Intraindividual histopathological comparison of embolized and nonembolized territories in recurrent chronic subdural hematoma after middle meningeal artery embolization: illustrative case.

backgroundMiddle meningeal artery (MMA) embolization is a minimally invasive treatment for chronic subdural hematoma (CSDH) that reduces recurrence by interrupting the vascular supply to the outer membrane; however, the mechanisms underlying recurrence remain incompletely understood. OBSERVATIONS: An 80-year-old man with recurrent CSDH after burr hole evacuation underwent MMA embolization. The anterior convexity branch, identified as the principal feeder, was successfully embolized using 20% n-butyl-2-cyanoacrylate (NBCA). The posterior convexity branch did not demonstrate membrane staining, and embolization was therefore concluded. Despite initial improvement, recurrence required craniotomy. Intraoperative indocyanine green angiography demonstrated no flow in the embolized territory, whereas preserved and newly developed blood flow was observed in the nonembolized territory. Histopathological examination revealed preserved vascular structures with abundant neovascularization in the nonembolized region. In contrast, the embolized region showed reticular NBCA casts within MMA branches and penetrating vessels, accompanied by endothelial desquamation, medial thinning, and mild lymphocytic infiltration. NBCA fragments were also identified within the outer membrane, associated with foreign body reactions. LESSONS: Recurrence may result from collateral flow from nonembolized MMA branches, even after technically successful embolization. Histological findings support NBCA penetration into the outer membrane and highlight the importance of tailored and potentially more comprehensive embolization strategies. https://thejns.org/doi/10.3171/CASE26423.