Muscle biopsy

Role

Muscle biopsy provides definitive histological diagnosis of inflammatory myopathies, distinguishing polymyositis, dermatomyositis, inclusion body myositis, and immune-mediated necrotising myopathy by their characteristic, distinct histological patterns
Performed when the clinical picture, CK elevation, and EMG findings support an inflammatory myopathy but definitive subtyping (with management and prognostic implications) requires tissue diagnosis

Each inflammatory myopathy subtype has a distinct immunopathological mechanism reflected in a characteristic biopsy pattern: dermatomyositis - perifascicular atrophy with a perivascular/perimysial inflammatory infiltrate (complement-mediated microvascular injury); polymyositis - endomysial CD8+ T cell infiltrate surrounding and invading non-necrotic fibres; immune-mediated necrotising myopathy - scattered necrotic fibres with minimal inflammatory infiltrate; inclusion body myositis - rimmed vacuoles and specific protein inclusions, alongside an inflammatory infiltrate

Biopsy site selection should target a clinically affected but not end-stage/severely wasted muscle (a muscle showing mild-moderate weakness) - a severely atrophied muscle may show only non-specific end-stage fibrofatty change rather than the diagnostic inflammatory pattern
Avoid biopsying a site very recently tested by EMG (needle-induced artefact can confound histological interpretation) - biopsy the contralateral or an adjacent, untested site instead

Confirmed subtype directly informs treatment approach and prognosis - e.g. inclusion body myositis is notably treatment-refractory to standard immunosuppression (unlike polymyositis/dermatomyositis, which generally respond to corticosteroids), a critical distinction biopsy can provide when the clinical picture is ambiguous

Dermatomyositis, polymyositis, inclusion body myositis (treatment-refractory), immune-mediated necrotising myopathy (including statin-associated), malignancy screening (particularly relevant once dermatomyositis is confirmed)

Recognising inclusion body myositis specifically via biopsy is clinically important given its poor response to immunosuppression - misclassifying it as polymyositis risks prolonged, ultimately ineffective immunosuppressive treatment with its attendant risks, without the expected benefit

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