Diagnostic Criteria for Dermatomyositis
The 2017 EULAR/ACR classification criteria use a weighted probability-based scoring system rather than traditional "major and minor" criteria, with a score ≥5.5 (without biopsy) or ≥6.7 (with biopsy) indicating probable idiopathic inflammatory myopathy. 1, 2
Understanding the Modern Classification System
The traditional Bohan and Peter criteria with "major and minor" categories are now obsolete. 1 The current standard uses a data-driven probability model that assigns specific weighted scores to clinical, laboratory, and histopathologic features. 2
Key Weighted Variables in the Scoring System
Cutaneous manifestations receive the highest weights in the probability score: 1, 2
- Heliotrope rash (violaceous periorbital erythema)
- Gottron papules (erythematous papules over extensor surfaces of joints)
- Gottron sign (erythematous macules over joints without papules)
- V-sign (erythema over anterior chest in V-distribution)
- Shawl sign (erythema over posterior shoulders and back)
Laboratory features contribute to the score: 2
- Anti-Jo-1 antibody (assigned 3.9 points)
- Elevated muscle enzymes including CK, aldolase, AST, ALT, LDH (assigned 1.3 points)
- Other myositis-specific autoantibodies
Muscle-related findings are scored: 1, 2
- Proximal muscle weakness (symmetric, upper and lower extremities)
- Neck flexor weakness greater than extensor weakness
- Dysphagia or esophageal dysmotility
Histopathologic features improve accuracy but are not mandatory: 1, 2
- Perifascicular atrophy
- Inflammation patterns
- Muscle fiber changes
Classification Thresholds
The probability-based model defines certainty levels: 1, 2
- Score ≥5.5 (without biopsy) or ≥6.7 (with biopsy) = "Probable IIM" (≥55% probability)
- Sensitivity: 87% / Specificity: 82% (without biopsy)
- Sensitivity: 93% / Specificity: 88% (with biopsy)
- Score ≥7.5 = "Definite IIM" (higher probability threshold)
Practical Application
A web-based calculator is available to compute the probability score based on patient data. 2 The criteria provide flexibility: higher probability cutoffs (≥90%) are recommended for clinical trials requiring high specificity, while lower cutoffs (≥55%) suit epidemiological studies requiring high sensitivity. 2
Critical Diagnostic Patterns
Concurrent heliotrope rash and Gottron papules have a 91% positive predictive value for pure dermatomyositis. 3 The presence of V-sign and/or shawl sign has a 100% positive predictive value for pure dermatomyositis. 3
Anti-Mi-2, anti-MJ, and anti-p155 autoantibodies are restricted to pure dermatomyositis (100% positive predictive value when present). 3
Performance and Validation
External validation demonstrated 100% sensitivity with no misclassification when sufficient data were available. 2 In the UK/Ireland Juvenile Dermatomyositis register, 92% of 332 cases could be classified with 100% sensitivity using the 55% probability cutoff. 2
The cross-validated area under the curve was 0.933 without biopsy and 0.942 with biopsy. 2
Important Caveats
Perifascicular atrophy is not specific for dermatomyositis—it occurs equally in overlap myositis with dermatomyositis features (30%) and pure dermatomyositis (17%). 3 The absolute specificity of the DM rash and perifascicular atrophy for pure dermatomyositis has been lost with modern understanding of overlap syndromes. 3
The criteria can be applied to patients with overlap diagnoses such as mixed connective tissue disease or systemic lupus erythematosus with myositis, since these patients were included among IIM cases in the derivation cohort. 1