Probability Scoring for Inclusion Body Myositis Using 2017 EULAR/ACR Criteria
The 2017 EULAR/ACR criteria use a two-step probability-based scoring system: first, patients must achieve ≥55% probability of having idiopathic inflammatory myopathy (IIM) with a score ≥5.5 without biopsy or ≥6.7 with biopsy, then IBM is sub-classified using a classification tree if finger flexor weakness with lack of treatment response OR muscle biopsy showing rimmed vacuoles is present. 1, 2
Step 1: Initial IIM Classification Probability Thresholds
The EULAR/ACR criteria establish three probability categories for IIM classification before subtyping to IBM 1:
"Possible IIM": Probability ≥50% and <55%
- Score ≥5.3 without biopsy
- Score ≥6.5 with biopsy 1
"Probable IIM": Probability 55-90% (recommended minimum cutoff)
- Score ≥5.5 and <7.5 without biopsy
- Score ≥6.7 and <8.7 with biopsy 1
"Definite IIM": Probability ≥90%
- Score ≥7.5 without biopsy
- Score ≥8.7 with biopsy 1
Step 2: IBM Sub-Classification After Meeting IIM Threshold
Once a patient meets the ≥55% probability threshold for IIM, they can be sub-classified as IBM using the classification tree if one of the following is present 1, 2:
- Finger flexor weakness AND lack of response to immunosuppressive treatment, OR
- Muscle biopsy demonstrating rimmed vacuoles 1, 2
Performance Characteristics for IBM
The EULAR/ACR criteria demonstrate superior performance for IBM compared to older criteria 3:
- Sensitivity for IBM: 98% 3
- Specificity for IBM: >98% 3
- The criteria correctly identified IBM with 92% agreement with physician diagnosis at the 55% probability cutoff, increasing to 93% agreement at the 90% probability cutoff 1
Clinical Context and Pitfalls
Critical diagnostic consideration: The older Bohan and Peter criteria frequently misclassify IBM patients as polymyositis, which is why these probability-based EULAR/ACR criteria were developed 2. IBM should be suspected in any adult patient presenting with chronic polymyositis unresponsive to immunosuppressive therapy 2.
The typical IBM patient profile includes 4, 5:
- Age at onset usually ≥40 years (mean 47 years) 2
- Insidious onset with painless weakness 4
- Early and often asymmetrical weakness of quadriceps and finger flexors 5
- Normal or mildly elevated creatine kinase 4
Important caveat: Up to 14% of IBM patients may present atypically (dysphagia, asymptomatic hyperCKemia, foot drop), with median time from symptom onset to diagnosis of 9 years 6. Clinical examination is often the key to diagnosis, as any given biopsy may lack characteristic histopathological abnormalities 5, 7. Adherence to strict histopathological criteria will exclude large numbers of patients with IBM who may be at earlier disease stages 7.