Life expectancy \<1 year

Unable to complete follow-up

Pregnant, lactating, or has a positive pregnancy test at time of admission

Diagnosed with acute SDH

Potentially dangerous anatomic variations leading to increased procedural risk or unsafe access for MMA embolization

Pre-randomized Markwalder Grading Scale score ≥ 3

Unmanaged, uncontrolled bleeding disorders/blood diathesis

Presumed septic embolus, or suspicion of microbial superinfection

Known active COVID-19 infection

CT or MRI evidence of intra-cranial tumor or mass lesion Contraindication to angiography

Participation in another clinical trial

Contraindicated for the use of Onyx™ LES

Cannot be taken off corticosteroids (intended to treat subacute or chronic SDH) for at least 90 days post-randomization

Embolization of the Middle Meningeal Artery With ONYX™ Liquid Embolic System for Subacute and Chronic Subdural Hematoma