Dermatomyositis

Dermatomyositis is an auto immune 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.

Pathology

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

Markers

• elevlated muscle enzymes (CK)
• elevated muscle specific antibodies 
  • anti RNA
  • anti Mi2

Associations

• interstitial lung disease ² : typically gives a patchy and subpleural consolidation with parenchymal bands. 
• internal malignancy ¹ : can occur as part of a paraneoplastic syndrome (e.g lung cancer)

Radiographic features

Plain film 

• 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
• acroosteolysis

Fluoroscopy

Barium swallow

• may show disordered peristalsis involving the upper oesophagus : portion supplied by skeletal muscle

MRI 

T2 : generally hyper intense 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

• polymyositis : doesn't affect skin