The Basic Process
Blood Flow Blockage. The brain receives about 25% of the body’s oxygen, but it cannot store it. Brain cells require a constant supply of oxygen to stay healthy and function properly. Therefore, blood needs to be supplied to the brain continuously through two main arterial systems:
- The carotid arteries come up through either side of the front of the neck. (The pulse of a carotid artery can be felt by placing the fingertips gently against either side of the neck right under the jaw.)
- The basilar artery forms at the base of the skull from the vertebral arteries, which run up along the spine, join, and come up through the rear of the neck.
- A reduction of, or disruption in, blood flow to the brain is the primary cause of a stroke. Blockage for even a short period of time can be disastrous and cause brain damage or even death.
- A stroke is usually defined as two types:
- Ischemic (caused by a blockage in an artery).
- Hemorrhagic (caused by a tear in the artery’s wall that produces bleeding in the brain).
The consequences of a stroke, the type of functions affected, and the severity, depend on where in the brain it has occurred and the extent of the damage.
Cell Death. In addition to oxygen cut-off to the brain, other factors are involved in the cycle of events leading to brain cell death after a stroke. The process is not altogether clear, but one hypothesis is as follows:
- When blood is cut off in the brain, proteins known as excitatory amino acids (such as glutamate and glycine) are released. These amino acids literally excite nerve cells and, when over produced, can kill them.
- These proteins open channels in the membranes that cover neurons (brain cells), allowing large amounts of calcium to flow in.
- Calcium reacts within the neurons to release harmful substances that damage cells.
Natural particles released by the body’s chemical processes, called oxygen-free radicals, may also play a role in the cell damage caused by a stroke. (Some oxygen-free radicals under study are called lipid peroxides.)
Another hypothesis focuses on an enzyme called PARP:
- PARP ordinarily makes minor molecular repairs.
- Confronted with substantial injury, however, PARP over-responds and takes up a substance called ATP, the major source of energy in cells.
- The cells die.
- Within minutes of a stroke, the zone of initial cell death is surrounded by additional damaged and dying brain cells. This process can continue for hours, leading to brain damage, perhaps irreversible.
Ischemic strokes are by far the more common type, causing over 80% of all strokes. Ischemia means the deficiency of oxygen in vital tissues. Ischemic strokes are caused by blood clots that are usually one of three types:
- Thrombotic stroke.
- Embolic stroke.
- Lacunar stroke.
Thrombotic or Large-Artery Stroke and Atherosclerosis. A thrombotic stroke usually occurs when an artery to the brain is blocked by a blood clot (a thrombi) that forms as the result of atherosclerosis, commonly known as hardening of the arteries. These strokes are also sometimes referred to as large-artery strokes. They account for about 60% of all strokes. The process leading to thrombotic stroke is complex and occurs over time:
- The arterial walls slowly thicken, harden, and narrow until blood flow is reduced, a condition known as stenosis.
- These now abnormal arteries become vulnerable to injury. This initiates a process called the inflammatory response , which may play a significant role in the stroke.
- The injuries to the arteries signal the immune system to release white blood cells (particularly those called neutrophils and macrophages) at the site.
- Macrophages literally “eat” foreign debris and become foamy cells that attach to smooth muscle cells of blood vessels causing them to build up.
- The immune system, sensing further harm, releases other factors called cytokines, which attract more white blood cells and perpetuate the whole cycle.
- As these processes continue, blood flow slows.
- In addition, injured inner walls fail to produce enough nitric oxide, a substance critical for maintaining blood vessel elasticity. The arteries become calcified and lose elasticity.
- The hardened and rigid arteries are even more susceptible to injury. If they tear, a blood clot, or thrombus, may form.
- The blood clot then completely blocks the already narrowed artery and shuts off oxygen to part of the brain. A stroke occurs.
Embolic Strokes and Atrial Fibrillation. An embolic stroke is usually caused by a dislodged blood clot that has traveled through the blood vessels (an embolus) until it becomes wedged in an artery. Embolic strokes account for about 25% of all strokes and may be due to various conditions:
- In about 15% of embolic strokes, the blood clots originally form as a result of a rhythm disorder known as atrial fibrillation : This abnormal rhythm is a rapid quivering beat in the upper chambers of the heart (the atria). Because of the irregular pumping, some blood may remain in the heart chamber where it forms clots, which can then break off and travel to the brain as emboli.
- Emboli can originate from blood clots that form at the site of artificial heart valves or as a result of heart valve disorders.
- Emboli can also occur after a heart attack or in association with heart failure.
- Rarely, emboli are formed from fat particles, tumor cells, or air bubbles that travel through the blood stream.
Lacunar Strokes. Lacunar infarcts are a series of very tiny, ischemic strokes, which cause clumsiness, weakness, and emotional variability. They are actually a subtype of thrombotic stroke and constitute about 38% of this major group. In some populations, such as among Japanese, they are the most common stroke subtype. They can also sometimes serve as warning signs for a major stroke.
Over 15% of strokes occur from hemorrhage (sudden bleeding) in the brain. In a healthy brain, brain cells called neurons are protected from exposure to blood by the blood-brain barrier , a wall of tiny vessels and structural cells. In a Hemorrhagic stroke, however, this barrier is broken.
Hemorrhagic strokes may be categorized by how and where they occur.
- Parenchymal, or cerebral, hemorrhage strokes. These strokes occur within the brain and account for about 10% of all strokes. They are most often the result of hypertension exerting excessive pressure on arterial walls already damaged by atherosclerosis. Heart attack patients who have been given drugs to break up blood clots or blood-thinning drugs have a slightly elevated risk of this type of stroke.
- Subarachnoid Hemorrhagic strokes . This other major Hemorrhagic stroke accounts for about 5% of all strokes. They occur when a blood vessel on the surface of the brain bursts, and blood leaks into the subarachnoid space , an area between the brain and the skull. They are usually caused by the rupture of an aneurysm, a weakening in the blood vessel wall, which is often an inherited trait.
- Arteriovenous malformation (AVM) is an abnormal connection between arteries and veins. If it occurs in the brain and ruptures, it can also cause a Hemorrhagic stroke.
Low Blood Pressure (Hypotension)
Less often, blood pressure that is too low can reduce oxygen supply to the brain and cause a stroke. This can occur from a heart attack, a major bleeding episode, an overwhelming infection, or rarely, from surgical anesthesia or from overtreatment of high blood pressure.
by Levon Ter-Markosyan, D.M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.