RheumatologyTier 2Disease (DEADMAN)

Myopathies - polymyositis

Description

Polymyositis - an idiopathic inflammatory myopathy causing symmetric, progressive proximal muscle weakness from CD8+ T cell-mediated muscle fibre injury - distinguished from dermatomyositis by the absence of the characteristic skin findings

Less common than dermatomyositis; adult-onset, with a female predominance, though less pronounced than in some other autoimmune rheumatic diseases

CD8+ cytotoxic T cells directly invade and destroy muscle fibres expressing MHC class I (upregulated in affected muscle) - a fundamentally different immune mechanism from dermatomyositis, which is primarily complement-mediated microvascular injury - explaining why the two conditions, though grouped together clinically, have distinct underlying pathology

Symmetric, progressive proximal muscle weakness (difficulty rising from a chair, climbing stairs, raising arms overhead) developing over weeks-months, without the skin findings of dermatomyositis (no heliotrope rash, no Gottron's papules)
Markedly elevated creatine kinase; EMG shows a myopathic pattern (see EMG and nerve conduction studies); muscle biopsy shows endomysial CD8+ T cell infiltrate surrounding and invading non-necrotic muscle fibres - a specific histological pattern distinguishing it from dermatomyositis's perifascicular/perivascular pattern
True isolated polymyositis is now considered less common than previously thought - many historically labelled cases are reclassified on modern testing as immune-mediated necrotising myopathy or another specific inflammatory myopathy subtype, given evolving diagnostic criteria and myositis-specific antibody testing

High-dose corticosteroids first-line, with a steroid-sparing immunosuppressive agent (methotrexate, azathioprine, or mycophenolate) added given the need for prolonged treatment and to allow steroid tapering
IVIG or rituximab for refractory disease
Physiotherapy to maintain function and prevent disuse atrophy, alongside pharmacological treatment

Interstitial lung disease (a shared complication with dermatomyositis, particularly with specific myositis-specific antibodies), malignancy association (present but generally considered less strong than in dermatomyositis), dermatomyositis (the key differential/related condition), immune-mediated necrotising myopathy

The historical category of "polymyositis" has narrowed considerably as myositis-specific antibody testing and more refined histological criteria have reclassified many previously labelled cases into more specific inflammatory myopathy subtypes - a genuine, evolving diagnosis rather than a fixed, unchanging entity

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