Embotrial-1 is an Italian multicenter prospective randomized clinical trial with open-label treatment and blinded outcome assessment (PROBE) to assess the superiority of MMA embolization compared to conservative treatment. The intervention group is MMA embolization and comparator control group is the conservative management with best medical treatment. Patients are randomized 1:1.

Patients randomized to the experimental arm will submit to cerebral angiography and bilateral MMA embolization of the cSHD, even if it is located to only one side. The ipsilateral MMA embolization of the cSDH is allowed when the endovascular procedure cannot be performed on both sides. Patients are preferably under monitored local anesthesia or alternatively under conscious sedation or general anesthesia. Briefly, through femoral or radial artery access a standard 5 French diagnostic catheter is placed in the proximal external carotid artery and a digital subtraction angiography (DSA) is performed. Then, a microcatheter is advanced selectively under roadmap guidance into the main trunk of MMA, and superselective angiography is performed to evaluate for potentially dangerous anastomoses between MMA and ophthalmic or petrous branches prior to embolization. After collateral vessels are excluded, the MMA is occluded with PVA particles or liquid embolizing materials . The successful embolization is defined as that both frontal and parietal branch of MMA are selectively occluded and for this purpose, the microcatheter should be placed as distally as possible. It is recommended to ensure embolic materials penetrating into the capillary network of the dura to be permanently blocked. Patients randomized to the conservative arm will be clinmically monitored and eventually receive a best supportive care according to the local practice.

The trial is powered to assess superiority. When assuming the event rate of 1% in intervention group and 10% event rate in the control group, with a power of 85% and two-sided alpha of 0.05, allowing for 10% drop-outs, the estimated sample size is 300 patients in total.

The safety and efficacy interim analysis will be planned when half of the target sample (150 patients) completed 6 months follow-up. The enrollment will be stopped if the futility assessment based on estimated conditional power will be <70%.

All patient data is registered in the electronic data capture software REDCap. This worldwide online system allows built-in logical checks and validations to promote data quality. All clinical data are entered via an encrypted connection, are anonymized, and fulfill the demands for data protection. All data entries and changes are logged in REDCap and meet the Good Clinical Practice (GCP) requirements for the use of the electronic case report form (eCRF) in medical trials. Trial coordinators, data managers and the investigators will be introduced to the platform and trained in data entry during the initial kick-of meeting prior to recruitment of the first patient. Trial staff will be provided with a personal ID.

Currently, 6 centers in Italy have been identified for the study. All the clinical investigators are board-certified and have renowned expertise in neurosurgery, neuroradiology and endovascular interventions.

Inclusion Criteria:

• No neurological deficits (Markwalder score ≤1);
• Unilateral or bilateral cSDH;
• Subdural Hematoma width ≤ 20 mm; midline shift ≤ 7mm;
• Independent functional status with mRS score ≤ 2 prior to symptom onset;
• Likely compliance of the participant in attending follow-up examination.

Exclusion Criteria:

• Radiological evidence of an acute or subacute subdural hematoma, subarachnoid hemorrhage, intracerebral hematoma or epidural hematoma;
• Contraindications to angiography (end stage chronic renal disease, any sign of anatomical variations that could make MMA embolization unsafe, pregnancy);
• Life expectancy < 1 year;
• Patients with any kind of ventricular derivation catheter.