What are the diagnostic criteria for polymyositis, including the Bohan‑Peter criteria and the required clinical, laboratory, electromyography, muscle biopsy, and autoantibody findings?

Polymyositis Diagnostic Criteria

The diagnosis of polymyositis requires four of the five Bohan-Peter criteria: (1) proximal muscle weakness, (2) elevated muscle enzyme levels, (3) myopathic pattern on EMG, (4) endomysial mononuclear inflammatory infiltrate on muscle biopsy, and (5) absence of characteristic skin findings (which would indicate dermatomyositis instead). 1

Bohan-Peter Criteria (Traditional Approach)

The classic diagnostic framework includes these five elements 1:

• Proximal muscle weakness – symmetric involvement of shoulder and pelvic girdle muscles developing over weeks to months 1
• Elevated muscle enzymes – creatine kinase (CK) typically elevated 10-50 fold, along with elevated lactate dehydrogenase, AST, ALT, and aldolase 2
• Myopathic EMG pattern – polyphasic motor unit action potentials of short duration and low amplitude, with increased insertional and spontaneous activity including fibrillation potentials, sharp waves, and repetitive discharges 1
• Muscle biopsy findings – endomysial mononuclear inflammatory infiltrate with CD8+ cytotoxic T cells invading non-necrotic muscle fibers 1
• Absence of dermatomyositis skin findings – no heliotrope rash, Gottron papules, or characteristic photosensitive eruptions 1

Critical Diagnostic Pitfalls

Polymyositis is significantly overdiagnosed, with more than half of suspected cases ultimately representing other conditions including inclusion body myositis, muscular dystrophies with inflammation, or unspecified myositis. 3 Several key considerations:

• Exclude inclusion body myositis (IBM) – patients over age 50 with finger flexor weakness, wrist flexor predominance over extensors, or quadriceps atrophy likely have IBM rather than polymyositis, even with inflammatory infiltrates on biopsy 1
• Rimmed vacuoles on biopsy indicate IBM, not polymyositis – this finding is pathognomonic for IBM and scores 3.1 points on the EULAR/ACR criteria 4, 5
• True polymyositis is rare – in one rigorous study, only 5% of patients initially diagnosed with polymyositis actually met strict diagnostic criteria at follow-up 3

2017 EULAR/ACR Classification Criteria (Modern Approach)

The 2017 EULAR/ACR criteria provide a more validated, weighted scoring system that outperforms traditional Bohan-Peter criteria and allows classification of "definite," "probable," and "possible" idiopathic inflammatory myopathy. 1

Scoring System

• Probability cutoff of ≥55% (score ≥5.5 without biopsy or ≥6.7 with biopsy) indicates "probable IIM" with sensitivity/specificity of 87%/82% without biopsy and 93%/88% with biopsy 1
• Each clinical, laboratory, and histopathologic feature receives a weighted score corresponding to probability of IIM 1
• Sub-classification into polymyositis, dermatomyositis, or IBM is performed using a classification tree after meeting the IIM threshold 1

Key Advantages Over Bohan-Peter Criteria

• Operationally defined elements reduce ambiguity in interpretation 1
• Validated methodology using 976 IIM patients and 624 mimicking conditions 1
• Better discrimination of IBM from polymyositis, reducing misclassification 1
• Incorporates modern findings including myositis-specific autoantibodies and MRI findings 6

Essential Laboratory Workup

• Serum CK level – typically elevated 10-50 fold in active polymyositis; minimal elevation suggests IBM or other diagnosis 2, 1
• Comprehensive metabolic panel – AST, ALT, and LDH are elevated due to muscle breakdown 2
• Autoantibody panel – myositis-specific antibodies (anti-Jo-1, anti-SRP, anti-Mi-2) help define clinical phenotypes and predict extramuscular manifestations, though their absence does not exclude polymyositis 6, 2
• Inflammatory markers – ESR and CRP may be elevated but are nonspecific 7

Electromyography Requirements

EMG demonstrates myopathic changes in approximately 45-75% of cases 7, 8:

• Myopathic motor unit potentials – polyphasic, short duration, low amplitude 1
• Increased spontaneous activity – fibrillation potentials and positive sharp waves indicating muscle fiber irritability 1
• Normal EMG does not exclude polymyositis – seen in 11% of confirmed cases 7
• Primary purpose – confirm myopathic (not neuropathic) process and guide muscle selection for biopsy 1

Muscle Biopsy Interpretation

Muscle biopsy remains the gold standard for confirming inflammatory myopathy and distinguishing polymyositis from mimicking conditions. 1

Polymyositis-Specific Findings

• Endomysial inflammatory infiltrate – predominantly CD8+ cytotoxic T cells invading non-necrotic muscle fibers expressing MHC class I 1, 4
• Absence of perifascicular atrophy – this pattern indicates dermatomyositis, not polymyositis 4
• No rimmed vacuoles – their presence mandates reclassification as IBM 4, 5
• Muscle fiber necrosis and regeneration – without prominent vacuolization 1

Biopsy Technique Optimization

• Target weak muscles identified by EMG abnormalities – biopsy the contralateral side of the muscle studied by EMG to avoid needle artifact 1
• Avoid end-stage atrophic muscles – these yield nondiagnostic findings 1
• Inflammatory infiltrates present in 91% of confirmed cases – absence does not exclude diagnosis if other criteria are met 7, 8

Differential Diagnosis Exclusions

Before diagnosing polymyositis, systematically exclude 1:

• Muscular dystrophies – limb-girdle dystrophy, dysferlinopathies; test dystrophin gene and look for dystrophin absence on biopsy 1
• Inclusion body myositis – age >50, finger flexor weakness, rimmed vacuoles, congophilic deposits 1
• Immune-mediated necrotizing myopathy – minimal inflammatory infiltrate, CK >10× normal, may be statin-induced with anti-HMGCR antibodies 1
• Mitochondrial myopathy – ragged red fibers on Gomori trichrome stain, cytochrome c oxidase deficiency 1
• Drug-induced myopathy – statins, colchicine; typically self-limited after drug discontinuation 1
• Endocrine myopathy – thyroid dysfunction, hyperparathyroidism 1

Diagnostic Algorithm

1. Establish proximal muscle weakness – symmetric shoulder and pelvic girdle involvement 1
2. Measure serum CK – expect 10-50 fold elevation; minimal elevation suggests alternative diagnosis 2, 1
3. Perform EMG – confirm myopathic pattern and identify biopsy site 1
4. Obtain muscle biopsy – look for endomysial CD8+ infiltrates invading non-necrotic fibers, exclude rimmed vacuoles 1, 4
5. Check autoantibody panel – myositis-specific antibodies support diagnosis and predict phenotype 6
6. Apply EULAR/ACR criteria – calculate weighted score; ≥5.5 (or ≥6.7 with biopsy) indicates probable IIM 1
7. Exclude mimicking conditions – particularly IBM in patients >50 years, muscular dystrophies, and drug-induced myopathy 1, 3

Common Diagnostic Errors

• Misclassifying IBM as polymyositis – always consider IBM in patients over 50, especially with distal weakness or finger flexor involvement 1, 3
• Diagnosing polymyositis without biopsy confirmation – clinical and laboratory findings alone are insufficient given the high rate of mimicking conditions 3
• Ignoring negative biopsy in appropriate clinical context – 17% of confirmed cases have normal muscle biopsy; diagnosis can still be made with 4/5 Bohan-Peter criteria 7, 2
• Failing to reassess diagnosis at follow-up – 25% of patients with "isolated unspecified myositis" subsequently develop connective tissue disease 3