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Causes
By Mayo Clinic staff
A stroke occurs when there's a problem with the amount of blood in your brain. The cause of the main type of stroke — ischemic stroke — is too little blood in the brain. The cause of the other type of stroke — hemorrhagic stroke — is too much blood within the skull.
Ischemic stroke
About 80 percent of strokes are ischemic strokes. They occur when the arteries to your brain are narrowed or blocked, causing severely reduced blood flow (ischemia). This deprives your brain cells of oxygen and nutrients, and cells may begin to die within minutes. The most common ischemic strokes are:
- Thrombotic stroke. This type of stroke occurs when a blood clot (thrombus) forms in one of the arteries that supply blood to your brain. A clot usually forms in areas damaged by atherosclerosis — a disease in which the arteries are clogged by fatty deposits (plaques). This process can occur within one of the two carotid (kuh-ROT-id) arteries of your neck that carry blood to your brain, as well as in other arteries of the neck or brain.
- Embolic stroke. An embolic stroke occurs when a blood clot or other particle forms in a blood vessel away from your brain — commonly in your heart — and is swept through your bloodstream to lodge in narrower brain arteries. This type of blood clot is called an embolus. It's often caused by irregular beating in the heart's two upper chambers (atrial fibrillation). This abnormal heart rhythm can lead to poor blood flow and the formation of a blood clot.
Hemorrhagic stroke
"Hemorrhage" is the medical word for bleeding. Hemorrhagic stroke occurs when a blood vessel in your brain leaks or ruptures. Hemorrhages can result from a number of conditions that affect your blood vessels, including uncontrolled high blood pressure (hypertension) and weak spots in your blood vessel walls (aneurysms). A less common cause of hemorrhage is the rupture of an arteriovenous malformation (AVM) — an abnormal tangle of thin-walled blood vessels, present at birth. There are two types of hemorrhagic stroke:
- Intracerebral hemorrhage. In this type of stroke, a blood vessel in the brain bursts and spills into the surrounding brain tissue, damaging cells. Brain cells beyond the leak are deprived of blood and are also damaged. High blood pressure is the most common cause of this type of hemorrhagic stroke. Over time, high blood pressure can cause small arteries inside your brain to become brittle and susceptible to cracking and rupture.
- Subarachnoid hemorrhage. In this type of stroke, bleeding starts in a large artery on or near the surface of the brain and spills into the space between the surfaces of your brain and your skull. This type of hemorrhage is often signaled by a sudden, severe "thunderclap" headache. This type of stroke is commonly caused by the rupture of an aneurysm, which can develop with age or be genetically inherited. After the hemorrhage, the blood vessels in your brain may widen and narrow erratically (vasospasm), causing brain cell damage by further limiting blood flow to parts of your brain.
Transient ischemic attack (TIA) and stroke
A transient ischemic attack (TIA, or ministroke) is a brief episode of symptoms similar to those you'd have in a stroke. The cause of a transient ischemic attack is a temporary decrease in blood supply to part of your brain. Most attacks last just a few minutes.
TIA has the same cause as an ischemic stroke. In ischemic strokes, which are the most common type of stroke, a clot blocks the blood supply to part of your brain. But in contrast to a stroke, which involves a more prolonged lack of blood supply and causes some permanent damage to your brain tissue, a TIA doesn't leave lasting effects to your brain. Still, if you've had a TIA, it means there's likely a blocked or narrowed artery leading to your brain, putting you at a greater risk of a full-blown stroke that could cause more permanent damage. If you're having a TIA, get emergency medical treatment and make sure your regular physician knows about it.
