Myositis (polymyositis and dermatomyositis)

Causes of myositis

The term myositis means "inflammation of the muscles". 

Myositis is usually caused by an underlying problem with the immune system, where it mistakenly attacks healthy tissue.

There are at least three distinct patterns that can occur in people with myositis. Some people have:

• severe symptoms (acute myositis) that may calm down in one to two years
• longer-lasting or persistent symptoms (chronic persistent disease)
• symptoms that tend to come and go (relapsing disease) 

Types of myositis

There are different types of myositis which all affect the muscles.

Polymyositis 

Polymyositis affects many different muscles, particularly the shoulders, hips and thigh muscles.

It's more common in women and tends to affect people aged 30 to 60.

Dermatomyositis

Dermatomyositis, as well as affecting the muscles, causes a rash.

It's also more common in women and affects both adults and children.

Post-infectious reactive myositis

You can get post-infectious reactive myositis after some viral infections. It causes the muscles to become inflamed.

This type of myositis is usually mild and settles without treatment.

Inclusion body myositis (IBM)

Inclusion body myositis (IBM) causes muscle weakness in:

• the quadriceps (main thigh muscles)
• the forearm muscles that flex the fingers
• the muscles below the knee, which can cause foot drop, making it difficult to lift the front part of your foot and toes and causing the foot to drag on the ground when walking

IBM is more common in men and tends to occur after the age of 50.

The UK-based charity, Myositis UK, has more information about the different types of myositis.

Symptoms of polymyositis

Polymyositis affects many different muscles, particularly around the neck, shoulders, back, hips and thighs.

Symptoms include:

• muscle weakness
• aching or painful muscles
• extreme tiredness 
• generally feeling unwell
• other more generalised symptoms, including joint pain and occasionally swelling, and shortness of breath

The muscle weakness may vary from week to week or month to month, although it tends to steadily get worse without treatment.

You may find it difficult to get up from a chair, climb stairs, lift objects, and comb your hair.

The muscle weakness can become so severe that even picking up a cup of tea can be difficult.

If your neck muscles are affected, you may find it hard to hold your head up.

If your throat or chest muscles are affected, you may also have breathing and difficulty swallowing.

Symptoms of dermatomyositis

Dermatomyositis also affects the muscles and the symptoms are similar to those of polymyositis. There's also a distinctive rash.

Before the muscle symptoms occur, a red or purple rash often appears on the face (eyelids, nose and cheeks), back, upper chest, elbows, knees and knuckles.

The rash can be itchy or painful. You may also get hard lumps of tissue under the skin called calcinosis.

Diagnosing myositis

Your GP will ask you about your symptoms and examine you. 

If they think you may have myositis, you’ll need to have some tests to help rule out other conditions with similar symptoms.

Tests you may have include:

• blood tests – to check for raised levels of enzymes and antibodies in your blood
• muscle or skin biopsy – a small sample of muscle tissue and/ or skin may be taken so it can be examined for inflammation, damage and other abnormalities
• magnetic resonance imaging (MRI) scans – scanners are able to accurately detect the extent of inflammatory changes in the muscles and distinguish areas of true inflammation from other areas close by, where the muscle fibres have been replaced by fat or fibrous tissue  
• electromyography (EMG) – a small needle-shaped electrode is inserted through your skin and into your muscle, using a local anaesthetic, to record the electrical signals from the nerve endings in your muscles; an abnormal pattern of electrical activity may show that you have polymyositis or dermatomyositis

Treating myositis

Exercise and physiotherapy

You must be very careful about exercising if you have severe symptoms of myositis, such as severe muscle pain and weakness.

Most specialists don't recommend exercising during this period.

However, it's very important to maintain gentle muscle and joint movement, particularly in cases where myositis develops during childhood.

This ensures that the joints which are moved by muscles don't become stiff and take on a fixed position.

As the condition is brought under control, a gentle exercise programme can be started and gradually increased following the advice of a physiotherapist.

Steroid medication

Steroids are the main type of medication used to treat polymyositis and dermatomyositis. They can include:

• steroid creams – which can be used to treat the affected areas of skin in dermatomyositis
• higher doses of steroid tablets if there's severe muscle weakness

High doses of steroids can cause side effects such as:

• weight gain
• high blood pressure
• diabetes
• cataracts (cloudy patches in the lens of the eye)
• osteoporosis (weakened bones)

Disease-modifying anti-rheumatic drugs (DMARDs)

If the inflammation in your muscles flares up, your doctor may prescribe a disease-modifying anti-rheumatic drug (DMARD).

DMARDs suppress your immune system and help reduce inflammation.

These medications are slow to act, but in the long term may allow the dose of steroids to be reduced, along with their side effects.

Intravenous immunoglobulin therapy

Intravenous immunoglobulin therapy may be needed in very severe cases of myositis where severe muscle weakness is causing life-threatening breathing or swallowing problems. 

It involves having an injection of normal antibodies from donated blood. This temporarily changes the way your immune system operates.

Intravenous immunoglobulin therapy can produce a rapid improvement in the symptoms of myositis, but it's very expensive and the benefits usually only last a few weeks.

Therefore, it isn't suitable as a long-term treatment, and is usually only used to treat people who are seriously ill while they're waiting for other treatments to work.

Biologic therapies

Biologic therapies, which are used to treat conditions such as rheumatoid arthritis and psoriatic arthritis, may also help people with myositis, whose symptoms don't respond to conventional steroids and immunosuppressive medication.

Outlook

Most people with myositis respond to a combination of steroid and immunosuppressive therapy, alongside carefully-controlled exercise.

Steroids are often needed, in very low doses, for several years as well as immunosuppressive medication.

This can lead to an increased risk of infection which in most cases can be easily managed with antibiotics if it becomes a problem.

Complications of myositis

Some people with myositis respond less well to treatment and find the condition significantly affects their everyday activities and quality of life. But continuing to exercise usually helps improve muscle strength.

If you have severe myositis, you may develop breathing and swallowing problems.

Speech and language therapy may be recommended if you're having problems swallowing or it's affecting your ability to communicate.

In rare cases, myositis can be associated with cancer, and you may be offered tests to check for cancer.

Help and support

Myositis UK provides further information, help and support for people who are diagnosed with myositis and their families.

More useful links

• How to use your health services