Polymyositis & Dermatomyositis

 

• Pathophysiology

  • Autoimmune diseases involving myositis (polymyositis) or myositis plus characteristic skin signs (dermatomyositis).
  • Polymyositis features T-cell-mediated muscle damage, while dermatomyositis features immune complex deposition in vessels.
  • Associated with underlying cancer in around 50% of patients, especially if adult-onset. Also associated with other inflammatory diseases ('overlap syndromes') e.g. RA, SLE, systemic sclerosis, sarcoidosis.

• Signs and symptoms

  Muscles and joints:

  • Weakness, especially proximal myopathy.
  • Polyarthritis

  Skin and hand signs in dermatomyositis:

  • Pink-purple ('heliotrope') rash around eyes ± periorbital oedema.
  • Gottron's papules: scaly red patched on knuckles.
  • Other rashes: 'V-sign' on neck and chest, 'shawl sign' on back of neck and shoulders.
  • Nail-fold capillary dilation.
  • Dermatomyositis can present with skin signs alone, while muscle symptoms may be sub-clinical.

  Organ involvement:

  • GI: dysphagia, reflux.
  • Lungs: ILD, aspiration pneumonia.
  • Cardiac and renal involvement is possible but rare.

• Investigations

  Bloods:

  • ↑WBC/ESR/CRP.
  • Muscle enzymes: ↑CK, ↑aldolase.
  • Auto antibodies: ANA (80% sensitive) and myositis-specific Abs such as anti-Jo-1 and anti-SRP.

  Other tests:

  • Electromyography (EMG) shows abnormal muscle activity.
  • Muscle and skin biopsy for definitive diagnosis.

• Management

  Myositis:

  • High dose PO steroids are first line.
  • Other immunomodulators such as methotrexate, tacrolimus, MMF, or azathioprine can be considered.
  • IVIg if refractory.

  Skin disease:

  • Topical steroids.
  • Encourage to use sunscreen all year.