Strokes can be divided into two types: ischemic and hemorrhagic. In ischemic strokes, damage is caused by too little blood in the brain; in hemorrhagic strokes, by too much blood in the skull.

Ischemic stroke
About 83% of all strokes are ischemic. Ischemic strokes are usually caused by atherosclerosis, when fatty deposits, made up mostly of cholesterol, form on the walls of the arteries. The arteries stiffen and become narrower. The resulting, severely reduced blood flow is called ischemia.

Arteries that have been narrowed by atherosclerosis can easily become blocked completely by blood clots, cutting off blood supply. Deprived of the blood that brings them oxygen and glucose (a sugar), brain cells become stressed or damaged. The amount of damage done to brain cells depends on how long they are deprived. If it's for only a brief time, they may recover. If they are deprived for a longer period of time (and this can mean just a few minutes) they may die, and some functions may be lost.

There are two types of ischemic stroke:

• Thrombotic stroke. A clot, or thrombus, forms in one of the arteries that supplies blood to the brain. This can occur in one of the arteries of the neck as well as in the brain itself.
• Embolic stroke. A thrombus forms in another part of the body, often in the heart, and travels through the arteries until it becomes stuck in the narrower arteries of the brain. A thrombus that breaks off and travels through the bloodstream is called an embolus.

Hemorrhagic Stroke
While hemorrhagic strokes occur less frequently than ischemic strokes, they have a much higher death rate. Hemorrhagic strokes occur when a blood vessel in the brain leaks or ruptures. Hemorrhagic strokes can result from a number of conditions that affect the blood vessels. Uncontrolled high blood pressure and the presence of an aneurysm are the most common causes.

There are two types of hemorrhagic stroke:

• Intracerebral hemorrhage. A blood vessel in the brain bursts, and the blood spills into the surrounding brain tissue. When blood comes into direct contact with brain tissue, it irritates it and can cause scarring. Brain cells that are not receiving blood from the damaged artery are damaged as well. Typically, this type of hemorrhagic stroke is caused by high blood pressure, which can cause arteries in the brain to become brittle.
• Subarachnoid hemorrhage. Bleeding occurs in a large artery on or near the surface of the brain and leaks into the space between the brain and the skull. Blood accumulates in this space and compresses the surrounding brain tissue. This type of hemorrhage often causes a sudden "thunderclap" headache. Usually, subarachnoid hemorrhage is caused by the rupture of a brain aneurysm: a bulge in an artery that may be genetic or develop with age. After the hemorrhage, blood vessels in the brain may vasospasm (widen and narrow erratically), further damaging the brain by limiting blood flow.

More on this topic

What Is Stroke? (VIDEO)
Your Brain Needs Oxygen
Clogged Carotids
Degree of Blockage
Types of Stroke
Risk Factors
Symptoms, Test & Diagnosis
Treating Stroke
Life After Stroke

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