Hypertension amplifies the association between systemic immune-inflammation index and cerebral ischemia: a 10-year population-based cohort study

Background The Systemic Immune-Inflammation Index (SII), derived from platelet, neutrophil, and lymphocyte counts, reflects both immune activation and thrombotic potential. Although its prognostic role in cardiovascular outcomes is recognized, evidence linking SII to cerebral ischemia remains limited and inconsistent. Purpose This study investigated whether hypertension modifies the association between SII and long-term risk of cerebral ischemia. Methods A total of 9,704 adults without prior brain ischemia were enrolled and followed for 10 years. Incident cerebral ischemia was confirmed through clinical and imaging criteria. Multivariable regression models were adjusted for age, sex, smoking, diabetes, and LDL cholesterol. Results During follow-up, 157 participants developed cerebral ischemia. In the overall population, log(SII) was not significantly associated with risk (RR = 2.23; 95% CI: 0.96–5.20; p = 0.063). In contrast, among hypertensive individuals, higher log(SII) was independently associated with an increased risk (RR = 4.19; 95% CI: 1.32–13.35; p = 0.015), whereas no association was observed among non-hypertensive participants (RR = 0.97; 95% CI: 0.29–3.22; p = 0.959). A significant interaction between log(SII) and hypertension was found (RR = 1.36; 95% CI: 1.18–1.57; p < 0.001). Other hematological indices neutrophil lymphocyte ratio (NLR), platelets lymphocyte ratio (PLR), and neutrophil HDL ratio (NHR) showed no independent association. Conclusions Elevated SII predicts long-term cerebral ischemia only in hypertensive individuals, suggesting that hypertension enhances the vascular vulnerability associated with systemic inflammation. SII may serve as a simple and accessible marker to refine cerebrovascular risk stratification in hypertensive patients.