Polymyositis and dermatomyositis
Myositis means inflammation in the muscles, which causes pain
and/or weakness in the affected muscles. Polymyositis and
dermatomyositis are autoimmune diseases. This means the immune
system attacks the body's own tissues.
Polymyositis and dermatomyositis are rare diseases, affecting
only 6–8 people out of every 100,000. They usually affect adults,
though there is a type of dermatomyositis that affects children
called juvenile dermatomyositis.
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Polymyositis and dermatomyositis symptoms
Polymyositis causes pain and/or weakness in
many muscles, particularly the larger muscles of the shoulders,
hips and thighs. Inflamed muscles may be tender to the touch, and
you may feel generally unwell and experience weight loss or night
sweats. In a few cases it can also affect breathing and
swallowing.
Dermatomyositis has similar symptoms but the
inflammation also causes a red/pink rash on the upper eyelids,
face, neck and the backs of the hands and fingers.
Symptoms range from mild pain and stiffness through to severe
weakness. If the symptoms appear very suddenly, and especially if
they are accompanied by breathing or swallowing difficulties, then
you may be admitted urgently into hospital until the condition has
been stabilized.
Other types of myositis
- Post-infectious reactive myositis can occur
after some viral infections. Inflammation is a normal part of the
body's defence against infections, but the body's immune system can
sometimes cause inflammation in tissues including the muscles even
after the virus has been dealt with. Symptoms are similar to those
of polymyositis but are usually mild, and will settle without
treatment
- Inclusion body myositis (IBM) causes muscle
weakness in much the same way as polymyositis and dermatomyositis.
However, in IBM the weakness usually affects muscles near the ends
of the limbs (hands, forearms and calves) and IBM only rarely
causes muscle pain.
What are the long-term risks?
In both polymyositis and dermatomyositis inflammation of the
lung tissues can occasionally lead to lung scarring and long-term
breathlessness.
Rarely myositis can be associated with cancer. Most people with
polymyositis and dermatomyositis don't develop cancer but your
consultant may arrange tests, such as a chest x-ray or an
ultrasound
scan of your abdomen and pelvis to be on the safe
side.
Children with dermatomyositis may develop painful calcium
deposits in damaged muscles. These deposits, combined with
immobility, can occasionally result in permanently bent joints
(flexion contracture).
What causes polymyositis and dermatomyositis?
The processes in the body that cause polymyositis and
dermatomyositis aren't well understood. Normally, the body's immune
system protects it against infections such as those caused by
bacteria and viruses. In polymyositis and dermatomyositis the
immune system attacks the body's own tissues, causing inflammation
in the muscles and possibly the skin.
What causes the immune system to turn against the muscles, skin
and blood vessels is not known. Genetic factors probably play a
part but the conditions are not directly inherited.
How are polymyositis and dermatomyositis diagnosed?
Muscle pain and weakness occur in many other conditions.
Tests will help your doctor decide whether you have
polymyositis or dermatomyositis and to rule out other possible
causes of your symptoms.
You will probably have the following blood tests:
- Erythrocyte sedimentation rate (ESR) – This
detects and measures inflammation, but inflammation occurs in other
conditions so a high ESR alone won't confirm polymyositis or
dermatomyositis
- Creatine phosphokinase – This is an enzyme
that leaks out of damaged muscles. If you have polymyositis and
dermatomyositis you're likely to have a high level of this enzyme
in the bloodstream.
These tests alone may not be enough to confirm polymyositis or
dermatomyositis. However, they can help to rule out other
conditions such as fibromyalgia where creatine phosphokinase and
ESR are usually normal.
You'll probably have repeat blood tests once your treatment
starts to make sure that it's working.
Sometimes further tests may be needed:
- Electromyography (EMG) – A thin electrode
probe is inserted into the muscle to record electrical discharges
from nerve endings which cause our muscles to move. An unusual
pattern of electrical activity in a number of different muscles
suggests you do have polymyositis or dermatomyositis.
- Muscle biopsy – A small sample of muscle,
usually taken from the thigh, is examined under a microscope. This
may show white blood cells sticking to the covering of the muscle
and damaging the muscle proteins.
You may need to have a repeat muscle biopsy if your condition
doesn't improve with standard treatments. This is to check for
inclusion body myositis (IBM). Very powerful microscopes allow
doctors to see abnormal clumps of proteins (inclusion bodies)
inside the muscle cells – these are not present in other types of
mysositis.
Polymyositis and dermatomyositis treatments
Polymyositis and dermatomyositis is treated using a
combination of drug therapies including steroid tablets,
disease-modifying drugs and infusions of
immunoglobulins.
Treatment is usually successful, though a few people do have
ongoing weakness.
Steroid tablets in relatively high-doses are usually the first
choice, and should reduce the inflammation very quickly. It will
take some weeks or months for the body to repair any muscle damage,
so don't expect your muscles to be back to full strength straight
away. High-dose steroids can have side-effects, including
osteoporosis. To minimize this risk, your doctor will
reduce the dose as quickly as possible.
If your symptoms flare up when the steroid dose is reduced then
your doctor may suggest drugs such as methotrexate or azathioprine,
which can reduce inflammation without having to increase your
steroid dose again. Methotrexate and azathioprine can also have
side-effects but you will have regular blood-monitoring tests as a
precaution against these.
Infusions of immunoglobulins are available if tablets alone
aren't controlling the disease. Immunoglobulins are antibodies that
suppress the immune system's attack on your own body tissues.
Infusions are given in hospital at monthly or 3-monthly
intervals.
At the start of more severe cases the muscles used for
swallowing and breathing may become very weak. People with these
conditions may require medical help to eat or breathe.
Even in severe cases myositis usually responds well to
treatment, though some people do need ongoing drug treatments to
keep the condition under control.
Self-help and daily living
You'll probably feel more tired than usual so you'll need to
pace yourself with your daily activities.
However, exercises will be an important part of your
recovery programme.
Exercise
When your myositis is very active, it's probably best to rest.
However, you'll need to start exercises as soon as possible once
the condition begins to settle to restore muscle strength and
improve your stamina. To start with this should be under the
supervision of a
physiotherapist.
Children will need more vigorous physiotherapy to avoid flexion
contracture where joints, especially the knees, become permanently
bent.