings that cause your muscles to move. An unusual pattern of electrical activity in a number of different muscles suggests you have polymyositis or dermatomyositis. This test isn’t painful but may be slightly uncomfortable.

Muscle biopsy

A small sample is taken out of one of your larger muscles (for example at the front of your thigh) and examined under a microscope. The site of the biopsy might be chosen using an MRI scan to assess the area. The part of the muscle that looks most badly affected on the scan will be the site of the biopsy. You’ll be given a local anaesthetic to numb the area while the sample is taken, but you may feel some discomfort for a few days afterwards.

The biopsy may show white blood cells sticking to the covering of your muscle and damaging the proteins that make your muscles contract. This causes weakness and fatigue.

You may need to have a repeat biopsy if your condition doesn’t improve with standard treatments. This is to check for the proteins found in people with inclusion body myositis (IBM).

Treatment

Learn more about the different treatments for polymyositis and dermatomyositis.

Drugs

The first choice of treatment for polymyositis and dermatomyositis is steroids, which are usually given in high doses to begin with. They can be given as tablets or injections. This should reduce the inflammation very quickly and settle muscle pain and the feeling of being unwell.

High doses of steroids can have side-effects so your doctor will reduce the dosage as quickly as possible. You may be given bisphosphonates, calcium tablets and vitamin D tablets to guard against osteoporosis.

Sometimes the inflammation can flare up when the steroid dosage is lowered. Your doctor may prescribe other drugs to help reduce the inflammation, for example methotrexate, azathioprine and ciclosporin. Cyclophosphamide may also be prescribed. A newer drug now being used is mycophenolate mofetil. These are all types of disease-modifying anti-rheumatic drugs (DMARDs), and you’ll need regular blood tests to check for any possible side-effects.

Occasionally the drugs mentioned previously don’t control the condition. A small number of people with severe muscle disease and complications have been treated with biological therapies like infliximab and rituximab. These work by blocking the process of inflammation.

Alert card – Steroids

When taking steroid tablets you must carry a steroid card, which records your dose and how long you’ve been taking them. If you become ill, or are involved in an accident in which you’re injured or become unconscious, it’s important for the steroid to be continued. The dose might also need to be increased because the treatment may prevent your body from being able to produce enough natural steroids in response to stress, as normally happens in this situation. Your doctor or rheumatology nurse can give you a steroid card, and they're usually available from pharmacies as well.

Other types of myositis

Post-infectious reactive myositis can occur after some viral infections. After killing the virus, your immune system may cause inflammation in some parts of your body for a few months. If the inflammation occurs in muscles, it causes myositis.

The symptoms are similar to those of polymyositis (muscle pain and weakness) and you may get positive results in some of the test used for polymyositis. But post-infectious reactive myositis is usually mild and settles without treatment. If you have this you may also get positive results for some tests that diagnose polymyositis.

Inclusion body myositis (IBM) causes muscle weakness. Tests to diagnose IBM show similar results as tests for polymyositis and dermatomyositis. In IBM the weakness usually affects muscles in your hands, forearms or calves.

The key difference in IBM is the abnormal lumps of protein (inclusion bodies) found in muscle cells. These lumps are so tiny that they can only be seen with special microscopes.

IBM doesn’t respond well to steroids, so if you’re diagnosed with polymyositis and steroids aren’t working you may need further tests for IBM.

IBM only rarely causes muscle pain and it generally affects people over 50.

Possible complications

In a few cases drug treatment doesn’t work fully and your muscles can remain weak. It's not yet clear why this happens, but moderate exercise (so you start breathing faster but aren’t straining yourself) will still help to improve muscle strength.

Occasionally polymyositis can also affect breathing and swallowing. This may happen at the start of severe cases, when the muscles used become very weak. It may also cause weakening of your heart, and inflammation of your lungs may cause scarring. This affects how your lungs work. Lung and heart conditions can cause long-term breathlessness.

Children with dermatomyositis may develop painful calcium deposits in damaged muscles. These deposits, combined with immobility, can occasionally result in the joints becoming permanently bent (flexion contracture).

Polymyositis occasionally occurs in people who also have another autoimmune disease such as rheumatoid arthritis, lupus and scleroderma.

On rare occasions, myositis can be associated with cancer. Most people with polymyositis and dermatomyositis don’t develop cancer, but your doctor might arrange tests such as a chest x-ray or an ultrasound scan of your abdomen and pelvis to be on the safe side.