Supplementary Material for: Febrile seizures at early childhood and subsequent fine motor skill delays at age six: Insights from a comprehensive population-based cohort study

Introduction: Febrile seizures, the most common childhood seizures, raise concerns about their impact on neurodevelopment due to their overlap with central nervous system development. This study examined the influence of febrile seizures on neurodevelopment. Methods: Data were analyzed from the Korean National Health Insurance System for children born between 2009 and 2011. The cohort included 10,442 exposed children with febrile seizures, compared to 291,724 unexposed children. Exposure was defined as visiting the emergency room or hospitalized with a main diagnosis of febrile seizures. Neurodevelopmental outcomes at age 6 were assessed across domains such as gross and fine motor skills, cognition, language, sociality, and self-care. Multivariable logistic regression was employed in the main analysis, controlling for multiple confounders. Results: Of 10,442 children with febrile seizures (53.5% male; median age 24.75 months [IQR, 14–32]), 332 (3.18%) demonstrated fine motor skills issues, compared to 7,375 out of 291,724 (2.53%) non-exposedchildren (absolute risk difference 6.5, 95% CI 3.4–9.6/1,000 person). Compared with the unexposed children, the children with febrile seizure observed increased the risk of fine motor skills deficits (aOR 1.18, 95% CI 1.05–1.33). These differences remained consistent even with partial adjustment for confounders and altering subject selection with a sophisticated statistical method. Risk was more pronounced in cases of single occurrences, subsequent non-febrile seizures, and those occurring in summer or winter. Conclusion: Delays in fine motor skill development were observed at six years of age in children with a history of febrile seizures.