Diagnostic Criteria for Polymyositis
Use the 2017 EULAR/ACR classification criteria, which require a probability score of ≥55% (aggregate score ≥5.5 without biopsy or ≥6.7 with biopsy) for "probable" polymyositis, with ≥90% probability (score ≥7.5 without biopsy or ≥8.7 with biopsy) for "definite" polymyositis 1.
Scoring System
The EULAR/ACR criteria use a weighted point system where each clinical feature contributes to a total score:
Age of Onset
- Age 18-40 years: 1.3 points (1.5 with biopsy)
- Age ≥40 years: 2.1 points (2.2 with biopsy)
Muscle Weakness Patterns
- Symmetric proximal upper extremity weakness: 0.7 points
- Symmetric proximal lower extremity weakness: 0.8 points (0.5 with biopsy)
- Neck flexors weaker than extensors: 1.9 points (1.6 with biopsy)
- Proximal leg muscles weaker than distal: 0.9 points (1.2 with biopsy)
Laboratory Findings
- Elevated CK, LDH, AST, or ALT: 1.3 points (1.4 with biopsy)
- Anti-Jo-1 antibody positive: 3.9 points (3.8 with biopsy)
Other Clinical Features
- Dysphagia or esophageal dysmotility: 0.7 points (0.6 with biopsy)
Muscle Biopsy Features (when performed)
- Endomysial mononuclear cell infiltration surrounding (not invading) myofibers: 1.7 points
- Perimysial/perivascular mononuclear infiltration: 1.2 points
- Perifascicular atrophy: 1.9 points
- Rimmed vacuoles: 3.1 points
Critical Diagnostic Algorithm
Step 1: Calculate the aggregate score based on clinical features present. Patients must have no better explanation for their symptoms 1.
Step 2: Determine classification level:
- Score 5.3-5.4 (6.5-6.6 with biopsy) = "Possible IIM" (50-54% probability)
- Score 5.5-7.4 (6.7-8.6 with biopsy) = "Probable IIM" (55-89% probability)
- Score ≥7.5 (≥8.7 with biopsy) = "Definite IIM" (≥90% probability)
Step 3: Sub-classify using the classification tree. For polymyositis specifically, patients must:
- Meet IIM criteria (probability ≥55%)
- Lack dermatomyositis skin manifestations (heliotrope rash, Gottron's papules, Gottron's sign)
- Exclude inclusion body myositis (no finger flexor weakness with treatment failure, no rimmed vacuoles on biopsy)
- Exclude immune-mediated necrotizing myopathy (though this distinction requires additional testing not in the criteria)
Critical Caveats
Polymyositis is a diagnosis of exclusion and is actually quite rare 2. The muscle biopsy hallmark is endomysial CD8+ T cell infiltrate invading healthy muscle fibers expressing MHC-I antigen 2. Before diagnosing PM, you must rule out:
- Inclusion body myositis (most commonly misdiagnosed as PM)
- Immune-mediated necrotizing myopathy (IMNM, now recognized as separate entity but included in PM category in these criteria) 1, 3
- Muscular dystrophies with inflammation
- Toxic, endocrine, and metabolic myopathies 2
- Overlap myositis (at least one-third of PM patients have or will develop associated connective tissue disease) 4
Important limitation: The 2017 criteria only included anti-Jo-1 antibody testing due to insufficient data on other myositis-specific antibodies 1. In clinical practice, testing for additional myositis-specific autoantibodies, MRI imaging, and EMG findings should supplement these criteria, though they weren't included in the validation 3.
For patients with pathognomonic DM skin rashes, muscle biopsy is not needed for classification. For suspected PM without skin manifestations, muscle biopsy is strongly recommended 1.