Abstract
Objectives
Juvenile idiopathic arthritis (JIA) in infants is extremely rare, making diagnosis particularly challenging. This study examines the characteristics of JIA in infancy, including early symptoms, time to diagnosis, JIA categories, treatment approaches and clinical outcomes.

Methods
Infants diagnosed with JIA were included in the study if enrolled in the German National Pediatric Rheumatology Database (NPRD) between 2011 and 2020 and followed prospectively. NPRD data were retrospectively supplemented using a dedicated infant-onset JIA module. To analyse differences in disease presentation, the infant-onset cohort was matched with NPRD patients who developed JIA between the ages of >1 and <6 years (toddler-onset JIA).

Results
Ninety individuals (62% female) with infant-onset JIA were identified across 18 pediatric rheumatology centres in Germany, with disease onset at 9.5 ± 2.64 months. Compared with toddlers, infants were more frequently affected by systemic JIA. Time from symptom onset to first rheumatology consultation was significantly longer in infants than toddlers (3.1 vs 2.3 months, P = 0.025). At follow-up, disease-modifying anti-rheumatic drugs (DMARDs) were prescribed in 66% of patients with infant-onset JIA and 59% with toddler-onset JIA. Although both groups exhibited similar disease activity at enrolment, the infant-onset group had significantly higher disease activity at follow-up (cJADAS10: 3.0 vs 2.1; P = 0.034).

Conclusion
Very early-onset JIA is often diagnosed late, delaying appropriate care. Our study underscores the need for improved awareness and earlier recognition of non-infectious arthritis in infants, along with timely initiation of effective treatment to minimize potential long-term consequences.