The Comprehensive Stroke Center at University Hospital
Types of Stroke
There are two main types of stroke — ischemic and hemorrhagic. Ischemic stroke is more common and occurs when blood flow to a part or parts of the brain is stopped by a blockage in a vessel. Hemorrhagic stroke is more deadly and occurs when a weakened vessel tears or ruptures, diverting blood flow from its normal course and instead leaking or spilling it into or around the brain itself.
|(left) Illustration of Ischemic Stroke showing blockage of one of the brain's arteries.(right) CT scan of Hemorrhagic Stroke. Arrow points to the area of blood in the brain.|
Treatment for each type is significantly different. In fact, treating an ischemic stroke as though it were hemorrhagic or vice versa could have life-threatening consequences. Therefore, a reliable determination (diagnosis) of which type has occurred is critical before treatment can begin.
There is a third type referred to as transient ischemic attack (TIA) or “mini-stroke.” While they are not true strokes because the symptoms are temporary, TIAs are usually a warning sign of a stroke to come. Heeding the warning signs of TIAs and treating the underlying risk factors that trigger them can prevent many strokes.
Transient Ischemic Attack (TIA)
A TIA is caused by a brief pause in blood flow to part of the brain — the result of a temporary or partial blockage. The symptoms of a TIA resemble those of a stroke but they do not last as long. Most symptoms disappear within an hour, although some may persist for up to 24 hours. Usually, no permanent brain damage occurs as a result of a TIA. According to the National Stroke Association, approximately 5 million Americans have experienced at least one TIA.
Patients suffering a TIA may describe a “veil” or “window shade” partly covering the vision of one eye that clears up spontaneously after several minutes. This represents the temporary blockage (occlusion) of the retinal artery to the eye. There may also be dizziness, imbalance, loss of coordination, confusion, difficulty speaking or understanding, and generalized weakness.
There is no way to differentiate the temporary symptoms of a TIA from those of an acute stroke. All patients need medical evaluation urgently. About one-third of those who have a TIA eventually will have an acute stroke. Many strokes can be prevented by heeding TIA warning signs and treating underlying risk factors.
The vast majority of strokes – approximately 83 percent — are ischemic. They are caused by an obstruction of an artery leading to or in the brain, preventing oxygenated blood from reaching parts of the brain that the artery feeds. Ischemic strokes are either thrombotic or embolic, depending on where the obstruction or clot (thrombus or embolism), causing the blockage originated:
Thrombotic Ischemic Stroke
Thrombotic stroke is caused by a thrombus (blood clot) that develops in an artery supplying blood to the brain — usually because of a repeated buildup of fatty deposits, calcium and clotting factors, such as fibrinogen and cholesterol, carried in the blood. The body perceives the buildup as an injury to the vessel wall and responds the way it would to a small wound — it forms blood clots. The blood clots get caught on the plaque on the vessel walls, eventually stopping blood flow.
There are two types of thrombotic stroke:
- Large vessel thrombosis,
the most common form of thrombotic stroke, occurs
in the brain’s larger arteries. The impact and
damage tends to be magnified because all the smaller
vessels that the artery feeds are deprived of blood.
In most cases, large vessel thrombosis is caused by
a combination of long-term plaque buildup (atherosclerosis)
followed by rapid blood clot formation. High cholesterol
is a common risk factor for this type of stroke.
- Small vessel disease (lacunar infarction) occurs when blood flow is blocked to a very small arterial vessel. It has been linked to high blood pressure (hypertension) and is an indicator of atherosclerotic disease.
Thrombotic disease accounts for about 60 percent of acute ischemic strokes. Of those, approximately 70 percent are large vessel thrombosis.
Embolic Ischemic Stroke
A blood clot that forms in one area of the body and travels through the bloodstream to another where it may lodge is called an embolus. In the case of embolic stroke, the clot forms outside of the brain – usually in the heart or large arteries of the upper chest and neck – and is transported through the bloodstream to the brain. There it eventually reaches a blood vessel small enough to block its passage.
Emboli can be fat globules, air bubbles or, most commonly, bits and pieces of atherosclerotic plaque, such as lipid debris, that have detached from an artery wall. Many emboli are caused by a cardiac condition called atrial fibrillation—an abnormal, rapid heartbeat in which the two small upper chambers of the heart (called the atria) quiver instead of beating. Quivers cause the blood to pool, forming clots that can travel to the brain and cause a stroke. Cardiac sources of embolism account for 80 percent of embolic ischemic strokes.
Ischemic Stroke Symptoms
The signs of ischemic stroke are similar to those of a TIA, except the damage can be permanent. The most common indicator is sudden weakness of the face, arm or leg, most often on one side of the body. Other warning signs may include:
- sudden numbness of the face, arm, or leg, especially on one side of the body;
- sudden confusion, trouble speaking or understanding speech;
- sudden trouble seeing in one or both eyes;
- sudden trouble walking, dizziness, loss of balance or coordination; and/or
- sudden severe headache with no known cause (most common with hemorrhagic stroke).
The symptoms depend on the side of the brain that's affected, the part of the brain, and how severely the brain is injured. Stroke may be associated with a headache, or may be completely painless. Therefore, each person may have different warning signs.
Hemorrhagic stroke occurs when a vessel in the brain suddenly ruptures and blood begins to leak directly into brain tissue and/or into the clear cerebrospinal fluid that surrounds the brain and fills its central cavities (ventricles). The rupture can be caused by the force of high blood pressure. It can also originate from a weak spot in a blood vessel wall (a cerebral aneurysm) or other blood vessel malformation in or around the brain.
Damage can be caused in two ways. As in the case of ischemic stroke, oxygen- and nutrient-rich blood is prevented from reaching the brain cells beyond the point of rupture. In addition, leaked blood can irritate and harm the brain cells in the areas where it accumulates.
It is the location of the hemorrhage, rather than the amount of bleeding, that tends to be the bigger factor in influencing the severity of the stroke. For example, bleeds in the brainstem, though relatively tiny, can be quite lethal, whereas the same-sized bleed in the frontal lobe may not even be noticeable.
There are two types of hemorrhagic strokes. They are differentiated by where the ruptured artery is located and where the resulting blood leakage occurs.
(also called Intraparenchymal hemorrhage or intracranial hematoma)
This type of stroke is caused by the sudden rupture of an artery or blood vessel within the brain. The blood that leaks into the brain results in a sudden increase in pressure that can damage the surrounding brain cells. If the amount of blood increases rapidly, the sudden and extreme buildup in pressure can lead to unconsciousness or death.
Approximately 10 percent of all strokes are intracerebral hemorrhages. They occur most commonly in the basal ganglia where the vessels can be particularly delicate.
High blood pressure (hypertension) is the most common cause of this type of stroke. Less common causes include trauma, infections, tumors, blood clotting deficiencies, and abnormalities in cerebral blood vessels.
Blood Vessel Abnormalities: Blood vessel abnormalities in the brain include arteriovenous malformations (AVMs) and arteriovenous fistulas (AVFs). AVMs and AVFs, also called lesions, are abnormal connections between cerebral arteries (which carry blood to the brain) and veins (which take blood away from the brain).
AVMs appear to be acquired prior to birth (congenital) and tend to form near the back of the brain. Although AVFs can be congenital, more often they are caused by a trauma that damages an artery and a vein which are side by side in the brain.
These blood vessel abnormalities can cause a host of problems, but the two most common are pressure against the adjacent parts of the brain, causing neurological problems (such as seizures, paralysis or loss of speech), and bleeding (hemorrhage) into surrounding tissues. Hemorrhage from cerebral arteriovenous abnormalities represents from 2 percent to 4 percent of all strokes.
Subarachnoid Hemorrhage (SAH)
Subarachnoid hemorrhage occurs when bleeding from a damaged vessel causes blood to accumulate between the brain and the skull, in the subarachnoid space, and press on the surface of the brain instead of dispersing into the tissue. The leaked blood can irritate, damage or destroy surrounding brain cells.
When blood enters the subarachnoid space, it mixes with the cerebrospinal fluid (CSF) that cushions the brain and spinal cord. This can block CSF circulation, which leads to fluid buildup and increased pressure on the brain. The open spaces in the brain (ventricles) may enlarge, resulting in a condition called hydrocephalus. This can make a patient lethargic, confused or incontinent. The large accumulation of blood increases the pressure surrounding the brain, interfering with brain function.
The leaked blood also can produce a condition called vasospasm in which the vessels narrow, impeding the flow of blood to the brain. This can result in an ischemic stroke. The condition typically develops five to eight days after the initial hemorrhage.
Most often, a subarachnoid hemorrhage occurs because a cerebral aneurysm, an abnormal bulging outward in the wall of an artery, ruptures. SAH also can occur because blood leaks from abnormal blood vessel connections (AVMs and AVFs) near the surface of the brain.
Cerebral Aneurysm: A brain aneurysm is a weak bulging spot on the wall of a brain artery very much like a thin balloon or weak spot on an inner tube. Aneurysms form from wear and tear on the arteries, and sometimes from injury, infection or an inherited tendency.
There are two types of aneurysm:
Saccular – This is the most common type. It has a neck and stem and is also known as a “berry” aneurysm because of its shape.
Illustration of a Saccular Aneurysm
Fusiform – This is a less common type of aneurysm. It is an outpouching of the wall on both sides of the artery and does not have a stem.
Illustration of a Fusiform Aneurysm
Aneurysms that cause subarachnoid hemorrhage are usually located at the base of the brain in the Circle of Willis. This is an area in which a lot of blood pressure changes occur and where a lot of vessels branch off, which can expose them to weakness.
Although it is not possible to predict whether an aneurysm will rupture, an aneurysm is more likely to do so when it has a diameter of 7 millimeters or more. Unruptured brain aneurysms can be medically treated to prevent a possible rupture.
Sudden & Severe Symptoms
Symptoms of a hemorrhagic stroke appear without warning. The sudden increase in blood volume within the rigid skull (cranium) creates intense intracranial pressure that cannot be released. This, in turn, may trigger a severe (“thunderclap”) headache, neck pain, double vision, nausea or vomiting, loss of consciousness or even death.
About 17 percent of strokes are hemorrhagic. The average age at which people suffer hemorrhagic stroke tends to be lower than for ischemic stroke. This is because many of the risk factors are related to unhealthy behaviors, such as smoking or drug use, rather than the effects on the body of aging. The fatality rate for hemorrhagic strokes is higher than for ischemic strokes and overall prognosis is poorer.