POLYMYOSITIS AND DERMATOMYOSITIS

What are polymyositis and dermatomyositis?

They are conditions in which inflammation of muscles may lead to quite severe disability but for which, fortunately, treatment is available.

Like other tissues of the body, muscles may suffer inflammation. Sometimes a sudden brief inflammation (or 'myositis') results from an acute infection by a virus or (rarely nowadays except in the tropics) by bacteria. Sometimes a muscle may be inflamed and painful after exceptional exercise. But polymyositis refers to a much more prolonged inflammatory illness of many of the body's muscles. Often this is associated with inflammation of the skin when the illness is called dermatomyositis.

What are the symptoms?

Polymyositis and dermatomyositis are often not painful and are only noticed when the muscles become weak gradually over a period varying from many days to several months. The weakness results in difficulty in walking, in lifting the arms and in getting up from the sitting and lying positions. In this respect these conditions may resemble other muscle diseases, including muscular dystrophy, and in some cases the distinction can be made only after careful medical investigation. Unlike most types of muscular dystrophy, however, polymyositis may cause some trouble in swallowing and the muscles, even if not painful, are sometimes tender on pressure.

In dermatomyositis the skin looks red in certain places, most often over the cheeks and eyelids, upper chest, elbows, knees, knuckles and nailbeds. These signs may be mistaken by people for an allergic reaction. Tiny blood vessels may be seen in the skin in the reddened areas, and sometimes the skin becomes thin, tight and shiny. Some affected people may develop joint pains and many show a tendency for individual fingers or the hands to go very white or blue in cold weather.

At what age does it usually start?

Both conditions may occur at any age. Dermatomyositis is more common than polymyositis in childhood and since the condition in children is often different in several respects from that in adults, a separate leaflet has been prepared on that condition.

How is the diagnosis made?

Although in typical cases doctors experienced in such cases may recognise dermatomyositis and even polymyositis after a simple clinical examination, this is rarely sufficient to prove the diagnosis. Usually the measurement of certain blood enzymes and other blood tests, examination of the electrical activity of the muscles (electromyography) and above all the examination of a small piece of muscle tissue under the microscope (muscle biopsy) will be necessary to identify the illness with certainty. In some unusual cases these tests give the first clue to its nature.

Is there any treatment?

Once diagnosis has been made, two principal types of drugs are used to treat both polymyositis and dermatomyositis. Steroid drugs such as prednisolone have been used for many years and recently more specific immunosuppressive drugs like azathioprine or cyclophosphamide (which, like steroids, suppress the body's immune responses and limit the inflammation) have been introduced as an additional or alternative treatment.

Most people begin to improve within days or weeks of beginning steroid treatment and if the condition is caught early, especially in young people, recovery is often complete. Opinions vary among doctors as to how long the treatment should be continued but most prefer to give drugs for at least two to three years. Occasional patients need longer courses of treatment, while in some the illness is suppressed in only a few months and treatment can be stopped more quickly.

The guiding principle must be to give the minimum dose necessary to control symptoms (as assessed by tests of muscle function) for as long as may be needed. In some cases permanent muscle damage seems to have occurred before treatment is started or to develop in spite of it, but the latter is relatively unusual.

In those few people in whom standard treatment fails to control the illness, short courses of 'pulsed' treatment with methylprednisone given intravenously may help; or a newer drug called cyclosporin, which is often used to suppress rejection after organ transplantation, may be effective but does have some serious side-effects.

In dermatomyositis, plasma exchange to remove antibodies from the circulating blood is useful in an emergency but is of no long-term benefit. Rarely, when other treatment has not been effective, whole body irradiation has been shown to be effective in some resistant cases.

How mild or severe can it be?

The pace of the illness varies enormously, some people becoming very ill and weak within days of the first symptoms and others failing to notice any problem until some episode draws attention to the wasting and weakness of the muscles which must have developed over many months or even years.

Eventually the untreated condition often stops deteriorating and sometimes it then slowly improves but may do so only after the muscles have been considerably damaged and weakened by the inflammation. It is usual for some weakness to become permanent if the condition is not treated and indeed the illness may be severe enough to put the person's life at risk.

Can any problems be anticipated?

Certain complications may occur in the course of the illness including quite often shortening of some affected muscles leading to 'contractures' or limitation of the range of movement at joints. A much rarer complication of dermatomyositis is the formation of little deposits of chalky material under the skin (called 'calcinosis'). When this occurs it tends to be at a time when the muscle inflammation is no longer active and it is much more common in children than in adults. Calcinosis tends to improve only over a very long period and in exceptional cases may require surgical removal of troublesome deposits.

Can exercise help?

During the early active phase of the illness rest is advisable. But once the inflammation has died down active exercise becomes important for without exercise the muscles become weak and wasted. The doctor will decide at what point to change from resting to exercising them. Stretching the muscles may be important to prevent or limit contractures. This is best done by someone else. A physiotherapist can advise on the best techniques and the needs vary from person to person.

Do:

• exercise as much as possible in moderation
• rest when feeling exhausted
• allow more time to wash and dress
• maintain your independence as far as possible
• find out what help is available and where to get it

Do not:

• sit for hours in one position
• lie in bed unnecessarily

Will any special equipment be needed?

Some people with polymyositis will need a wheelchair and other supportive equipment while the illness is at its height. An unfortunate few may need more permanent help if serious permanent muscle damage results. An occupational therapist from the hospital or Local Authority can often help to advise about equipment and, if necessary, adaptations to the house to enable people to remain as active and as independent as possible during treatment.

What of the future?

Immunology (the scientific study of immunity and its disorders) is a rapidly developing field and we can be confident that the next few years will bring new understanding of the causes of polymyositis and dermatomyositis and that ultimately even better methods of treatment will be devised.

Other forms of polymyositis?

Some rare varieties of polymyositis occasionally occur which are rather different in presentation:

• Localised nodular myositis - the condition begins with one or more painful tender lumps appearing in single muscles or muscle groups.
• Granulomatous myositis - in this condition the changes seen with the microscope in a muscle biopsy specimen are distinctive.

The treatment of both these conditions is similar to that of the more common varieties of polymyositis and dermatomyositis.

Inclusion body myositis - this very rare condition is a painful and progressive form which occurs usually in older people and affects the forearm muscles and those below the knees more severely than those of the shoulders, upper arms, and thighs. Muscle biopsy in such cases often shows characteristic structures called inclusion bodies in muscle, suggesting that the condition is probably due to a virus but this has not yet been fully confirmed. Unfortunately inclusion body myositis is usually progressive and uninfluenced by treatment.

What is the cause of polymyositis and dermatomyositis?

These conditions belong to a group of disorders which are called auto-immune. These occur when the complex mechanisms which the body has to combat and provide us with immunity against infections and other foreign matter, are directed against our own tissues and may damage them.

Sometimes a specific type of white blood cell (the so-called T lymphocyte) becomes sensitised against the body's own tissues (sometimes a single tissue such as a muscle) and in consequence attacks and damages it (this is called cell-mediated immunity).

Similar damage can result from antibodies which develop against the tissue and are then found in the blood plasma (this is humoral immunity). These conditions are not due to a specific infecting agent but sometimes the autoimmune process appears to be initiated by the presence in muscle of a virus which normally causes only a short, self-limiting illness (such as that of influenza, for example).

There is also good evidence that in polymyositis, in which the inflammation occurs only in skeletal muscle, the condition is largely T cell-mediated, while in dermatomyositis, in which skin and blood vessels are involved, the process is largely humoral. In a few people with dermatomyositis there may also be features of another autoimmune disease such as, for example, systemic lupus erythematosus, scleroderma or rheumatoid arthritis.