Diagnosing muscle disease in a cohort of classic dermatomyositis patients seen at a rheumatologic dermatology outpatient clinic

Background: Existing criteria to improve the probability of capturing dermatomyositis (DM) include muscle biopsy but little is known about whether less invasive diagnostic procedures may be just as useful. Objective: We aimed to determine whether skin biopsy, electromyography, or magnetic resonance imaging of the involved muscle could be done in lieu of muscle biopsy. Methods: Two hundred and seventy-five patients were reviewed to investigate the presence of cutaneous and muscle disease, their timing in relation to diagnosis, and results of skin biopsies, muscle biopsies, magnetic resonance imaging, and electromyography. Results: Of the cases with findings consistent with DM on muscle biopsy, 65% were in agreement with diagnostic features on electromyography or magnetic resonance imaging. Results of skin and muscle biopsies supported DM in 67% of patients who underwent both procedures. Limitations: A limited number of patients had muscle biopsies. Conclusion: In the presence of DM-specific skin findings, less invasive procedures may be sufficient to diagnose DM and guide its management.