What are the European Community Study on Clinical and Epidemiological Assessment of Inclusion Body Myositis (ECMA) diagnostic guidelines for Inclusion Body Myositis (IBM)?

ECMA Diagnostic Guidelines for Inclusion Body Myositis

The question appears to reference "ECMA" (European Community Study on Clinical and Epidemiological Assessment) diagnostic guidelines for IBM, but the current gold standard classification system is the 2017 EULAR/ACR classification criteria, which supersedes older diagnostic frameworks including ECMA criteria. 1

Current Classification Framework: 2017 EULAR/ACR Criteria

For classifying Inclusion Body Myositis, you must first establish that the patient meets criteria for idiopathic inflammatory myopathy (IIM) using the EULAR/ACR probability-score model, then sub-classify to IBM using the classification tree. 1

Step 1: Establish IIM Classification

The EULAR/ACR criteria use a weighted scoring system where each clinical feature receives points that correspond to a probability of IIM 1:

• "Probable IIM": Score ≥5.5 without biopsy (or ≥6.7 with biopsy), corresponding to ≥55% probability 1
• "Definite IIM": Score ≥7.5 without biopsy (or ≥8.7 with biopsy), corresponding to ≥90% probability 1
• "Possible IIM": Score 5.3-5.5 without biopsy (or 6.5-6.7 with biopsy), corresponding to 50-55% probability 1

The scoring system includes weighted points for age of onset, muscle weakness patterns, laboratory findings, and histopathological features 1.

Step 2: Sub-classify to IBM

Once IIM criteria are met (≥55% probability), IBM is diagnosed using the classification tree if ONE of the following is present 1:

1. Finger flexor weakness AND lack of treatment response (no improvement with immunotherapy) 1

   OR

2. Muscle biopsy showing rimmed vacuoles 1

Key Clinical Features of IBM

Characteristic Presentation

• Insidious onset of slowly progressive muscle weakness, typically over months to years 2, 3
• Age at onset: Usually ≥40 years (mean 47 years in validation cohorts) 1, 2
• Pattern of weakness: Most commonly proximal and symmetrical (58%), but can be distal (6%) or asymmetrical (19%) 2
• Selective muscle involvement: Finger flexors and quadriceps are characteristically affected 3
• Depressed or absent tendon reflexes, especially in lower limbs 2

Atypical Presentations (14% of cases)

IBM can present atypically, which delays diagnosis by a median of 9 years 3:

• Dysphagia (50% of atypical presentations) 3
• Asymptomatic hyperCKemia (24%) 3
• Foot drop (12%) 3
• Proximal arm weakness, axial weakness, or facial diplegia (6%, 4%, and 4% respectively) 3

Laboratory Findings

• Serum creatine kinase: Normal in 11% or mildly elevated in 89% of cases 2
• Cytosolic nucleotidase-1A (cN1A) antibodies: Elevated in 56% when tested 3

Electromyography Findings

EMG shows mixed myogenic and neurogenic patterns that vary by disease stage 4:

• Early stage: May show neurogenic suggestive patterns even in IBM 4
• Late stage: Myogenic changes predominate in biceps and gastrocnemius with weakness 4
• Myopathic features present in approximately 80% of patients 2
• Abnormal MUP can occur even in muscles without clinical weakness 4

Histopathological Features

The definitive histopathological triad for IBM includes 2:

1. Rimmed vacuoles in muscle fibers (100% of cases) 2
2. Groups of atrophic fibers (92%) 2
3. Endomysial inflammatory infiltrates composed mainly of CD8+ T cells (89%) 2

Electron microscopy reveals characteristic intracytoplasmic filamentous inclusions (100% when performed) 2

Important Clinical Caveats

Treatment Response

IBM characteristically does NOT respond to immunosuppressive therapy 2, 3:

• Only 1 of 26 patients (4%) showed minimal improvement with immunotherapy in one cohort 3
• Lack of treatment response is actually a diagnostic criterion in the EULAR/ACR classification tree 1
• Immunosuppressive treatment failed to prevent disease progression in all but one patient (who had associated Sjögren's syndrome) 2

Disease Progression

• Progressive decline in strength with a summated score decline of 0.082/month 3
• Swallowing impairment develops in 86.5% when formally tested 3
• Mobility decline: 16% eventually require walking aids 3

Diagnostic Pitfalls

The older Bohan and Peter criteria frequently misclassify IBM patients as polymyositis 1, which is why the 2017 EULAR/ACR criteria were developed to better distinguish IBM from other IIM subgroups 1.

IBM should be suspected in any adult patient presenting with chronic polymyositis that is unresponsive to immunosuppressive therapy 2.