Non-O blood groups as independent predictors of chronic subdural hematoma recurrence in antithrombotic-naive patients: risk mitigation by prior aspirin use and new perspectives for stratification.

OBJECTIVE: Recurrence after surgical treatment of chronic subdural hematoma (CSDH) remains insufficiently understood. Microvascular pathology of the CSDH membrane is a suspected driver of this recurrence, supported by reduced recurrence following middle meningeal artery embolization. Given the known differences in clotting propensity among ABO blood groups and the frequent use of antiplatelet and/or anticoagulant therapy (AAT) in the CSDH population, this study aimed to investigate the impact of ABO blood groups on postoperative recurrence risk. METHODS: The authors conducted a retrospective analysis of patients with symptomatic CSDH who underwent surgery at their institution between June 2012 and December 2023. Prior AAT was discontinued at hospital admission and reinitiated 3 weeks postoperatively, if CSDH resolution was complete. Recurrence was defined as an episode of radiological hematoma progression, with or without the development of symptoms, that required reoperation. RESULTS: A total of 756 patients with surgically treated CSDH (mean age 75.1 [SD 11.9] years, female-to-male ratio 1:2.1) were included. In patients without prior AAT use (n = 360), univariate analysis showed that non-O blood groups had significantly higher recurrence rates than blood group O (19.4% vs 8.7%, p = 0.006). Multivariate analysis confirmed non-O (p = 0.002, adjusted odds ratio [aOR] 3.5), and particularly A (p < 0.001, aOR 5.1), blood groups were independent predictors of recurrence. Comparing patients with prior acetylsalicylic acid (aspirin) use (n = 190) with those without prior AAT use, univariate analysis revealed that prior aspirin therapy was associated with a significantly lower recurrence rate among non-O (6.9% vs 19.4%, p = 0.002), and particularly A (8% vs 21%, p = 0.011), blood groups. Multivariate analysis identified prior aspirin use in non-O (p = 0.004, aOR 0.284), and particularly A (p = 0.015, aOR 0.324), blood groups as an independent predictor of reduced recurrence. CONCLUSIONS: This large-scale study demonstrated that non-O blood groups, and particularly A blood groups, are independent predictors of increased CSDH recurrence in patients without prior AAT use. These findings support the development of a scoring system based on blood groups, particularly for patients with groups A and O, to improve recurrence risk stratification and to tailor treatment plans. In contrast, prior aspirin use in these groups was an independent predictor of markedly reduced recurrence rates, comparable to those of blood group O.