Advances in JDM: Biomarker and MRI sensitivity, outcomes and steroid management in a Dutch national prospective cohort

Objectives JDM is a rare chronic inflammatory disorder of childhood. The aim of this prospective study was to describe the characteristics and outcomes of the Dutch JDM population. Methods Demographics, clinical features, diagnostic test results and treatment were prospectively evaluated at diagnosis and during follow-up visits in JDM patients diagnosed between 2007 and 2024 in the study-coordinating centre and between 2015 and 2024 in five other tertiary referral hospitals. Results A total of 83 patients were included (65% female). Median age at diagnosis was 6.0 years (IQR 4-9). Median follow-up was 3.2 years (IQR 1.4-5.8). The most common features at diagnosis were proximal muscle weakness (94.9%) and Gottron's papules/sign (79.7%). While CK was abnormal in 73.1%, IFN-related biomarkers Galectin-9 and CXCL10 were elevated in 98.3% and 93.3% at diagnosis, respectively. Whole-body MRI showed muscle oedema in 30/33 (90.9%) patients. Corticosteroids could be tapered to 0.2 mg/kg at 6 months in 41.5%. Of all patients, 27 (32.5%) experienced one or more flares and 24 (28.9%) had a refractory disease course. Calcinosis occurred in 12 patients (14.5%). At last follow-up, 28.9% of patients had clinically inactive disease without medication. No patient died. Conclusion This study demonstrates the superior diagnostic sensitivity of Galectin-9 and CXCL10 over CK, the high sensitivity of MRI and the frequent occurrence of refractory disease and flares despite treatment. Reliable predictors are needed to identify patients who can safely taper immunosuppressants versus those needing continued treatment, and who may benefit from targeted therapies initiated upfront.