Strokes are classified based on the timing of the signs and symptoms, which vary widely depending on where and how badly the brain is damaged. TIAs often reoccur because blood flow to the same brain area is reduced and may eventually be followed by more serious strokes. In a progressive stroke, signs and symptoms develop gradually and get worse over time. If the signs and symptoms develop quickly and show very little or no improvement over time, a completed stroke has occurred. Most strokes result from atherosclerosis in arteries either within the brain or leading from the heart to the brain.
Atherosclerosis leads to ischemic strokes by making arteries narrow, rough, stiff and weak.
Narrowness reduces blood flow directly. Stiffness causes inadequate blood flow by preventing arteries from dilating when a region of the brain becomes more active and needs more oxygen or when blood pressure drops. Weakness in arteries can lead to reduced blood flow if the weak area bulges outward, forming an aneurysm . Though the region with the aneurysm is initially wider than normal, blood swirling in aneurysm can clot. As the clot enlarges, it can partially or completely block blood flow through the artery.