Immune-mediated necrotizing myopathy diagnostic criteria

Diagnostic categories	Criteria
Clinical criteria	Inclusion criteria: Age >18 years Subacute or insidious onset Symmetric proximal muscle and neck flexor weakness > distal and neck extensor weakness Exclusion criteria: Clinical features of IBM Ocular weakness, isolated dysarthria, neck extensor > flexor weakness Toxic myopathy, active endocrinopathy, amyloidosis, family history of muscular dystrophy or proximal motor neuropathies (SMA)
Elevated CK	Generally >1000 international units/L
Laboratory criteria (1 of 3)	Positive EMG – Fibrillation potentials, positive sharp waves, or complex repetitive discharges; short-duration, small-amplitude, polyphasic MUAPs Muscle MRI – Increased signal (edema) within muscle on STIR images Myositis-specific antibodies detected in serum (eg, anti-HMGCR antibody, anti-SRP antibody)
Muscle biopsy	Prominent muscle fiber necrosis Sparse inflammatory infiltrate, no perimysial infiltrate MAC deposition on small vessels or pipestem capillaries Rare tubuloreticular inclusions in endothelial cells

CK: creatine kinase; EMG: electromyography; IBM: inclusion body myositis; MAC: membrane attack complex; MRI: magnetic resonance imaging; MUAPs: motor unit action potentials; SMA: spinal muscular atrophy; STIR: short tau inversion recovery.

Reference:

Allenbach Y, Mammen AL, Benveniste O, et al. 224th ENMC International Workshop: Clinico-sero-pathological classification of immune-mediated necrotizing myopathies Zandvoort, The Netherlands, 14-16 October 2016. Neuromuscul Disord 2018; 28:87.

Adapted from: Hoogendijk JE, Amato AA, Lecky BR, et al. 119th ENMC international workshop: trial design in adult idiopathic inflammatory myopathies, with the exception of inclusion body myositis, 10-12 October 2003, Naarden, The Netherlands. Neuromuscul Disord 2004; 14:337.

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Immune-mediated necrotizing myopathy diagnostic criteria