Beyond Middle Meningeal Artery: Paradoxical Predominance Sign Predicts Additional Collaterals Supplying Chronic Subdural Hematoma.

BACKGROUND AND PURPOSE: Middle meningeal artery embolization (MMAE) is an emerging treatment for chronic subdural hematoma (CSDH). However, hematoma recurrence remains a challenge, often due to collateral vascular supply from heterogenous sources. We aimed to explore the correlation between small MMA and collaterals, and identified a distinct angiographic phenomenon termed the "paradoxical predominance sign" where the contralateral MMA is larger than the ipsilateral MMA and hypothesized that this sign predicts the presence of collateral supply. MATERIALS AND METHODS: We performed a retrospective analysis of 125 patients with CSDH who underwent embolization at our center between October 2022 and August 2025. Patients were categorized based on the paradoxical predominance sign defined as a contralateral MMA diameter exceeding the ipsilateral MMA by ≥ 0.2 mm. We investigated the concordance between MMA diameter and the presence of contributing collaterals, comparing supply patterns between patients with typical ipsilateral predominance and those exhibiting the paradoxical sign. RESULTS: In the general cohort, a smaller ipsilateral MMA (≤ 1.384 mm) was predictive of additional collaterals (p < 0.001). The paradoxical predominance sign was identified in 16 patients (12.8%). Patients in this group exhibited a significantly higher prevalence of contributing collaterals compared to the ipsilateral predominance group (93.75% vs. 35.78%; p < 0.001) and were more likely to have multiple collateral sources (p < 0.001). Furthermore, within the ipsilateral predominance group, a diameter of ipsilateral MMA ≤ 1.462 mm indicated a significantly higher risk of collateral supply (53.70% vs. 18.18%; p < 0.001). Clinical outcomes, including recurrence and hematoma resorption, were comparable between groups, suggesting that successful embolization of identified collaterals neutralized the risk. CONCLUSIONS: The hematoma membrane arterial supply in CSDH is heterogeneous, with additional collaterals contributing to MMAE recurrence risk. An ipsilateral MMA diameter ≤1.384 mm correlates with more collaterals, supporting prior hypotheses but with unproven generalizability. The paradoxical predominance sign is a robust hemodynamic biomarker for identifying additional collaterals. Assessing relative MMA asymmetry instead of absolute diameters enables simple patient stratification, and recognition of this sign guides comprehensive hematoma membrane arterial embolization, thereby improving endovascular treatment efficacy for CSDH.