Polymyositis-Dermatomyositis

Aqeel A. Chowdhry, MD

Tan-Lucien H. Mohammed, MD, FCCP

Key Facts

Terminology

Polymyositis: Autoimmune inflammatory myopathy (limb girdle & anterior neck muscles)
Dermatomyositis: Polymyositis + skin rash

Imaging Findings

Nonspecific, multiple patterns based on prevailing histology: NSIP, UIP, COP, DAD
Aspiration common and may alone result in pulmonary disease or be superimposed on other patterns
Hypoventilation also common and may result in atelectasis (discoid or long lines)
Distribution similar irrespective of pattern; primarily involves basilar lung
Dystrophic muscle calcification in polymyositis
Skin or subcutaneous calcifications in dermatomyositis
Whole-body turbo STIR helpful in demonstrating soft tissue inflammatory burden

Top Differential Diagnoses

Clinical Issues

No established association between interstitial lung disease and extent of muscle or skin findings
Polymyositis: Weakness is painless in 66%
Dermatomyositis: Violaceous heliotrope rash over anterior edge of upper eyelids

Frontal radiograph shows nonspecific interstitial pneumonitis from polymyositis. Note the diffuse peripheral interstitial thickening

Axial HRCT shows subpleural ground-glass opacities and reticular opacities

. Traction bronchiectasis is out of proportion to reticular opacities. Note the NSIP pattern in polymyositis-dermatomyositis.

TERMINOLOGY

Abbreviations and Synonyms

Polymyositis-dermatomyositis (PM-DM), dermatomyositis-polymyositis (DM-PM), dermato-polymyositis (DPM)
Nonspecific interstitial pneumonia (NSIP), cryptogenic organizing pneumonia (COP), usual interstitial pneumonia (UIP), diffuse alveolar damage (DAD)

Definitions

Polymyositis: Autoimmune inflammatory myopathy (limb girdle & anterior neck muscles)
Dermatomyositis: Polymyositis + skin rash

IMAGING FINDINGS

General Features

Best diagnostic clue
- Lung disease in patient with muscle weakness
  - Hint: Elevated diaphragms and subsegmental atelectasis

CT Findings

Nonspecific, multiple patterns based on prevailing histology: NSIP, UIP, COP, DAD
- Aspiration common and may alone result in pulmonary disease or be superimposed on other patterns
- Hypoventilation also common and may result in atelectasis (discoid or long lines)
Distribution similar irrespective of pattern; primarily involves basilar lung
NSIP
- Ground-glass opacities > reticular opacities in bronchovascular distribution
- Traction bronchiectasis out of proportion to reticular opacities
UIP
- Reticular opacities and honeycombing in subpleural distribution
- Traction bronchiectasis
COP
- Subpleural focal areas of consolidation
- Reverse halo sign
DAD
- Acute diffuse ground-glass opacities often with traction bronchiectasis
Treatment
- Ground-glass opacities, consolidation may resolve
Extrathoracic manifestations
- Skin or subcutaneous calcifications in dermatomyositis
- Dystrophic muscle calcification in polymyositis, uncommon
Other
- High incidence of malignancy, particularly bronchogenic carcinoma

Radiographic Findings

10% have normal chest radiographs
Lung volumes reduced
- Elevated hemidiaphragms due to respiratory muscle weakness, atelectasis
Lungs
- Nonspecific symmetric, basal reticular pattern of parenchymal involvement, may progress to honeycombing (less often)
- Aspiration
  - Variable appearance, aspiration segments, waxing and waning opacities eventually resulting in fibrosis
Soft tissue calcifications
- Often over bony prominences
- More common in younger patients

Fluoroscopic Findings

Esophagram
- Upper esophagus predominantly involved may result in reflux or aspiration
- Swallowing studies useful for diet
- Obstruction due to stricture or scarring, late

MR Findings

Imaging Recommendations

Best imaging tool
- CT: Affects patient prognosis by demonstrating extent of pulmonary involvement, chronicity of disease process, and response to treatment
- MR imaging useful to demonstrate areas of muscular involvement

DIFFERENTIAL DIAGNOSIS

Nonspecific Interstitial Pneumonitis