Juvenile dermatomyositis across diagnostic eras: treatment evolution and long-term outcomes in a 25-year cohort

Juvenile dermatomyositis (JDM) is a rare autoimmune myopathy with heterogeneous phenotypes and variable long-term outcomes. We aimed to summarize a 25-year single-center cohort and to evaluate clinical characteristics, investigations, autoantibody profiles, treatments and determinants of cumulative damage. We retrospectively included 104 patients fulfilling the 2017 EULAR/ACR criteria (2000-2025) with ≥6 months follow-up. Clinical features, complications, investigations (EMG, MRI, biopsy), treatments and myositis-specific/associated antibodies were recorded. Diagnostic eras were defined as 2000-2007, 2008-2014 and 2015-2025. Damage was assessed using standardized and normalized MDI extent/extended scores and MyoDAM severity. The cohort showed female predominance (66.3%). Mean age at onset and diagnosis were 7.7 ± 4.1 and 8.5 ± 4.2 years; median follow-up was 52.6 months. Skin involvement (91.3%) and proximal muscle weakness (79.8%) were the most frequent presenting features. Autoantibody frequencies were comparable to previous reports, although phenotype associations differed. Calcinosis increased from 13.5% at diagnosis to 40.4% overall; skin ulceration from 3.8% to 12.5%. Nailfold capillaroscopy abnormalities (57.5%) were associated with calcinosis (p = 0.031). Calcinosis frequency did not differ across eras (p = 0.535), and time-to-calcinosis curves were similar (log-rank p = 0.235). In multivariable analyses, diagnostic delay ≥6 months, chronic active course, dysphagia and cutaneous ulceration were independently associated with higher damage (adjusted R²: 0.416 for MDI extent; 0.403 for MDI extended; 0.449 for MyoDAM severity). Cumulative damage was primarily driven by delayed diagnosis, persistent disease course and severe vasculopathic/systemic features, underscoring the importance of early recognition and treat-to-target escalation to prevent long-term damage.