Etiology and Genetic Predisposition
Both conditions belong to the group of idiopathic inflammatory myopathies. The exact causes remain incompletely understood, but several trigger factors are identified:
- Infectious agents: Viral involvement is suspected (Coxsackie B, picornaviruses including ECHO viruses and cardioviruses), alongside certain bacterial and parasitic infections.
- Drugs: Penicillamine and zidovudine can trigger myopathy.
- Physical factors: Excessive physical exertion sometimes precedes disease onset.
Genetics plays a substantial role. Dermatomyositis patients frequently carry HLA-B8 and DR3 antigens, whereas overlap syndromes (coexistence with other connective tissue diseases) reveal HLA-B14 and B40. The presence of specific HLA antigens directly correlates with the production of myositis-specific autoantibodies.
Pathogenesis: Autoimmune Mechanisms
The pathogenesis is driven by autoimmunity, likely mediated by molecular mimicry—structural similarities between unidentified infectious agents and host tissues. Autoantibodies to cytoplasmic proteins and RNA are detected in the serum of 90% of patients.
Key differences in tissue damage mechanisms:
- Dermatomyositis: The primary target is the intrafascicular capillaries. Injury begins with the deposition of membrane attack complex components in small vessels (occurring 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 the leading role. T-cell cytotoxicity develops against muscle cells expressing autoantibodies along with class I HLA molecules. The inflammatory infiltrate is localized to the endomysium and consists primarily 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 calcinosis, areas of stone-like consistency form within the tissues.
Microscopic changes show distinct 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. The skin shows productive and productive-necrotic dermal vasculitis. End-stage skin changes resemble 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 necrosis and regeneration. Vascular involvement is not typical for this form.
Systemic Manifestations and Complications
Beyond the musculature, the heart, lungs, and joints are most frequently involved. The gastrointestinal tract and kidneys are affected much less frequently.
Disease severity and mortality risk are driven by severe complications:
- Cardiovascular system: Myocarditis, progressive cardiosclerosis.
- Respiratory system: Respiratory failure (arising from marked respiratory muscle weakness), secondary bronchopneumonia, and interstitial lung fibrosis. Drug-induced toxic lung injury is also possible as a side effect of aggressive pharmacotherapy.