Clinical meaning
Stroke is classified as ischemic (87%) or hemorrhagic (13%). Ischemic stroke results from thrombotic occlusion of a cerebral artery (large vessel atherosclerosis or small vessel lacunar infarction) or embolic occlusion (cardiogenic emboli from atrial fibrillation, valvular disease, or paradoxical embolism through patent foramen ovale). Within seconds of occlusion, neurons in the ischemic core lose ATP and undergo irreversible injury through glutamate-mediated excitotoxicity, calcium influx, free radical production, and membrane failure. Surrounding the core is the penumbra—tissue with compromised but viable perfusion sustained by collateral flow. The penumbra is the therapeutic target: reperfusion within the time window can salvage this tissue. Time is brain: approximately 1.9 million neurons die per minute in untreated large vessel occlusion. Hemorrhagic stroke occurs from intracerebral hemorrhage (ICH, most commonly from hypertensive arteriolar damage in basal ganglia, thalamus, pons, or cerebellum) or subarachnoid hemorrhage (SAH, from ruptured berry aneurysm at the Circle of Willis). Transient ischemic attack (TIA) produces temporary neurological deficit lasting <24 hours (typically <1 hour) without permanent infarction on imaging.