Criteria
Formal diagnostic criteria for idiopathic inflammatory myopathy (IIM) do not exist and classification criteria are used for guidance instead.[1] The 2017 EULAR/ACR classification for adults and juvenile IIM and their subgroups can aid in identifying the major myositis subtypes.
The European League Against Rheumatism and American College of Rheumatology (EULAR/ACR) classification for adults and juvenile idiopathic inflammatory myopathies and their subgroups[3]
These classification criteria are used when no better explanation for the patient’s symptoms and signs exists.
Score points are allocated as follows for different variables, with or without muscle biopsy:
Age of onset
Age of onset of first symptom assumed to be related to the disease ≥18 years and <40 years
With muscle biopsy: 1.3
Without muscle biopsy: 1.5
Age of onset of first symptoms assumed to be related to the disease ≥40 years
With muscle biopsy: 2.1
Without muscle biopsy: 2.2
Muscle weakness
Objective symmetric weakness, usually progressive, of the proximal upper extremities
With muscle biopsy: 0.7
Without muscle biopsy: 0.7
Objective symmetric weakness, usually progressive, of the proximal lower extremities
With muscle biopsy: 0.8
Without muscle biopsy: 0.5
Neck flexors are relatively weaker than neck extensors
With muscle biopsy: 1.9
Without muscle biopsy: 1.6
In the legs, proximal muscles are relatively weaker than distal muscles
With muscle biopsy: 0.9
Without muscle biopsy: 1.2
Skin manifestations
Heliotrope rash
With muscle biopsy: 3.1
Without muscle biopsy: 3.2
Gottron’s papules
With muscle biopsy: 2.1
Without muscle biopsy: 2.7
Gottron’s sign
With muscle biopsy: 3.3
Without muscle biopsy: 3.7
Other clinical manifestations
Dysphagia or oesophageal dysmotility
With muscle biopsy: 0.7
Without muscle biopsy: 0.6
Laboratory measures
Anti-histidyl-tRNA synthetase (anti-Jo-1) autoantibody present
With muscle biopsy: 3.9
Without muscle biopsy: 3.8
Elevated serum levels of creatine kinase or lactate dehydrogenase or aspartate aminotransferase or alanine aminotransferase
With muscle biopsy: 1.3
Without muscle biopsy: 1.4
Additional points are allocated as follows if muscle biopsy features the presence of:
Endomysial infiltration of mononuclear cells surrounding, but not invading, myofibres: 1.7
Perimysial and/or perivascular infiltration of mononuclear cells: 1.2
Perifascicular atrophy: 1.9
Rimmed vacuoles: 3.1
Scoring
Probability of IIM including muscle biopsy = 1/[1+exponential (5.33–score)]
or
Probability of IIM without muscle biopsy = 1/[1+exponential (6.49–score)]
≥50% and <55% = possible IIM
≥55% and <90% = probable IIM
≥90% = definite IIM
An online scoring calculator is available:
Patients classified with IIM by the EULAR/ACR classification criteria can be further subclassified using a classification tree that includes:
Amyopathic dermatomyositis
Dermatomyositis
Inclusion body myositis
Immune-mediated necrotising myopathy
Juvenile dermatomyositis
Polymyositis
The European Neuromuscular Centre (ENMC) inclusion body myositis (IBM) research diagnostic criteria[90]
Clinico-pathologically defined IBM
Clinical and laboratory features:
Duration >12 months
Age of onset >45 years
Knee extension weakness/hip flexion weakness and /or
Finger flexion weakness/shoulder abduction weakness
Serum creatine kinase no greater than 15 × the upper limit of normal
Pathological features include all of the following:
Endomysial inflammatory infiltrate
Rimmed vacuoles
Protein accumulation* or 15-18 nm filaments
Clinically defined IBM
Clinical and laboratory features:
Duration >12 months
Age of onset >45 years
Knee extension weakness/hip flexion weakness and
Finger flexion weakness/shoulder abduction weakness
Serum creatine kinase no greater than 15 × the upper limit of normal
Pathological features include one or more, but not all of the following:
Endomysial inflammatory infiltrate
Up-regulation of MHC class I
Rimmed vacuoles
Protein accumulation* or 15-18 nm filaments
Probable IBM
Clinical and laboratory features:
Duration >12 months
Age of onset >45 years
Knee extension weakness/ hip flexion weakness or
Finger flexion weakness/shoulder abduction weakness
Serum creatine kinase no greater than 15 × the upper limit of normal
Pathological features include one or more, but not all of the following:
Endomysial inflammatory infiltrate
Up-regulation of MHC class I
Rimmed vacuoles
Protein accumulation* or 15-18 nm filaments
* Demonstration of amyloid or other protein accumulation by established methods (e.g., for amyloid Congo red, crystal violet, thioflavin T/S, for other proteins p62, SMI-31, TDP-43). Current evidence favors p62 in terms of sensitivity and specificity but the literature is limited and further work required.
The Myositis Association diagnostic criteria for immune-mediated necrotising myopathy[91]
A diagnosis of immune-mediated necrotising myopathy should be considered for patients presenting with no skin symptoms and the following criteria:
Symmetrical muscle weakness in the shoulders/upper arms or hips/upper legs and trunk
Elevation of serum levels of skeletal muscle-associated enzymes: CK, aldolase, lactate dehydrogenase (LD or LDH), transaminases (ALT/SGPT and AST/SGOT)
Muscle pain on grasping or spontaneous pain
The triad of muscle-related changes on EMG:
Short, small, low-amplitude polyphasic motor unit potentials
Fibrillation potentials, even at rest
Bizarre high-frequency repetitive discharges
Positive for any of the myositis-specific autoantibodies
Non-destructive arthritis or arthralgias
Signs of systemic inflammation
Fever
Elevated serum C-reactive protein (CRP) level, or
Accelerated erythrocyte sedimentation rate (ESR)
Muscle biopsy findings that include many necrotic muscle fibres as the predominant abnormal histological feature. Inflammatory cells are or only sparse perivascular, perimysial infiltrate is not evident. Membrane attack complex deposition on small blood vessels or pipestem capillaries on endomysium may be seen, but tubuloreticular inclusions in endothelial cells are uncommon or not evident.
Diagnostic criteria for anti-synthetase syndrome[92]
The presence of an anti-aminoacyl-tRNA synthetase antibody plus two major criteria or one major criteria and two minor criteria.
Major criteria:
Interstitial lung disease that is not explained by environmental, occupational, or drug exposures and not related to any other underlying disease
Polymyositis or dermatomyositis
Minor criteria:
Arthritis
Raynaud’s phenomenon
Mechanic’s hands
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