Dermatomyositis

Dermatomyositis is an autoimmune inflammatory myositis.

Epidemiology

There is a recognised female predilection. It has a bimodal age of presentation depending on the variant:

  • juvenile dermatomyositis (JDM): affects children and tends to be more severe
  • adult dermatomyositis (ADM): typically affects adults around the age of 50

Clinical presentation

The classic presentation is proximal, symmetrical motor weakness with cutaneous changes. Dysphagia, myalgia, fevers, and loss of weight are other features 7.

Pathology

There is cell mediated injury targeted at striated muscle with resultant atrophy, oedema, coagulation necrosis, fibrosis and calcification.

Markers
  • elevated muscle enzymes (CK)
  • elevated muscle specific antibodies
    • anti-RNA
    • anti-Mi2
Associations

Radiographic features

Plain radiograph
  • typically shows dystrophic calcification in muscles and soft tissues (calcinosis universalis)
    • sheet-like although at least four patterns have described with childhood dermatomyositis 4
    • classically seen affecting the thigh regions
  • chest radiograph may show diaphragmatic elevation
  • acro-osteolysis
Fluoroscopy
Barium swallow
  • may show disordered peristalsis involving the upper oesophagus: portion supplied by skeletal muscle
MRI
  • T2: generally hyperintense signal throughout the affected muscles; calcific areas may be low signal; perimuscular oedema may additionally appear as high signal; signal intensity may return to normal after treatment 4

Differential diagnosis

General imaging differential considerations include:

Practical points

MRI T2 weighted sequences are useful to guide muscle biopsy: 

  • areas of oedema related to the active inflammatory process
  • avoid the nonspecific end-stage fatty atrophic muscle should be avoided 

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