Myositis

Myositis (plural: myositides) is the subset of myopathy characterised by inflammation of skeletal muscle.

Pathology

Aetiology

Myositides can be generally categorised by aetiology as follows ¹:

• inflammatory myositis

  • idiopathic inflammatory myopathy ⁵

    • dermatomyositis

      • amyopathic dermatomyositis

    • anti-synthetase syndrome

    • immune-mediated necrotising myopathy

    • inclusion body myositis

    • (polymyositis)

  • overlap myositis (myositis associated with connective tissue diseases) ⁵

  • proliferative myositis

  • eosinophilic myositis

  • granulomatous myositis

    • sarcoid myositis

    • chronic graft versus host disease ²

• infectious myositis ³

  • bacterial myositis (pyomyositis)

  • viral myositis

    • influenza myositis

    • coxsackievirus B virus-related pleurodynia (Bornholm disease)

    • virus-associated rhabdomyolysis

  • parasitic myositis (a cause of eosinophilic myositis)

    • cysticercosis

    • trichinosis

  • fungal myositis (rare)

• drug-related myositis

  • statin-related myositis

  • immune checkpoint inhibitor-related myositis

• trauma or iatrogenic myositis

  • ​myositis ossificans

  • radiation-induced myositis ⁴

Radiographic features

MRI is the gold standard modality for non-invasive evaluation of myositis, which shows a classic oedema signal pattern (high T2WI and STIR signal) in affected muscles. Inflamed muscles demonstrate contrast enhancement.

If chronic, T1WI will show high muscle signal indicating fat replacement and atrophy.