Background/Purpose: Amyopathic dermatomyositis (ADM) is a subset of DM in which cutaneous findings occur without muscle disease for >6 months after diagnosis. However, there is limited data on the mean time to onset of muscle symptoms in classic DM (CDM). Differentiating ADM and CDM may affect treatment, screening for comorbidities including malignancy and interstitial lung disease, and clinical trial inclusion.

Methods: We performed a retrospective cohort study of adult DM patients at an academic tertiary referral center to determine the mean time to onset of muscle symptoms in CDM and characterize the presentation of patients who develop muscle symptoms >6 months after diagnosis. Patients were reviewed to confirm diagnostic and demographic details, including the timing of muscle symptom onset relative to first cutaneous or pulmonary symptom onset and the time from presentation to formal diagnosis.

Results: 223 patients with DM were identified, of which the majority had CDM (172; 77%). 154 CDM patients had a clearly identifiable date of symptom onset. Median time to muscle symptoms was 0 months, with 24 (15.6%) developing muscle symptoms >6 months after first symptom onset. The onset of muscle symptoms in the delayed group ranged from 6.4 months to 42.4 months (median 13.3 months). Those with late onset muscle symptoms did not differ demographically from typical CDM or ADM patients, but they experienced a higher rate of ILD (33%) than either typical CDM (29%) or ADM (18%) patients and lower rate of DM-related death (0%) than either typical CDM (11%) or ADM (5.8%) patients. Those with late onset muscle symptoms also experienced a longer mean delay from presentation to diagnosis (8.4 months) than either ADM (4.3 months) or typical CDM (1.9 months) patients (ANOVA p=0.024).

Conclusion: Our results suggest that even patients diagnosed with ADM should be monitored for muscle weakness and pain. This study sheds further light on a subpopulation of DM patients with potentially disparate outcomes from either CDM or ADM, which could affect counseling, treatment, and clinical trial inclusion.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/delayed-onset-myopathy-in-dermatomyositis-a-retrospective-cohort-study/