Normalized air/brain volume ratio (Air–Brain Index) as a postoperative marker of delayed chronic subdural hematoma after clipping of unruptured cerebral aneurysms

Background: Chronic subdural hematoma (CSDH) remains a delayed complication after aneurysm clipping. Quantitative evidence linking postoperative pneumocephalus to CSDH is limited. Objective: To evaluate the association of a normalized CT index—the Air–Brain Index (ABI)—and intracranial volume (ICV) with postoperative CSDH, with prespecified sex adjustment. Methods: Single‑center retrospective cohort of adults undergoing clipping. Day‑1 CT underwent standardized segmentation to derive ABI (air/brain) and ICV (air + brain). Multivariable logistic regression included age and sex; sex‑stratified analyses and ROC curves assessed performance. Results: Among 68 patients, 18 developed CSDH. Higher ABI was associated with CSDH in univariable analysis; however, after adjustment for age and sex, ABI was no longer significant, whereas older age and male sex remained independent predictors. Conclusions: Although ABI was not independently associated with CSDH after adjustment for age and sex, it demonstrated a significant univariable relationship and may serve as a descriptive postoperative marker of residual intracranial air burden for hypothesis-generating risk stratification.