Etiology and genetic predisposition
Both nosologies belong to the group of idiopathic inflammatory myopathies. The exact causes of their development are not fully understood, but several groups of trigger factors are distinguished:
- Infectious agents: a role is hypothesized for viruses (Coxsackie B, picornaviruses including ECHO viruses and cardioviruses), as well as certain bacterial and parasitic infections.
- Drugs: myopathy can be triggered by D-penicillamine and zidovudine.
- Physical factors: the development of the disease is sometimes preceded by excessive physical exertion.
Genetics plays a significant role. Patients with dermatomyositis frequently carry HLA-B8 and DR3 antigens, while overlap syndromes (combinations with other connective tissue diseases) reveal HLA-B14 and B40. It has been proven that the presence of specific HLA antigens directly correlates with the production of myositis-specific autoantibodies.
Pathogenesis: autoimmune mechanisms
The pathogenesis is based on autoimmunity, likely caused by molecular mimicry—structural similarity between unidentified infectious agents and the body's own tissues. In 90% of patients, autoantibodies to cytoplasmic proteins and RNA are detected in the serum.
Key differences in damage mechanisms:
- Dermatomyositis: the main target is intrafasicular capillaries. Damage begins with the deposition of membrane attack complex components in small vessels (this occurs even before inflammation appears). The infiltrate accumulates in the perivascular space and perimysium, dominated by B lymphocytes and CD4+ T lymphocytes.
- Polymyositis: cell-mediated immunity plays a leading role. T-cell cytotoxicity develops against muscle cells that express autoantibodies along with class I HLA molecules. The inflammatory infiltrate is localized in the endomysium and consists predominantly of cytotoxic CD8+ T lymphocytes.
Pathological anatomy and morphogenesis
The macroscopic picture in both forms is identical: skeletal muscles become edematous and acquire a pale yellow hue. Due to the development of calcinosis, areas of stony consistency form in the tissues.
Microscopic changes have specific features:
- In dermatomyositis: infiltrates of plasma cells, B cells, and CD4+ T cells form around small vessels in the perimysium. The most characteristic histological sign is perifascicular atrophy of muscle fibers. Productive and productive-necrotic vasculitis of the dermis is observed in the skin. The final stage of skin changes resembles systemic sclerosis.
- In polymyositis: skeletal muscle pathology is significantly more pronounced and segmental in nature. Infiltrating cells (macrophages, histiocytes, CD8+ T cells, eosinophils) actively invade necrotic muscle fibers, causing necrotic and regenerative changes. Vascular involvement is not typical for this form.
Systemic manifestations and complications
In addition to the muscles, the heart, lungs, and joints are most frequently involved in the pathological process. The gastrointestinal tract and kidneys are affected much less frequently.
The severity of the condition and the risk of fatal outcome are due to the development of severe complications:
- Cardiovascular system: myocarditis, progressive cardiosclerosis.
- Respiratory system: respiratory failure (arising due to marked weakness of the respiratory muscles), secondary bronchopneumonia, development of interstitial pulmonary fibrosis. Toxic lung damage is also possible as a side effect of aggressive drug therapy.
Antibodies are easy to remember by their targets: in dermatomyositis, 2 systems are affected (skin and muscles) — anti-Mi2 antibodies are characteristic. In polymyositis, only one system is affected, but there are two types of antibodies — anti-Jo1 and anti-SRP.
Class and exam questions
What is the main difference between polymyositis and dermatomyositis?
In polymyositis, skin lesions are completely absent; however, skeletal muscle pathology is significantly more pronounced, and inflammation is localized predominantly in the endomysium.
What specific antibodies are detected in these diseases?
Myositis-specific antibodies are found in 40% of patients. Anti-Mi2 serves as a marker for dermatomyositis, while anti-Jo1 and anti-SRP serve as markers for polymyositis.
How do muscles change macroscopically?
They become edematous and acquire a pale yellow color. Due to the development of calcinosis, characteristic areas of stony consistency appear in the muscle tissue.
What is the most frequent cause of patient death?
A fatal outcome is usually associated with severe complications: respiratory failure, bronchopneumonia, myocarditis, or drug-induced toxic lung damage.
What else to study on this topic
- Full classification of idiopathic inflammatory myopathies (juvenile, paraneoplastic, inclusion body myositis).
- Overlap syndromes: combination of myositis with other diffuse connective tissue diseases.
- Mechanisms of molecular mimicry and the involvement of the idiotypic network in the development of autoimmunity.
- Morphology of productive-necrotic vasculitis of the dermis.
- Toxic lung injury secondary to pharmacotherapy.
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