Polymyositis

Definition

Polymyositis is a rare autoimmune disease affecting the muscles. It is part of the group of diseases called Idiopathic Inflammatory Myopathy (IMM).

Women are affected twice as often as men.

Polymyositis is a type of autoimmune myositis along with dermatomyositis, immune-mediated necrotizing myopathy, and inclusion body myositis.

Symptoms

Symptoms of polymyositis are mostly related to muscle weakness, but other parts of the body can also be affected. They can appear gradually over a period of 3 to 6 months.

The symptoms include:

• Difficulty lifting objects, raising the arm above the head, kneeling, going up / down the stairs
• Joint pain
• Tiredness
• Weight loss
• Difficulty swallowing
• Difficulty raising the head

Causes and risk factors

The causes of polymyositis are currently unknown. The symptoms appear to be caused by an inflammation process that destroys and blocks the regeneration of muscle components.

Genetic predisposition as well as some environmental factors (certain infections, for example) could be the origin of this inflammation.

Diagnosis

Most patients are between 45 and 60 years old, but the disease can also develop in children between 5 and 15 years old.

Certain ethnic groups may be more affected.

The diagnosis of polymyositis is based on the presence of four criteria:

• Weakness of the muscles around the trunk (called proximal muscles): muscles of the shoulders and arms, muscles of the hips and thighs
• Blood test results showing elevated levels of muscle enzymes (specific to muscles) 
• Abnormalities discovered during muscle examination (Electromyogram / EMG or MRI), such as changes in electrical activity in muscles.
• Abnormalities discovered during microscopic examination of muscle cells (muscle biopsy) revealing, in particular, the presence of cell infiltrate, responsible for the inflammation.

The presence of anti-nuclear antibodies (antibodies that react with certain components of healthy cells, causing inflammation) may serve as additional evidence, but their presence is not specific to polymyositis.

Treatments

The goal of the treatment is to suppress inflammation and allow the muscles to regain all of their capacities.

The treatment consists primarily of high doses of oral corticosteroids. Immunosuppressants can also be used to soothe the immune system and reduce inflammation that causes  the symptoms.

In severe cases, immunoglobulins (a type of antibodies that help modulate the dysfunction of the immune system) or corticosteroids may be injected. The injection allows for a faster action compared to oral treatments.

Physiotherapy is also part of therapeutic strategy to re-educate muscles affected by the disease.

Living with the disease

A relapse of polymyositis is possible but does not occur in all cases. Physiotherapy is necessary for proper muscle recovery.

Because of the possible damage to other organs and tissues, it is necessary to carry out follow-up examinations: chest X-ray, lung examination, cardiac ultrasound and screening for the most frequent cancers depending on the patient's age and sex.

The prognosis is generally good. It depends on the severity of the symptoms, the response to treatment, and whether or not the disease affects other parts of the body besides the muscles.

Last updated: 12/3/21