Clinical, histologic and prognostic features of clinically amyopathic dermatomyositis

To characterise clinical amyopathic dermatomyositis (CADM) from a clinical, histological, and prognostic perspective.

We retrospectively recorded data from our DM cohort. Patients were categorised into three groups: classic DM, hypomyopathic DM (HDM), characterised by normal muscle strength and evidence of muscle involvement in laboratory tests and/or instrumental examinations and CADM, featured by normal muscle strength and unremarkable findings in both laboratory tests and instrumental examinations. Available muscle biopsies from each group were also compared.

Our cohort included 63 DM (69.2%), 12 HDM (13.2%) and 16 CADM (17.6%) patients. Compared to DM, CADM patients were younger at onset and diagnosis (45.5±17 vs. 57±18, and 46±17 vs. 58±18 years, respectively; p<0.05). They were more likely to test positive for anti-MDA5 (37.5% vs. 4.8%) and anti- TIF1-γ (31.3% vs. 6.3%), had a higher incidence of arthritis (37.5% vs. 12.6%) and interstitial lung disease (ILD) (43.8% vs. 15.9%) (all comparisons with p<0.05). Muscle biopsies were available for 44 DM, 7 CADM, and 11 HDM patients, revealing similar sarcolemma MHC-I expression rates. Five-year survival rates were comparable across groups (DM: 74.6%, CADM: 75%, HDM: 83.3%). Cox analysis indicated the main mortality predictors in overall cohort were ILD (HR: 3.57, CI: 1.11-11.5) and cancer (HR: 3.67, CI: 1.17-11.5), not CADM (HR: 1.46, CI: 0.33-6.68).

CADM patients differ in disease onset, autoantibody profiles, joint and lung involvement. While laboratory and instrumental tests have not shown muscle involvement in CADM, many muscle biopsies have shown MHC-I overexpression.