Clinical Features and Outcomes of Pediatric Lyme Arthritis in Europe: A Large Swiss Cohort Study.

Background: Lyme arthritis (LA) in children typically manifests as monoarthritis of a large joint. Although the prognosis is generally good, persistent arthritis occurs in 10%-23% of patients despite antibiotic treatment. Most described pediatric LA cohorts are from the United States, with limited European data available. This study aimed to describe clinical features and outcomes of pediatric LA in Europe using a large Swiss cohort. Methods: Children diagnosed with LA at the University Children's Hospital Zurich, Switzerland between 1 January 2006 and 31 December 2020 were included. Data on clinical disease presentation, laboratory findings, and treatment courses were compared between outcome groups using descriptive statistics and multivariable logistic regression models. Results: A total of 107 LA patients were included. Persistent arthritis, defined as symptoms lasting ≥2 months after antibiotics, occurred in 26 patients (24.3%; 95% CI, 16.8%-33.7%). Characteristics associated with persistent arthritis included older age, symptoms >14 days before presentation, delayed LA-specific antibiotic treatment >30 days, absence of empirical antibiotics, and lower blood monocyte counts. Among patients with persistent arthritis, 10 (38.5%) recovered (9 after intra-articular corticosteroid injections), whereas 16 (61.5%) were eventually diagnosed with juvenile idiopathic arthritis (JIA). Conclusions: This study provides a comprehensive characterization of pediatric LA in the largest European cohort to date, complementing existing data from the United States. Early clinical and laboratory features did not reliably differentiate children who achieved resolution from those who developed persistent arthritis. The finding that a substantial proportion of children with persistent arthritis were later diagnosed with JIA highlights the urgent need to identify predictors of disease persistence and to optimize management strategies.