Polymyositis and dermatomyositis

What are they?

Within the defense mechanisms of the immune system we have some types of lymphocytes, B lymphocytes, which are transformed into plasma cells, the cells responsible for generating antibodies, compounds that detect parts of agents external to the body - antigens -, in order to be able to do so. identify potential threats and attack them, as they do with bacteria and viruses.

Under normal conditions, plasma cells only recognize foreign elements as foreign to the body and have mechanisms to also recognize cells and proteins of the body. Now, when for various reasons there is an alteration in this ability to recognize organic components as their own, plasma cells can generate antibodies against the body itself and cause various alterations depending on the type of tissue, cell or protein that they attack. When this happens, it is called an autoimmune disease.

muscles are not oblivious to these possible alterations and when the body creates antibodies that attack muscle fibers, which causes the death of muscle cells, we speak of inflammatory myopathies, among which the most common are polymyositis and dermatomyositis.

How are they produced?

In some cases, inflammatory myopathies have an undetermined genetic component. Likewise, the importance of certain previous viral infections that can alter the recognition of muscle tissues as their own by antibodies has been postulated.

Within the inflammatory myopathies, it is worth highlighting some specific entities, namely:

• polymyositis
• childhood dermatomyositis
• adult dermatomyositis
• inclusion body myositis
• myositis associated with neoplasms
• myositis associated with vasculitis
• myositis associated with collagen diseases

Polymyositis usually appears in young people, especially women, as well as adult and childhood dermatomyositis. Inclusion body myositis tends to affect young men more.

In inflammatory myopathies, cellular immunity mechanisms have been detected, that is, defense cells such as T8 lymphocytes, polymorphonuclear cells and macrophages, as well as antibodies, are characteristic of these diseases, anti-Mi- 2, anti-Jo-1 (especially in polymyositis) and anti-SRP antibodies, the presence of which usually reflects severe muscle and cardiac involvement.

Symptoms

The main symptoms of inflammatory myopathies derive from muscle involvement, from chronic inflammation that occurs in the muscles and leads to necrosis of muscle tissue.

Polymyositis represents approximately 35% of myopathies and is characterized by symmetric muscle involvement with proximal onset that develops over weeks or months. Patients complain of muscle weakness that prevents them from performing daily activities and only 20% of patients report muscle pain. It is usually associated with dysphagia that can lead to aspiration episodes, with consequent respiratory infections. At the pulmonary level, there can be so much involvement of the intercostal muscles, which makes breathing difficult, such as interstitial pneumonitis. The heart can also be affected with myocaridopathies, arrhythmias or pericarditis. The ocular musculature is rarely affected.

Dermatomyositis accounts for one third of patients with inflammatory myopathies. It is characterized by muscular involvement equal to that of polymyositis but with associated skin manifestations. In areas exposed to the sun, an erythema appears that is classically called heliotrope and it is characteristic that it occurs around the eyes. On the bony prominences, such as the knuckles, erythematous spots appear called Gottron's papules. The hands often have thick, cracked skin. These patients can present muscle pain with exertion, as well as muscle edema and atrophy in the long run. Calcifications often occur in the affected muscles. The organic manifestations of dermatomyositis are similar to those of polymyositis.

In inclusion body myositis, the onset of symptoms is more insidious, lasts for years, and the muscle involvement, unlike the previous ones, is both proximal and distal.

The neoplasms most frequently associated with inflammatory myopathies are those of the lung, ovary, breast, stomach, and myeloproliferative syndromes. Tumor-associated myopathies represent less than 10% of the total.

Inflammatory myopathies associated with collagen disease account for a fifth of the total, and the diseases most frequently associated with them are systemic sclerosis, rheumatoid arthritis, systemic lupus erythematosus, and mixed connective tissue disease.

Diagnosis

The diagnosis of inflammatory myopathies is suspected in any young patient who presents symptoms of generalized muscle weakness that impede activities of daily living without other apparent causes and with or without the presence of skin manifestations suggestive of dermatomyositis. The possible existence of a neoplasm, a vasculitis or a disease of the connective tissue should be investigated.

The blood test will show an elevation of muscle enzymes due to inflammation and destruction of muscle tissue. You will see an increase in creatine kinase (CK), lactate dehydrogenase (LDH) and aldolase. Common autoantibodies in inflammatory myopathies will be specifically searched for, such as those already mentioned anti Mi-2, anti Jo-1 or anti SRP.

An electromyogram will show muscle involvement without changes in innervation, with characteristic alterations.

The muscle biopsy of an affected muscle will allow to observe inflammatory cells as well as a process of destruction of muscle fibers. In inclusion body myositis, a characteristic material will be seen that will form corpuscles in the muscle tissue.

Treatment

Treatment will be based on the administration of corticosteroids at high doses and for months to reduce symptoms and prevent the appearance of flare-ups. In case of outbreaks, intravenous corticosteroids will be administered in large doses.

If the clinical picture does not improve within three months, immunosuppressants will be administered, especially azathioprine.

Hydroxychloroquine can improve skin involvement.

Precautionary measures

Inflammatory myositis are diseases of autoimmune origin, so there are no preventive measures to avoid them.