There are two major types of strokes:

• Ischemic stroke. An ischemic stroke is due to restriction of blood flow to the brain. It usually involves a blood clot or blockage in an artery (blood vessel that carries blood away from the heart) in the brain. The majority – almost 90 percent – of all strokes that occur in the United States are ischemic strokes, according to the American Heart Association. There are two kinds of ischemic strokes:
  
  
• Thrombotic strokes. Stroke due to blood vessel blockage that begins in the brain. A thrombotic stroke can occur when a blood clot (thrombus) forms in an artery in the brain, which can result from a hardening of the arteries (atherosclerosis). Most ischemic strokes are thrombotic.
  
  
• Embolic strokes. Stroke due to blood vessel blockage (embolism) that travels through the bloodstream to the brain from another part of the body. Embolic strokes are frequently the result of blood pooling in the upper chambers of the heart (the atria) in people who have abnormal heart rhythms (arrhythmias), such as atrial fibrillation. In addition, blood clots may form in the area due to a weakened heart muscle, or may travel from a vein to the heart (e.g., due to a hole in the wall between the right and left atria [atrial septal defect]). The swelling of blood flow in the heart raises the risk of a blood clot forming and of it traveling into the carotid arteries (the arteries that carry blood from the heart to the brain).
  
  
• Hemorrhagic stroke. A hemorrhagic stroke is caused by excessive bleeding (hemorrhaging) within or around the brain. In addition, this loss of blood supply from cells in the brain can prevent the cells from properly functioning. Blood that accumulates may pressure surrounding brain tissue, which can lead to further interference with brain activity. There are two kinds of hemorrhagic strokes:
  
  
• Cerebral hemorrhage. Bleeding within the brain. This may occur when a defective artery within the brain bursts, which is often a complication of high blood pressure. Blood that leaks from a blood vessel in the brain can accumulate over a matter of minutes or hours. Other causes of cerebral hemorrhage include cerebral aneurysms (blood-filled pouches that balloon out from weak spots in an arterial wall), arteriovenous malformations and brain tumors. Cerebral hemorrhage is the most common type of hemorrhagic stroke, accounting for just under 10 percent of all strokes in the United States, according to the American Heart Association.
  
  
• Subarachnoid hemorrhage. Bleeding around the brain. This occurs when a blood vessel on the brain’s surface ruptures and bleeds into the space between the brain and skull, but not into the brain itself. It may result from various conditions, including a ruptured cerebral aneurysm or a head injury.