A blood clot or other tissue in the blood (such as fat or air) from a part of the body other than the brain can travel through blood vessels and become wedged in a smaller brain artery. This free-roaming clot or tissue is called an embolus (plural: emboli). Emboli often form in the heart. They also commonly form in the neck arteries or within the aorta. A stroke caused by an embolus is called an embolic stroke.

 

There are several conditions that increase a person's chances for developing an embolus. Some of the most common include atrial fibrillation, sick sinus syndrome, and other irregular heart rhythms. These conditions can cause poor blood flow, which allows harmful clots to form. Emboli are also more likely to form in people:

 

A blood clot can also form in one of the brain arteries (called cerebral arteries), but instead of breaking free remains attached to the artery wall until it grows large enough to block blood flow. This type of stroke is known as a thrombotic stroke.

 

Most of the time, a thrombus occurs within an area of the brain damaged by atherosclerosis. In atherosclerosis (also known as hardening of the arteries), deposits of plaque (a mixture of fatty substances, including cholesterol and other lipids) build up along the inner walls of large- and medium-sized arteries, causing thickening, hardening, and loss of elasticity of artery walls and decreased blood flow. This increases the risk that a thrombus will form, causing severe narrowing or complete blockage of the affected blood vessel.