Atherosclerosis in arteries within the heart (coronary arteries) can also lead to an ischemic stroke by first reducing blood flow to the heart muscle. As the heart becomes weaker, blood flow to the brain decreases. If a heart attack occurs near the inner surface of the heart, it can cause a rough spot and clot formation inside the heart (mural thrombus). A heart weakened by atherosclerosis may also bulge outward (ventricular aneurysm), leading to clot formation. In either case, the clot from the heart may embolize into arteries leading to or through the brain. Ischemia then follows when the embolus blocks an artery. Sometimes many emboli develop, causing multiple strokes.
When atherosclerosis weakens an artery in the brain, the artery can burst, causing hemorrhaging within the brain. Congenital defects in brain arteries can also lead to hemorrhagic strokes. These types of strokes can develop in several ways. The area to which the artery was carrying blood becomes immediately ischemic. At the same time, the blood spurting from the artery can directly damage nerve cells. Indeed, hemoglobin leaking from dying red blood cells injures and kills neurons. Part of this injury is from free radicals formed in the presence of iron in the hemoglobin.
As the blood mass (hematoma) enlarges, it is the pressure on the surrounding neurons that injures and kills them. Also, the enlarging hematoma may push the brain toward one side of the skull. And as the brain shifts across the rough base of the skull, more injury occurs to brain cells and to nerves and vessels attached to the brain. Because the skull is a rigid container, further enlargement of the hematoma causes an increase in pressure throughout the skull, leading to more neuron injury and death. Inflammation develops in the injured areas, resulting in brain swelling, additional increases in intracranial pressure, and more neuron injury and death. As intracranial pressure rises, brain vessels become compressed, causing more ischemia throughout the brain. If the intracranial pressure becomes great enough, portions of the brain may be forced outward through skull openings, such as the foramen magnum at the base of the skull.
More on what causes strokes from Harold Adams, a professor of neurology at the University of Iowa:
The term stroke encompasses a number of blood vessel diseases of the brain. Approximately 20 percent of strokes are called hemorrhages, during which a blood vessel ruptures and bleeding occurs into or around the brain. The leading causes of hemorrhages are high blood pressure, disorders of bleeding (tendency to bleed) and abnormalities of the blood vessels, including blisters (aneurysms) or tangles of blood vessels (vascular malformation).
The other 80 percent of strokes are due to infarctions. In this situation, a blood vessel becomes blocked and blood can not reach an area of the brain. The area of the brain that does not receive adequate blood dies because of a lack of oxygen and other vital nutrients. Brain infarction is quite similar to an infarction in the heart, or heart attack. The most common cause of brain infarction is hardening of the arteries (atherosclerosis). However, a number of other conditions--including heart diseases, which produce clots that move to the brain (embolism), other diseases of blood vessels and disorders of blood that promote clotting--can also lead to brain infarction.
What happens to the body during a stroke? The brain is the most sophisticated computer ever created. Just as a computer can lose part of its programs due to a faulty chip or circuit, the brain too can lose functionality from stroke-induced damage. Because the brain controls the electronic circuits that make up the nervous system, damage to it presents problems such as loss of language, paralysis, loss of vision, poor balance and sensory loss. Clinical findings vary greatly depending on the location and extent of brain damage done by a stroke. A very severe stroke may even cause a loss of consciousness. As the name stroke implies, the event usually occurs suddenly. A patient can be literally struck down.