There are two main types of stroke: ischemic and hemorrhagic.

Roughly 80 percent of strokes are ischemic or caused by a severe episode of cerebral ischemia, according to the National Institutes of Health (NIH). During cerebral ischemia, the brain is deprived of oxygen-rich blood. This is usually because a blood clot is blocking an artery leading to the brain. A blood clot that remains in the blood vessel where it originated is called a thrombus. When a thrombus develops in an artery of the brain, it can cause a thrombotic stroke. Alternatively, the blood clot may lodge in the artery after traveling through the bloodstream from another part of the body (an embolism). This results in an embolic stroke.

Embolic strokes are usually the result of blood pooling in the upper chambers of the heart (atria). They are more common in people who have conditions that cause abnormal heart rhythms (arrhythmias), such as atrial fibrillation. This pooling of blood in the heart elevates the risk of a blood clot forming in the atria and traveling through the aorta and up into the carotid arteries. Blood clots may also originate in lower-left chamber of the heart (left ventricle), particularly in patients with a weakened heart muscle.

A hemorrhagic stroke is caused by excessive bleeding (hemorrhaging) within or around the brain. Bleeding within the brain is known as a cerebral hemorrhage, which can be a complication of high blood pressure. Cerebral hemorrhage occurs when blood leaks from the small arteries in the brain and accumulates over the course of minutes or hours. Hemorrhagic strokes account for about 20 percent of all strokes, according to the NIH. About half of these are due to cerebral hemorrhage. Bleeding around the brain is known as a subarachnoid hemorrhage, which can be caused by a ruptured cerebral aneurysm, a head injury or other causes.

Another type of stroke is called a transient ischemic attack (TIA). These are also sometimes referred to as “mini-strokes” because symptoms are less severe than an acute ischemic stroke, and go away fairly quickly (within minutes, in most cases). However, TIAs may indicate a serious medical complication that may cause another, more serious stroke at some point in the future.

Finally, just as some people experience silent heart attacks with no symptoms, people may also experience a silent stroke. A silent stroke is a stroke in which brain damage occurs, but the person does not show any obvious symptoms. Roughly one-third of elderly people may have had a silent stroke, which often damages their cognitive abilities.

Studies have shown that people who experience silent strokes have twice the risk of developing dementia. In addition to the elderly, other people at higher risk of a silent stroke are those who smoke and those with diabetes and high blood pressure.