Polymyositis

Alternative names: When associated with skin rash, it is referred to as Dermatomyositis.

Polymyositis is an inflammatory disease of muscle, its cause unknown.  It begins when white blood cells – the immune cells of inflammation – spontaneously invade muscles.  The muscles affected are mostly those closest to the trunk or torso.  This in turn results in weakness which can be severe.  It is a chronic illness with periods of increased symptoms, called flares or relapses, and decreased symptoms, known as remissions.

Polymyositis can affect other areas of the body and is, therefore, a systemic illness.  Occasionally it is associated with cancer, or with other diseases of the connective tissues (for example, systemic lupus erythematosus, scleroderma or rheumatoid arthritis).

Aside from diseases with which polymyositis can be associated, many other diseases and conditions can mimic polymyositis.  These include nerve-muscle diseases (such as muscular dystrophies), drug toxins (such as alcohol, cocaine, steroids and lovastatin), metabolic disorders (where muscle cells are unable to process chemicals normally), hormone disorders (such as abnormal thyroid, calcium, and magnesium conditions), and infectious diseases (such as influenza virus, AIDS, streptococcus and Lyme bacteria, pork tapeworm and schistosomiasis).

Incidence; Causes and Development

Polymyositis is slightly more common in females.  It affects all age groups, although its onset is most common in middle childhood and in the twenties.  Polymyositis occurs throughout the world.

To date, no cause of polymyositis has been isolated by medical researchers.  There are indicators of heredity (genetic) susceptibility that can be found in some patients.  There is also indirect evidence of infection by a virus that has yet to be identified in a form of polymyositis that is particularly resistant to treatment, called inclusion body myositis.  This form of polymyositis is diagnosed by a pathologist – a physician specialist who interprets the microscope findings of muscle tissue.  The muscle tissue in this form of polymyositis displays clear areas within the muscle cells (called vacuoles) when viewed under a microscope.

Signs and Symptoms

Weakness of muscles is the most common symptom of polymyositis.  The muscles involved usually are those that are closest to the trunk of the body.  The onset can be gradual or rapid, resulting in varying degrees of loss of muscle power.  The loss of strength can be noticed as difficulty getting up from chairs, climbing stairs or lifting above the shoulders.  Trouble with swallowing and weakness lifting the head from the pillow can occur.  Occasionally (in some 25% of patients), the muscles ache and are tender to the touch.

Patients can also feel fatigue, a general feeling of discomfort and have weight loss and/or low-grade fever.  With skin involvement (dermatomyositis), the eyes can be surrounded by a violet discoloration with swelling.  There can be scaly reddish discoloration over the knuckles, elbows and knees (Gottron's sign).  There can also be reddish rash on the face, neck and upper chest.  Hard lumps of calcium deposits can develop in the fatty layer of the skin, most commonly in childhood dermatomyositis.

Heart and lung involvement can lead to irregular heart rhythm and shortness of breath.  Both polymyositis and dermatomyositis can sometimes be associated with cancers, including lymphoma, breast, lung, ovarian, and colon cancer.  The cancer risk is reported to be much greater with dermatomyositis than polymyositis.

Diagnosis and Tests

When a patient first sees a doctor, recent symptoms – especially those concerning weakness – will be discussed.  The condition of many other body areas might be reviewed, for example the skin, heart, lungs and joints.  An examination will further focus on these and other systems.  Various measures of strength might be noted.

Blood testing usually reveals abnormally high levels of muscle enzymes, CPK or creatinine phosphokinase and aldolase.  These enzymes are released into the blood by muscle that is being damaged by inflammation.  They can also be used as measures of the activity of the inflammation.  Other routine blood and urine tests can also look for internal organ abnormalities.  Chest X-rays, mammograms, PAP smears and other screening tests might be considered.

An electromyography (EMG) and nerve conduction velocity are electrical tests of muscle and nerves that can show abnormal findings typical of polymyositis as well as exclude other nerve-muscle diseases.

A muscle biopsy is used to confirm the presence of muscle inflammation typical only of polymyositis.  This is a surgical procedure whereby muscle tissue is removed for analysis by a pathologist.  Muscles often used for biopsy include the quadriceps muscle of the front of the thigh, the biceps muscle of the arm and the deltoid muscle of the shoulder.

Prognosis

Patients can ultimately do well, especially with early medical treatment of disease and disease flares.  The disease frequently becomes inactive, and rehabilitation of atrophied muscle becomes a long-term project.  Inclusion Body Myositis is often more resistant to treatment than the usual form of disease.