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

Inclusion Criteria: * Pre-morbid Modified Rankin Score ≤3 * Confirmed diagnosis of subacute or chronic subdural hematoma * Completed informed consent * Meets criteria for Surgery or Observation Cohort Exclusion Criteria: * 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