Dermatomyositis is an idiopathic inflammatory myopathy, presumably autoimmune in aetiology, which carries an increased risk of malignancy. It is considered a distinct condition to anti-synthetase syndrome.

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 years

Associations

• interstitial lung disease ²: typically gives a patchy and subpleural consolidation with parenchymal bands

• malignancy ¹: can occur as part of a paraneoplastic syndrome (e.g. lung cancer)

Clinical presentation

The classic presentation is that of a myalgic symmetrical proximal myopathy with associated dermatological changes which includes a dusky-red rash over the face (e.g. heliotrope rash), arms, hands (e.g. Gottron papules), legs (e.g. holster sign) and other features (e.g. V sign and shawl sign) ¹⁰. Dysphagia, myalgia, fever and weight loss are other features ⁷.

Complications

Malignancy

There is a sixfold increased risk of malignancy in dermatomyositis (cf. twofold in polymyositis) ⁸. Multiple risk factors for the development of malignancy have been identified ⁸:

• >60 years old

• male

• dysphagia

• necrosis of the skin

• cutaneous vasculitis

• accelerated onset of disease

• increased creatine kinase (CK) levels

• increased ESR and C-reactive protein (CRP) levels

• certain antibodies in adult-onset disease: anti-TIF1γ, anti-NXP2 ¹¹

Pathology

There is cell-mediated injury targeted at striated muscle with resultant atrophy, oedema, coagulation necrosis, fibrosis and calcification. Additionally, it is thought that enhanced type 1 interferon signalling plays a critical role in its pathogenesis ¹².

Markers

• elevated muscle enzymes (e.g. CK)

• elevated myositis-specific antibodies

  • anti-Mi2

  • anti-MDA5

  • anti-TIF1γ

  • anti-NXP2

  • anti-SAE

Subtypes

• hypomyopathic dermatomyositis

• clinically amyopathic dermatomyositis (CADM) or amyopathic dermatomyositis

  • anti-melanoma differentiation-associated gene 5 (MDA5) antibody-positive clinically amyopathic dermatomyositis (CADM) 

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 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 i.e. the portion supplied by skeletal muscle

MRI

• T2: generally hyperintense signal throughout the affected muscles; calcific areas may be low signal; peri-muscular oedema may additionally appear as high signal; signal intensity may return to normal after treatment ⁴

Treatment and prognosis

Management of myositis is primarily with immunosuppression. Options include corticosteroids and steroid-sparing agents (e.g. azathioprine, mycophenolate, methotrexate, etc.) ⁹.

Differential diagnosis

General imaging differential considerations include:

• polymyositis: does not affect the skin

• anti-synthetase syndrome

• immune-mediated necrotising myopathy

Practical points

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

• areas of oedema 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.