SAH and ICH are two entirely different entities. In SAH, the bleed is arterial, transient and the diagnostics are to correlate the presenting symptoms of HA, hemiparesis, stupor, and/or coma with a pathology. Noncontrast CT, MRI, and LP are the diagnostic procedures for SAH. If a patient is even a candidate for an LP there is realistically no way an expanding hematoma with impending herniation is possible. To humor your point, true, an aneurysm can re-rupture. A CTA in this scenario would neither predict for, nor catch the event in any meaningful way greater than a repeat noncontrast CT brain. An ICH, on the otherhand, is diagnosed with noncontrast head CT and is prone to expansion. A repeat noncontrast CT brain at an interval to assess expansion is typically conducted, and if there is a focal neurologic exam change in the interim the repeat CT is expedited. A patient herniating in under 3 hours undoubtedly would have progressive exam changes. FWIW, other active arterial bleeds such as epidural hematoma (EDH) are constrained by fixed structures such as the cranial vault and the dura and do not expand at the same rate as an unsecured active aneurysm bleed. An aneurysm bleeds in to the subarachnoid space, stops bleeding, and is at high risk of rebleeding. If the initial bleed doesn't stop there is a large amount of SAH on the initial CT. If the patient rebleeds, there is a sudden deterioration and more than likely herniation is not the issue but instead hydrocephalus from obstructive intraventricular hemorrhage.