What are the types of stroke?
Strokes are usually classified by what mechanism caused the loss of the blood supply, either ischemic or hemorrhagic. A stroke may also be described by what part of the brain was affected (for example, a right temporal stroke) and what part of the body stopped working (stroke affecting the left arm).
An ischemic stroke is caused by an artery in the brain being obstructed or blocked, preventing oxygen-rich blood from being delivered to brain cells. The artery can be blocked in a couple of ways. In a thrombotic stroke, an artery can narrow over time because of cholesterol buildup, called plaque. If that plaque ruptures, a clot is formed at the site and prevents blood from passing to brain cells downstream, which are then deprived of oxygen.
In an embolic stroke, the artery is blocked because of debris or a clot that travels from the heart or another blood vessel. An embolus or embolism is a clot, a piece of fatty material or other object that travels within the bloodstream that lodges in a blood vessel to cause an obstruction.
Blood clots that embolize usually arise from the heart. The most common cause of these blood clots is a heart arrhythmia called atrial fibrillation, where the upper chambers of the heart, the atria, do not beat in an organized rhythm. Instead, the chaotic electrical rhythm causes the atria to jiggle like a bowl of Jell-O. While blood still flows to the ventricles (the heart's lower chambers) to be pumped to the body, some blood along the inner walls of the atrium can form small blood clots. If a clot breaks off, it can travel or embolize to the brain, where it can block blood flow to a part of the brain and cause a stroke.
The carotid arteries are two large blood vessels that provide the brain with blood supply. These arteries can narrow, or develop stenosis, with cholesterol plaque that may build up over time. The surface of the plaque is irregular and bits of debris can break off and embolize to the brain to block blood vessels downstream and deprive brain cells of oxygen-rich blood.
When a blood vessels leaks and spills blood into brain tissue, those brain cells stop working. The bleeding or hemorrhage is often due to poorly-controlled high blood pressure that weakens the wall of an artery over time. Blood may also leak from an aneurysm, a congenital weakness or ballooning of an artery wall or from an AVM (arteriovenous malformation), a congenital abnormality where an artery and vein connect incorrectly. The bleeding can form a hematoma that directly damages brain cells and may also cause swelling that puts further pressure on surrounding brain tissue.
Describing a stroke by anatomy and symptoms
There are four major arteries that supply the brain with blood.
- The right and left carotid artery are located in the front of the neck and their pulse can be felt with the fingers.
- The right and left vertebral arteries are encased in bone as they run through the vertebrae in the neck. As the two enter the brain, they join to form the basilar artery.
- The carotid arteries and the vertebrobasilar arteries join to form the Circle of Willis at the base of the brain and from this circle, arteries branch off to supply the brain with blood.
The left side of the brain controls the right side of the body and vice versa. Speech tends to be located in the dominant hemisphere, most often the left brain.
The anterior and middle cerebral arteries provide blood supply to the front two-thirds of the brain, including the frontal, parietal, and temporal lobes. These parts of the brain control voluntary body movement, sensation, speech and thought, personality, and behavior.
The vertebral and basilar arteries are considered the posterior circulation and supply the occipital lobe where vision is located, the cerebellum that controls coordination and balance, and the brainstem that is responsible for the unconscious brain functions that include blood pressure, breathing, and wakefulness.
Picture of brain anatomy.
Strokes may be described based upon the function of the body that is lost and by the area of the brain that is affected. Most commonly in strokes that involve the brain, the symptoms involve either the right or left side of the body. In strokes that affect the brainstem or the spinal cord, symptoms may present on both sides of the body.
Strokes may affect motor function or the ability for the body to move. Part of the body may be affected, like the face, a hand, or an arm. An entire side of the body may be affected (for example, the left part of the face, left arm, and left leg). Weakness on one side of the body is called hemiparesis (hemi= half + paresis=weak) and paralysis is hemiplegia (hemi=half +plegia=paralysis).
Similarly, sensory function -- the ability to feel -- can affect the face, hand, arm, trunk, or a combination of these.
Other symptoms like speech, vision, balance, and coordination help locate the part of the brain that has stopped working and helps the health care professional make the clinical diagnosis of stroke. This is an important concept since not all loss of neurologic function is due to stroke and if the anatomy and physiology do not match the loss of body function, other diagnoses may be considered that can affect both brain and body.
A lacunar stroke describes the blockage of a single tiny penetrating artery branch in the brain. The area of the involved brain is small but can still cause significant neurologic deficits, just like a stroke involving a larger blood vessel and more brain tissue. In some cases, however, the stroke is silent, meaning that no obvious body function is lost and an old lacunar stroke can be seen as an incidental finding on a CT or MRI scan of the head that might be done for other reasons. The term lacune means empty space and a tiny empty space of an old lacunar stroke can be seen on imaging where brain tissue has been lost.