Introduction

The revised European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) 2017 criteria and European Neuromuscular Centre’s (ENMC) 2018 criteria have improved diagnostic accuracy for dermatomyositis (DM). However, electromyography (EMG) was not included in the diagnostic criteria. We evaluated the usefulness of EMG under both criteria in the classification of DM and its amyopathic subtypes for patients who had rash with or without muscle weakness.

Methods

In this retrospective study, 20 patients who were classified into DM and alternative diagnoses groups based on the Bohan and Peter (BP) criteria and histopathological findings of muscle and/or skin were included. We then compared the diagnostic accuracy to that of the EULAR/ACR and ENMC criteria. Differences in proportion were determined using Fisher’s exact test.

Results

Two patients who had weakness with elevated creatine kinase but no typical DM features on muscle biopsies were incorrectly excluded as alternative diagnoses using BP criteria. Both EMGs demonstrated presence of myopathic recruitment pattern and spontaneous activity in the proximal and distal muscles, consistent with the clinical phenotype. Two patients with a final diagnosis of systemic lupus erythematosus were incorrectly classified as DM by EULAR/ACR and ENMC criteria, but EMG demonstrated no evidence of muscle inflammation.

Conclusion

Electromyography is valuable in identifying mild myopathy among DM patients with subtle clinical muscle weakness, allowing better classification of DM subtypes. Absence of EMG signs indicating muscle involvement provides clues for alternative diagnosis. In addition, EMG aids in interpretation of inconclusive muscle biopsy results. Therefore, EMG should be performed for every suspected DM patient.

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