The “Paradoxical Dilation” Sign as an Indicator for Overlooked Collateral Arterial Supply in Chronic Subdural Hematoma

INTRODUCTION: Middle meningeal artery embolization (MMAE) is a safe and effective established treatment for chronic subdural hematoma (CSDH). However, treatment failures and recurrences remain a concern, with underlying causes still unclear. Collateral arterial supply, while sporadically reported, have not yet been attached full importance. CASE REPORT: A 60-year-old male presenting with left CSDH exhibited a marked morphologic asymmetry of middle meningeal artery (MMA) on 7T time-of-flight MR angiography (TOF-MRA), with notably right MMA dilation. Digital subtraction angiography (DSA) confirmed this paradoxically dilated right MMA as the primary arterial supply to the hematoma membrane. Subsequent bilateral MMA embolization resulted in immediate symptom resolution and near-complete hematoma regression at 1-month follow-up. CONCLUSION: "Paradoxical dilation" sign might indicate overlooked collateral arterial supply in CSDH, necessitating systematic vascular evaluation to achieve the "hematoma membrane supply embolization" over unilateral MMA embolization. Noninvasive imaging such as TOF-MRA could facilitate preoperative detection of this collateral arterial supply.