Polymyositis and Dermatomyositis

Polymyositis: idiopathic inflammatory myopathy that causes symmetrical, proximal muscle weakness
Dermatomyositis: clinically similar to polymyositis but also has typical cutaneous manifestations

Aetiology

  • Idiopathic, autoimmune
  • Higher prevalence in females (2:1)
  • Peak incidence age 40-50

Pathophysiology

  • T cell mediated process against (unidentified) muscle antigens
  • CD8+ T cells and macrophages surround, invade and destroy healthy, non-nectrotic muscle fibres
  • An autoimmune response to nuclear and cytoplasmic autoantigens is detected in about 60-80% of patients with polymyositis and dermatomyositis

Clinical presentation

Symptoms

  • Symmetrical, proximal muscle weakness in the upper and lower extremities
    • Insidious onset, worsening over months
    • Often specific problems e.g. difficulty brushing hair, climbing stairs
  • Myalgia in 25-50% (usually mild)

Signs

  • Muscle wasting
  • Confrontational testing - direct testing of power
  • Isotonic testing - 30 second sit to stand

Skin involvement in dermatomyositis

notion image

Other organ involvement

  • Lung
    • Interstitial lung disease (10% of patients, especially those positive for anti-Jo-1 antibody)
    • Respiratory muscle weakness
  • Oesophageal - dysphagia (1/3 patients, poor prognostic sign)
  • Cardiac - myocarditis
  • Other - fever, weight loss, Raynauds phenomenon, inflammatory arthritis

Investigations

Bloods

  • Muscle enzymes e.g. creatine kinase (often raised 10x normal limit)
  • Inflammatory markers
  • Electrolytes, calcium, PTH, TSH to exclude other causes
  • Autoantibodies:
    • Non-specific - ANA, anti-RNP
    • Myositis specific - Anti-Jo-1, anti-SRP

Electromyography (EMG)

  • Abnormal in almost all patients
  • Various abnormalities depending on the stage of the disease

MRI

  • Used to localise the extent of muscle involvement
  • Show signal intensity abnormalities due to muscle inflammation, oedema, fibrosis and calcification

Muscle biopsy

  • Definitive test
  • Perivascular inflammation and muscle necrosis

Management

  1. Prednisolone - aim to reduce dose and eventually stop
  1. Alternative treatments if not responsive:
    1. Immunosuppression - azathioprine/methotrexate/ciclosporin
    2. IV immunoglobulin
    3. Biological therapy e.g. rituximab (B cell depleting therapy)

Complications

  • Associated increased risk of malignancy
    • 15% in dermatomyositis, 9% in polymyositis
    • Ovarian, breast, stomach, lung bladder and colon cancers most commonly
    • Greatest risk in males > 45 years