- CT-confirmed SAH with most recent ictus less than 24 hours ago Definition: subarachnoid hemorrhage is a bleeding pattern on computed tomography with hyperdensity in the basal cisterns and/or Sylvian or interhemipheric fissures or a intraparenchymal hyperdensity consistent with a hematoma from an anterior, a pericallosal, a posterior or a middle cerebral artery aneurysm.