Polymyositis, dermatomyositis, immune-mediated necrotizing myopathy, sporadic inclusion body myositis.

Myositis is a muscle inflammation that is classified according to various criteria. The most common are polymyositis, dermatomyositis, immune-mediated necrotizing myopathy, and sporadic inclusion body myositis. They belong to the group of inflammatory muscle diseases that are not caused by pathogens such as bacteria, viruses or parasites.

Overview: What is myositis?

Myositis is an inflammation of the skeletal muscles. The name is derived like this: “Myo” stands for muscle and the ending “itis” means inflammation. Several types of muscle inflammation fall into the myositis group. Polymyositis (PM), dermatomyositis (DM), immune-mediated necrotizing myopathy (IMNM), and inclusion body myositis (IBM) are the major forms of muscle inflammation that are not caused by pathogens (bacteria, viruses, or parasites). The exact causes of these muscle diseases are not yet clear. However, it is known that the immune system plays a role in its development. For as yet unknown reasons, it attacks and destroys the body’s own structures – in this case, the muscles. In addition, myositis occurs in the context of other diseases, for example in autoimmune diseases such as rheumatoid arthritis, primary Sjögren’s syndrome or systemic lupus erythematosus.

The symptoms of myositis are loss of strength and weakness of the muscles, often in the trunk area. Muscle pain and degeneration are also common. In addition to the muscles, the skin, joints, vessels or internal organs such as the lungs or heart can also be affected. This then leads to further symptoms.

There are several ways to treat myositis. The goal is to stop or mitigate the activation of the immune system that causes inflammation and damage. Drugs that modulate the immune system are used for this purpose. These are sometimes referred to as immunosuppressants. The muscles can be strengthened again with the help of physiotherapy.

Myositis – frequency and age

Myositis, which is caused by autoimmune processes, is a comparatively rare disease. Experts estimate that around 2 to 7 out of every 1,000,000 inhabitants worldwide contract the disease each year.

Myositis can affect people of different ages. It can occur in childhood, but also in adulthood. Women are more often affected than men with the exception of inclusion body myositis, This usually occurs beyond the age of 50 years.

Myositis: causes and risk factors

Autoimmune myositis: immune system out of control

In autoimmune myositis, the immune system is out of control: It mistakenly attacks the body’s own structures – in this case primarily the muscles, but in some cases also the skin, joints, vessels or internal organs such as the heart and lungs, and less frequently the intestines. If this attack is not stopped, damage to the muscles or other structures/organs may occur. It is largely unknown why the defense system reacts so excessively. A genetic predisposition present at birth is likely a contributing factor.

What forms of myositis are there?

  • Dermatomyositis (DM): Here, skin and muscle involvement is often present in parallel. In dermatomyositis, the immune system’s attack is thought to be directed primarily against the blood vessels responsible for supplying oxygen and nutrients to the skin and muscles. The vascular damage then impairs the blood supply. Thus, the skin and muscles take damage indirectly.
  • Polymyositis (PM): the immune system probably damages the muscle fibers directly. Special white blood cells – the cytotoxic T cells – penetrate the muscle fibers and damage them.
  • Inclusion body myositis (IBM): Here, too, the muscle fibers are directly damaged. The name comes from the fact that proteins (inclusion bodies) deposited inside the cell can be seen under the microscope. It is not clear whether inflammation is present first, which then leads to muscle tissue breakdown over time, or whether muscle breakdown and inclusion body formation reactively cause inflammation.
  • Immune-mediated necrotizing myositis (IMNM): In this form of myositis, very extensive muscle damage occurs.

In addition, myositis still occurs in association with other autoimmune diseases such as Systemic Lupus Erythematosus (SLE), mixed connective tissue disease, systemic sclerosis, primary Sjögren’s syndrome, or rheumatoid arthritis.

Sometimes immune-mediated myositis may also occur in the context of malignant tumor disease or be triggered by drugs.

Pathogen-induced myositis: infections as causes

Muscle inflammation can also occur with infections. Some examples:

  • Viruses (e.g. influenza, HIV)
  • Bacteria (e.g. tetanus, leprosy, syphilis)
  • parasites (e.g., schistosoma (schistosomiasis), pork tapeworm (cysticercosis)).

Symptoms: Myositis often causes muscle weakness or pain

Autoimmune myositis often develops insidiously. Symptoms increase over time as the disease progresses. Common to many myositides is a general feeling of illness, sometimes a slight fever, muscle pain like soreness, and muscle weakness. There may be other symptoms that affect other parts of the body.

The following signs may indicate muscle inflammation:

  • Muscle weakness and atrophy – affecting muscles on the trunk and extremities near the center of the body
  • Muscle pain
  • Movement limitations: Problems climbing stairs or getting up from a chair or lifting or carrying objects, tripping, falling, dropping objects.
  • Dysphagia when the muscles of the pharynx are affected.

General symptoms may also occur:

  • Tiredness, fatigue
  • Weight loss
  • Fever or elevated temperature

Symptoms that may indicate the involvement of other structures / organs:

  • Joint pain
  • Raynaud’s syndrome (disorder of blood circulation especially in fingers and toes, which can lead to discoloration and pain)
  • skin rash (increased sensitivity to UV light), calcification under the skin
  • Shortness of breath
  • Dry cough
  • Palpitations/stumbling
  • Digestive problems / abdominal pain.

Diagnosis myositis

Because the symptoms often develop insidiously at the beginning of the disease and are non-specific, months to years can pass before a diagnosis is made. The diagnosis of “myositis” is challenging. There are a variety of non-inflammatory muscle diseases that present with similar symptoms, for example, hereditary muscle wasting diseases, metabolic disorders of the muscles, or drug side effects. Detailed expertise is needed to rule out other muscle diseases as the cause of the symptoms. Diagnostics includes a wide variety of examinations, in which specialists from several disciplines may be involved at the USZ, in particular from rheumatology, neurology, (neuro)pathology and dermatology.

Diagnostics always begins with a discussion of your medical history (anamnesis).

We ask you the following questions

  • What exactly are your symptoms?
  • Where are the complaints localized?
  • How pronounced are the symptoms?
  • When did you first notice the discomfort?
  • Do you have any known diseases? If yes: Which ones?
  • Are there any diseases in your family? If yes: Which ones?
  • Do you take medication regularly? If yes: Which ones and since when?

Diagnosis myositis

This is followed by a physical examination. We palpate the muscles and joints to see if there are any abnormalities. We also test the mobility and strength of the extremities to detect limitations. Further information is provided by a skin examination. We assess your gait pattern by having you walk a few steps in the room and check reflexes and touch sensation. In addition, we listen to the heart and lungs and also examine the abdomen and kidneys. In addition, we check the blood flow by palpating the pulses. Measurement of blood pressure and temperature may also be part of the examination.

A blood test is also part of the routine. In doing so, we determine, among other things:

  • the inflammatory signs such as C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and leukocyte count,
  • certain muscle values: Creatine kinase – an enzyme that destroyed muscle fibers release – is important. The value allows conclusions to be drawn about the activity of the disease. The values for lactate dehydrogenase, aspartate aminotransferase (AST) or alanine aminotransferase (ALT) as well as myoglobin and troponin also help us.
  • Certain autoantibodies typical of myositis, for example, anti-Mi2, anti-Jo1 antibodies, anti-SRP, anti-c1NA antibodies.

This is usually followed by further examinations. The most important are:

  • Muscle strength and function test: Using internationally standardized and validated tests, we record limitations in muscle strength and endurance and can take this into account when making treatment decisions.
  • Magnetic resonance imaging (MRI = magnetic resonance imaging): Radiologists use strong magnetic waves and record the body “slice by slice”. This gives them detailed cross-sectional images of different regions of the body. Muscles can be easily visualized and changes such as inflammation or degradation can be detected.
  • Electromyography: An examination that uses a thin needle to determine the electrical activity of individual muscle groups.
  • Muscle biopsy: This examination may be necessary if the previously mentioned tests do not allow a clear diagnosis. Here, a small piece is taken from the muscle under local anesthesia. It is then examined for changes under the microscope in the pathology department. A previously performed MRI may reveal areas that are good candidates for muscle biopsy. Biopsy results can help distinguish inflammatory (autoimmune) from other causes. This information can be very important in making treatment decisions.

The final diagnosis “myositis” can be made from the synopsis of all examination results.

Myositis: prevention, early detection, prognosis

The causes and mode of development of muscle inflammation, which is not caused by pathogens, are still largely unknown. Therefore, there are no preventive measures.

Auch spezielle Früherkennungsmassnahmen gibt es bei der Myositis nicht. Therefore, the general advice is to always seek medical advice promptly if you are experiencing symptoms such as a severe feeling of illness, muscle pain, or muscle weakness.

Course and prognosis in myositis

Myositis is a chronic disease that usually progresses without treatment. However, muscle inflammation can often be stopped by timely and sufficient treatment. The course of myositis varies widely and depends on the type, severity of muscle damage and involvement of other organs.

Often, we can achieve significant improvement with medication in the majority of cases, sometimes even freedom from symptoms. Then those affected can live an almost normal everyday life and the quality of life is maintained.

Inclusion body myositis, on the other hand, has a less favorable prognosis. After five years, many sufferers are dependent on walking aids, and after about ten years on a wheelchair. It can bring many other problems, such as difficulty swallowing and shortness of breath.

Myositis: Treatment means slowing down muscle inflammation and restoring muscle strength

Due to the impairments that are often relevant to everyday life, the recording of all manifestations of the disease and the quality of life plays a major role. At the USZ, the primary therapy goal is therefore the creation of an individual treatment concept, which, in addition to drug therapy, also includes an adapted training plan. Advanced therapies allow most patients with myositis to reduce symptoms and improve quality of life. Because the diseases are so rare, no drugs have been approved for them, although their efficacy has been proven. We therefore use these drugs “off label” after obtaining the cost approval for the person concerned. This is what our expert knowledge of these rare diseases and detailed expertise and experience with these therapies enable us to do.