The Combination of Atrial Fibrillation and Occlusion Site Predicts In Situ Atherosclerotic Thrombosis in Basilar Artery Occlusion

<b>Background/Objectives</b>: The accurate evaluation of stroke etiology in basilar artery occlusion (BAO) remains underexplored. This study aimed to explore a simple and practical method for predicting the in situ atherosclerotic thrombosis (ISAT) subtype of BAO. <b>Methods:</b> We retrospectively analyzed patients diagnosed with BAO at our institution between April 2015 and April 2025. ISAT was characterized by moderate-to-severe stenosis (>50%) at the occlusion site on angiography, while the cardioembolism (CE) subtype was defined as sudden-onset arterial occlusion with evidence of a cardiac source of embolism. The location of BAO was classified based on cerebral angiography findings as proximal, middle, or distal. Clinical and imaging characteristics were compared between CE and ISAT subtypes. <b>Results:</b> Among 33 patients, 8 (24%) were classified as having the ISAT subtype. Multivariable analyses revealed that the presence of atrial fibrillation (AF) (OR 0.03; 95% CI, 0.00-0.56) and a non-proximal occlusion site (i.e., middle or distal) (OR 0.02; 95% CI, 0.00-0.27) were independently associated with ISAT. Patients were stratified into four groups based on the presence or absence of proximal occlusion and AF. CE subtypes predominated in groups with either AF or no proximal occlusion (25/27, 93%), whereas ISAT was present in all patients with both proximal occlusion and absence of AF (6/6, 100%). The area under the curve for this classification was 0.955. The sensitivity, specificity, positive predictive value, and negative predictive value for diagnosing ISAT were 75%, 100%, 100%, and 93%, respectively. <b>Conclusions:</b> A simple classification based on the presence of proximal occlusion and AF status suggested the potential to facilitate preprocedural differentiation of ISAT subtype in BAO.