Linklater H, Pipitone N, Rose MR, et al., Classifying idiopathic inflammatory myopathies: comparing the performance of six existing criteria. Clin Exp Rheumatol. 2013 Jun 14. [Epub ahead of print]
Diagnosis by a specialist is considered the gold standard for classifying inflammatory myopathies. But a comparison of different criteria finds that one proposed in 2003 by Dalakas comes close, say British rheumatologists.
At present, no single set of criteria has gained universal acceptance for classifying the idiopathic myositides (IIMs), polymyositis (PM), and dermatomyositis (DM), note the authors from the rheumatology department at Kings College Hospital in London. However, a comparison of six proposed classification systems among 52 patients with a specialist diagnosis of PM, DM, inclusion body myositis (IBM), and non-inflammatory myopathy, finds that the criteria proposed in 2003 by neuromuscular specialist Marinos C. Dalakas MD, then at the National Institutes of Health, shows substantial agreement with a specialist diagnosis – with sensitivity of 77% and specificity of 99%.
Specialists typically use a combination of clinical, laboratory and electromyography findings, with muscle biopsy as the definitive diagnostic test for myopathies. But biopsy is only one of the criteria set by Dr. Dalakas, who is now at the Imperial College of London. He proposes a definite diagnosis for PM that include:
• Myopathic muscle weakness (affecting proximal rather than distal muscles, with subacute onset)
• Myopathic electromyographic findings
• High muscle enzymes (up to 50 times normal)
• Muscle-biopsy findings including: primary inflammation, with the CD8/MHC-1 complex and no vacuoles
• Absence of rash or calcinosis.
The current study shows that the 2004 European Neuromuscular Centre criteria (ENMC) demonstrate only fair agreement with a specialist diagnosis, with sensitivity of 71% and specificity of 82%.
The 1975 Bohan and Peter PM/DM criteria demonstrate poor specificity of only 29%.