Dermatomyositis and Polymyositis
(Etiology, Pathogenesis, Clinical Manifestations)
Definition
Both are idiopathic inflammatory myopathies, meaning chronic muscle inflammation of unknown cause.
Etiology
Common Causes/ Associations:
- Autoimmune origin (T-cell mediated in polymyositis; humoral in dermatomyositis).
- Viral infections (Coxsackie, HIV, HTLV- 1, hepatitis B/ C).
- Drugs (statins, hydralazine, penicillamine).
- Paraneoplastic syndromes – especially in dermatomyositis.
- Genetic predisposition (HLA associations).
Pathogenesis
Polymyositis
- T-cell–mediated cytotoxicity against muscle fibers:
- CD8+ T-cells invade the endomysium (between individual muscle fibers ).
- Direct attack → muscle fiber necrosis → weakness and inflammation.
Dermatomyositis
- Humoral (antibody + complement-mediated) immune response.
- Perimysial inflammation with B-cells and CD4+ T-cells.
- Capillaries damaged by complement → ischemia of muscle.
- Also targets skin capillaries → skin rash.
Clinical Manifestations
Skin Signs in Dermatomyositis (KEY!)
- Heliotrope rash: violet rash over eyelids.
- Gottron’s papules: scaly rash over knuckles.
- Shawl sign: rash on shoulders/ upper back.
- Photosensitivity: rashes worsen with sun exposure.
Diagnosis Support
- ↑ Creatine kinase (CK)
- ↑ Aldolase, AST, ALT
- Positive ANA, anti-Jo-1 antibody (esp. in polymyositis)
- EMG changes
- Muscle biopsy (definitive)
- MRI shows muscle inflammation