Spatial and seasonal patterns of pediatric idiopathic nephrotic syndrome in France.

BACKGROUND AND HYPOTHESIS: Idiopathic nephrotic syndrome (INS) is the most frequent glomerular disease in children, yet robust population-based data remain limited, particularly at the national level. We hypothesized that nationwide health data could provide reliable estimates of INS incidence and help characterize its spatial and temporal patterns. METHODS: We conducted a nationwide cohort study using the French National Health Data System (SNDS), which covers nearly the entire population and captures hospital, emergency, and day-care activity. Children and adolescents < 20 years with an in-hospital diagnosis of nephrotic syndrome between 2013 and 2023 were identified and validated through oral corticosteroid (OCS) dispensing. A 7-year wash-out period excluded prevalent cases. We estimated national incidence, identified spatial clusters using SaTScan, and assessed seasonality using Seasonal-Trend Decomposition (STL). RESULTS: The analytic cohort included 4 475 incident INS cases (62% boys; median age, 6 years). External validation against the NEPHROVIR cohort showed 84% sensitivity with comparable age and sex distributions. The national mean annual incidence was 2.49 per 100 000 person-years (95% CI, 2.41-2.56), higher in boys (3.05) than in girls (1.93). Two significant high-risk clusters were identified: the Paris area (standardized incidence ratio [SIR], 1.59; P < 0.001) and south-western France (SIR, 1.34; P < 0.001). These clusters were consistent across sensitivity analyses. STL analysis revealed a reproducible bimodal seasonal pattern with peaks in winter and late spring. CONCLUSIONS: This nationwide study provides robust estimates of pediatric INS incidence in France and identifies geographic clustering and consistent seasonal patterns, suggesting a potential role for environmental and infectious factors.