Strokes can damage brain tissue in the outer part of the brain (the cortex) or deeper structures in the brain underneath the cortex. A stroke in a deep area of the brain (for example, a stroke in the thalamus, the basal ganglia or pons) is called a lacunar stroke. These deeper structures receive their blood flow through a unique set of arteries. Because of the characteristics of these arteries, lacunar strokes happen a little bit differently from other strokes.
A lacunar stroke occurs when one of the arteries that provide blood to the brain's deep structures is blocked. These arteries are small, and are uniquely vulnerable. Unlike most arteries, which gradually taper to a smaller size, the arteries of a lacunar stroke branch directly off of a large, high-pressure, heavily muscled main artery. High blood pressure (hypertension) can lead to lacunar strokes because it causes a pounding pulse. Since the arteries don't gradually taper down in their size, high blood pressure can directly damage these arteries. High blood pressure also can cause atherosclerosis, a condition in which fatty deposits (plaques) build up along the walls of blood vessels. When atherosclerosis is present, a clot can form inside of one of these small arteries, blocking blood flow in the artery.
Unlike strokes that damage the cortex, lacunar strokes are only rarely caused by a blood clot (also called a "thrombus") that forms elsewhere in the body, such as the neck or heart, and travels through the bloodstream to the brain. After a clot (or any debris) begins to travel through the bloodstream it is called an embolus. It is difficult for an embolus to make its way into the small arteries that can cause a lacunar stroke.
Many of the deep brain organs that can be injured by a lacunar stroke help to relay communication between the brainstem and the brain cortex, or help to coordinate complicated body movements. In a lacunar stroke, brain cells in a relatively small area (measuring from 3 millimeters to as much as 2 centimeters across) are damaged or killed by lack of oxygen. Such a small area of brain destruction is called a lacune. A lacunar stroke involves only a small area of the brain, but it can cause significant disability.
Lacunar strokes account for about 20 percent of all strokes in the United States.