Myopathy is a group of diseases that affect the muscles. Myopathy can have a genetic origin, affecting the production of proteins involved in the formation of muscles. Myopathy can take many forms.

One of the forms of myopathy is muscular dystrophy, characterized by progressive degeneration of muscle cells. There are different types of muscular dystrophy, the most common being Duchenne muscular dystrophy.

There are also myopathies caused by infection, injury or autoimmune disease. Those inflammatory myopathies are rare diseases that cause muscle pain and weakness and can progress to decreased muscle mass and severe disability. The most common are polymyosite dermatomyosite and the inclusion myositis. These three diseases affect the skeletal muscles (= muscles of voluntary movements).

Other myopathies can be caused by metabolic (glycogenosis, enzyme deficiency) or endocrine (dysfunction of the tyroid gland or adrenal gland) or be caused by the consumption of certain drugs (amphetamines, heroin) or drugs ( antipsychotics, statins, some antidepressants, colchicine).

Causes

The exact cause of inflammatory myopathies is unknown. According to research, they have an autoimmune component, which occurs when the immune system, which normally protects the body against infections, goes wrong and attacks the body’s cells, then destroys them. 

Inflammatory myopathies are not considered a genetic disease, but there is a hereditary component that predisposes some people to the development of autoimmune diseases.

Prevalence

Inflammatory myositis are rare diseases. The prevalence of dermatomyositis and polymyositis is estimated to be around 6 to 7 cases per 100 people. The prevalence of inclusion myositis would be 000 to 5 cases in 9 people.

Complications

Inflammation of the muscles caused by dermatomyositis can lead to:

• Difficulty swallowing. When the muscles of the esophagus are affected, difficulty swallowing (dysphagia) can cause weight loss and malnutrition.
• Aspiration pneumonia. Difficulty swallowing can contribute to the aspiration of food or fluids, including saliva, into the lungs, which can lead to pneumonia.
• Difficulty breathing. When myopathy affects the muscles of the chest, those affected may experience difficulty breathing or shortness of breath.
• Gastrointestinal problems, such as ulcers or perforations in the intestines causing bleeding.
• Skin complications, such as calcium deposits in muscle, skin or tissue called calcinosis as the disease progresses.
• A greater risk of infection especially in the respiratory and digestive system.

People with dermatomyositis are also at greater risk of developing:

• Raynaud’s disease.
• Other connective tissue diseases, such as lupus, rheumatoid arthritis, scleroderma, and Gougerot-Sjögren syndrome.
• Cardiovascular disease, for example myocarditis, heart failure, or cardiac arrhythmia
• Pulmonary diseases, for example interstitial lung disease.
• Cancer, and in particular cancer of the cervix, lung cancer, pancreatic cancer, breast cancer, ovarian cancer and cancer of the gastrointestinal tract.