Inflammatory Myopathy: Polymyositis and Dermatomyositis

Background/Epidemiology

  • Related disorders characterised by inflammation of the skeletal muscle +/- skin.
  • Both are very rare (2-10/1000000/year)- dermatomyositis slightly more common than polymyositis (although polymyositis is more frequently misdiagnosed)
  • Can be idiopathic or associated with other autoimmune diseases e.g. SLE, systemic sclerosis and Sjogren’s syndrome (particularly polymyositis)
  • Usually occurs between 40-60 years old
  • Important diagnoses as both can be associated with malignancy
    • Especially dermatomyositis (10-20%)
    • Often warrants further investigations
    • Common associated cancers include breast, lung, ovarian and gastric

Clinical Presentation

  • Gradual onset (weeks); symmetrical proximal muscle weakness (usually lower limbs > upper limbs)
    • Difficulty standing from a sitting position unaided, difficulty climbing stairs
    • Pain is relatively uncommon but some patients report aches/cramps
    • Difference between polymyositis and dermatomyositis: polymyositis weakness tends to be more gradual (months) compared to dermatomyositis (weeks)
  • In Dermatomyositis, skin lesions are characteristic
    • Gottron’s papules- scaly, erythematous/violaceous, psoriaform plaques occurring on the extensor surfaces of the PIP and DIP joints
    • Heliotrope rash (violaceous discolouration of the eyelid and periorbital oedema)
    • Rash also occurs on the upper back, chest and shoulders (cape/shawl distribution)
    • Other features include nail fold capillary changes (tortuous and enlarged)
    • The GI tract may also be involved (dysphagia) and interstitial lung disease may also be present
  • Systemic features such as fever, weight loss, anorexia etc can also be present

Investigations

  • FBC, CRP, U&Es and LFTs (particularly kidney function and inflammatory markers)
  • Other bloods include serology
    • Anti-Jo-1 (polymyositis > dermatomyositis; particularly found in patients with associated ILD)
    • Antinuclear antibody is more common in dermatomyositis but is also found in SLE
    • Anti-PM/Scl antibodies may also suggest polymyositis
    • Anti-SRP and Anti-Mi-1 may also be found and usually carry a poorer prognosis
  • Creatine Kinase
    • Although not specific, CK in both conditions is usually elevated way into the thousands (may also help distinguish between IBM and Polymyositis)
  • Electromyography can often be useful
  • However, muscle biopsy is the key investigation
  • Other investigations include screening for malignancy (CT chest/abdomen/pelvis, upper and lower GI endoscopy, mammography, etc

Management

  • Most patients will respond to steroids
    • Usually start high dose (possibly even high-dose IV methylprednisolone)
      • e.g. 1mg/kg per day as a single dose
    • Once CK has reduced, the dose may be reduced slowly and eventually, then need for any treatment can be reviewed
    • Make sure to cover risk of osteoporosis e.g. with calcium, vitamin D and weekly bisphosphonate
  • Immunosuppressive agents may be useful

Inclusion Body Myositis

  • Similarly, inflammatory myopathy of unknown cause.  It is similar to polymyopathy but with some differences:
    • Pathology: on muscle biopsy, there are characteristic inclusion bodies
    • Presentation: rarely associated with connective tissue disease (usually isolated); also much more gradual progression (often over 5-10 months); distal muscle involvement may also occur (particularly, for example, wrist flexion (>extension) and knee extension can be weaker than hip flexion), and dysphagia is more common
    • Investigation: CK only moderately raised (e.g. 1000)
    • Treatment: often refractory to treatment- steroids and immunosuppression can be tried, as can IVIg, although the effect is minimal
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