Seasonal Variation Impacts Outcomes of Mechanical Thrombectomy in Ischemic Stroke

Introduction/Purpose Mechanical thrombectomy (MT) is the standard treatment for acute ischemic stroke due to large vessel occlusion, yet outcomes remain variable and may be influenced by environmental and seasonal factors. Prior studies have shown higher stroke incidence and mortality in colder months, but it is unclear whether seasonal variation also affects outcomes after MT, which this study aims to investigate. Materials/Methods This retrospective single‐center study was conducted using the prospective registry of stroke patients who underwent mechanical thrombectomy at the Stroke Center between January 2017 and January 2025. We analyzed the effect of season on the modified Rankin Scale (mRS) score of 0 to 2 at 90 days (mRS90 ≤2), 90‐day mortality, and NIHSS≤5 at discharge or on day 5. To account for the effect of treatment timing, we adjusted the model for onset‐to‐door time (defined as the interval from last known well or symptom onset to arrival at our center) and door‐to‐puncture time. Other covariates included in the adjusted model were age, sex, coronary artery disease, atrial fibrillation, smoking status, hypertension, diabetes mellitus, hyperlipidemia, and presenting NIHSS score. Two sensitivity analyses were conducted: first, by grouping months into warm (April to October) and cold (November to March) categories, and second, by restricting the cohort to patients with M1 and M2 occlusions, the most common anterior LVOs treated with mechanical thrombectomy. Results A total of 901 patients were included in the study. The median onset‐to‐door time was 320 minutes (IQR, 126–735) in winter, 206 minutes (144–787) in spring, 241 minutes (117–656) in summer, and 234 minutes (120–606) in autumn (p = 0.513). Patients treated in summer had significantly higher odds of achieving mRS90 ≤2 compared to those treated in winter (OR = 2.247, 95% CI: 1.199, 4.212). Patients who underwent mechanical thrombectomy in summer (OR = 0.432, 95% CI: 0.242, 0.772) and spring (OR = 0.404, 95% CI: 0.223, 0.731) had lower odds of death compared to winter. In sensitivity analysis, 90‐day mortality was lower in warm vs. cold months (OR = 0.52, 95% CI: 0.35, 0.77). After restricting the analysis to M1 and M2 occlusions, patients treated in spring (OR = 0.390, 95% CI: 0.191, 0.797) and summer (OR = 0.408, 95% CI: 0.167, 0.720) also had lower odds of death compared to winter. Discussion This study suggests that stroke patients who underwent mechanical thrombectomy in winter had worse mechanical thrombectomy outcomes. Further research is needed to explore the underlying causes of this association.