Dermatomyositis

Dermatomyositis is an autoimmune inflammatory myositis, which like its closely-related condition polymyositis, carries an increased risk of malignancy.

Epidemiology

There is a recognized 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
Associations

Clinical presentation

The classic presentation is that of a symmetrical proximal myopathy with associated dermatological changes which includes a dusky-red rash over the face, arms, hands, legs and other features (e.g. Gottron papules). Dysphagia, myalgia, fever and weight loss are other features .

Complications
Malignancy

There is a sixfold increased risk of malignancy in dermatomyositis (cf. twofold in polymyositis) . Multiple risk factors for the development of malignancy have been identified :

Several factors decrease the risk of malignancy :

Pathology

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

Markers
  • elevated muscle enzymes (e.g. CK)
  • elevated muscle specific antibodies
    • anti-RNA
    • anti-Mi2
Subtypes

Radiographic features

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

Differential diagnosis

General imaging differential considerations include:

Practical points

MRI T2-weighted sequences are useful to guide muscle biopsy:

  • areas of edema related to the active inflammatory process
  • non-specific end-stage fatty atrophic muscle should be avoided 

Further imaging in the form of a contrast-enhanced CT of the chest, abdomen and pelvis may be undertaken to exclude an associated primary malignancy.