Polymyositis is a rare systemic rheumatic disease characterized by a degenerative inflammatory process of the muscles.
Rheumatic diseases are those that affect the musculoskeletal system and generally are autoimmune, in which the body confuses its own tissue with is foreign ones, triggering an immunological reaction where the organism attacks its own structures. Although there is no reliable data on the prevalence of polymyositis, it can develop at any age. However, it is more common in people around 50 years old and in children between the ages of 5 and 15 years of age. It occurs more frequently in women than in men.
The exact cause of this disease is unknown. However, since it’s an autoimmune reaction, the probability of suffering from it increases in an individual that is genetically susceptible. What do we mean when we talk about a genetically susceptible individual? To understand this, we must first discuss the system of human leukocyte antigens (HLA). This is also known as the major histocompatibility complex (MHC) and refers to the genes that code these antigens.
The HLA system is basically a set of molecules that are found on the surface of practically all the human body’s cells. The function of this system is to discern between what is ‘self’ and what is foreign. Each individual presents a unique HLA constituted by the combination of a set of antigens. When we speak of individuals genetically susceptible, we refer to those whose HLA is formed by at least one subtype of antigen which predisposes the individual to suffer from autoimmune diseases. These subtypes are: DR3, DR52 and DR6.
This does not imply that individuals whose HLA is constituted by any of these antigenic subtypes will suffer an autoimmune disease. For this to happen, a triggering phenomenon must take place, such as a muscular inflammation due to a viral infection or an underlying cancer. In fact, there are studies that talk about similar pathologies in animals triggered by viruses. However, the exact type of trigger virus is not known.
The pathophysiology of polymyositis has common aspects to many other autoimmune disease, including muscular atrophy, where the loss of muscle tissue releases substances, triggering an inflammatory process whose severity is directly proportional to the seriousness of the disease. Muscular atrophy usually affects all the muscles of the body. However, the muscles of the extremities and face seem to be the least affected. If not treated properly, the involvement of the muscles of the esophagus, pharynx and / or heart can compromise the function of these organs, resulting in fatal consequences for the individual.
The symptoms of this disease can appear in a short period of time, but it can also appear gradually. The main symptom is joint and muscle pain, characteristic of almost any inflammatory disease. Muscle weakness is a direct consequence of polymyositis. It becomes more evident when the immune system has destroyed almost half of the individual’s muscle fibers, a symptom of advanced polymyositis. In the most severe of cases, the patient may lose the ability to walk and be forced to use mobility aids, especially if the neck muscles have been affected to such an extent that the patient is unable to hold their own head. Joint pain, known as polyarthralgia (when present in more than five joints) is another consequence of the inflammatory process that polymyositis causes. Generally, the individual is forced to take analgesics and anti-inflammatories to be able to enjoy a normal life.