Interaction between age and atrial fibrillation on ischemic stroke severity: a cross-sectional analysis

Background While atrial fibrillation (AF) and older age are established independent risk factors for greater ischemic stroke severity, their interactive effect remains poorly characterized. This study aimed to evaluate the interaction between age and AF status on stroke severity, as measured by the National Institutes of Health Stroke Scale (NIHSS). Methods We conducted a cross-sectional study using a prospectively collected institutional stroke registry comprising 5,044 patients with acute ischemic stroke from 2019–2023. The primary exposures were AF status and age, categorized as < 65 and ≥65 years, while stroke severity at admission measured by the NIHSS served as the outcome. NIHSS was modeled using a negative binomial distribution within PROC GLIMMIX with a logarithmic link function to account for overdispersion in the NIHSS scores. Model included age, AF, and their interaction term, along with relevant co-variates such as gender, race and premorbid mRS. Results AF was present in 17.3% of patients. Stroke severity was significantly greater in patients with AF across both age groups: IRR 1.88 (95% CI, 1.44–2.45; p < 0.0001) in patients < 65 years, and IRR 2.13 (95% CI, 1.89–2.39; p < 0.0001) in patients ≥65 years. The interaction between AF and age on stroke severity was not statistically significant ( p for interaction = 0.40). Conclusions AF is associated with greater stroke severity in both younger and older adults. There was no statistically significant interaction between age and AF status on NIHSS scores, indicating no evidence of effect modification by age within limits of our data. These findings underscore the need to consider stroke severity in risk stratification, especially in younger AF patients who often experience greater treatment burden.