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What Is Stroke – Stroke Types – Stroke Causes
Classification of Strokes – Major Types of Strokes – Most Common Causes of Strokes
What is Stroke?
Stroke is a permanent damage to some portion of the brain or spinal cord tissues produced by either interruption of blood supply or bleeding.
The word “permanent” is critical. Nervous tissue is very metabolically active. It is absolutely unable to tolerate glucose and oxygen shortage. Most of brain nerve cells die within 4 minutes after the blood flow has stopped.
Small stroke symptoms may often be short lasting with full recovery of the lost function. Recovered function has nothing to do with the brain tissue recovery. Once the nerve cells are gone, there is no way to resurrect them. The lost function comes back because surviving cells are taking over the functions of the deceased ones. This is why the best stroke treatment is prevention rather than dealing with consequences of the damage.
Please, bear in mind that even though the dead cells do not recover, not all the cells get damaged at the same time. In ischemic stroke, some nerve cells die right away and some will die later. This is due to uneven loss of blood flow in the stroke area. Some blood is shunted from the neighboring regions, or the blood vessel is not completely blocked initially. One way or the other, in substantial percentage of stroke patients, restoration of blood flow within short period of time dramatically reduces the extent of tissue damage and level of future disability. If you suspect someone having a stroke, call 911 ASAP.
Short lasting lack of blood flow may cause a transient stroke-like episode with full recovery and no evidence of brain damage. It is called a Transient Ischemic Attack or TIA. True TIAs are relatively rear. More often “TIA” ends up being a small stroke with fast recovery.
Strokes are divided into ischemic, venous, and hemorrhagic types of stroke. I don’t follow any particular classification here, since the purpose of the article is to make the reader understand the basics of stroke mechanisms.
Ischemic strokes happen when some blood vessel supplying a portion of the tissue is blocked either partially or completely. There are a few mechanisms of this blockage.
People with abnormal cholesterol and fat metabolism often develop a condition called atherosclerosis. Cholesterol gets deposited in the arterial walls. Arteries are the blood vessels, which carry blood from the heart to the tissues. These deposits are called atherosclerotic plaques. Plaques gradually narrow the blood vessel eventually leading to ischemic strokes. Atherosclerotic plaque may disintegrate and send its fragments (emboli) down the blood stream. These fragments eventually block some thinner branch(s) and cause a stroke in the area supplied by this branch(s).
Blood clots formed in the heart or veins can also cause strokes. Small vessel disease due to diabetes, hypertension, and smoking often cause small (lacunar) strokes. Please see the appropriate articles regarding those conditions.
Blockage of veins draining blood from the brain is another type of stroke. They most commonly happen in people with excessive blood clotting. Pregnancy is also a risk factor for this type of stroke. Brain tissue is damaged by inefficient circulation caused by poor drainage of the blood. This stroke type is often associated with a constant dull headache, which is often the first symptom.
Hemorrhagic stroke is a bleeding into the brain tissue. Brain damage is caused by compression of the tissues neighboring the blood clot. Symptom severity depends on the amount of blood in the hematoma. Prognosis of this type of stroke is variable.
On one hand, large bleeds may damage a large portion of the brain, leading to increased pressure inside the skull. The outcome may be devastating – from severe disability to death. On the other hand, since the blood supply to the tissue is preserved, smaller bleeds may damage only a tiny portion of the brain in spite of bad symptoms initially. Those patients who recover, may have minimal problems afterwards. Some conditions, such as hypertension, cause chronic changes in the blood vessels. They become fragile and easy to rupture.
Another cause of bleeding inside the skull is a subarachnoid hemorrhage due to an aneurysm rupture. Blood usually does not damage brain tissue directly (unless it is hit by a blood jet). In subarachnoid hemorrhage, there is a leak into so called subarachnoid space. Blood accumulates of the surface of the brain. Large subarachnoid bleeds often lead to death due to rapidly expanding volume with subsequent compromise the blood and cerebrospinal fluid circulation.
Survivors are often hit by the second wave of problems. The blood clot irritates major arteries at the base of the brain. It causes inflammation in their walls narrowing the lumen. This process is a very common cause of strokes after an aneurysm rupture.
Strokes may also happen in the nerves. This type of stroke is typical in patients with diabetes. The symptoms are usually severe pain, numbness and loss of muscle power in some portion of the body (most commonly in the thigh).
There is no single treatment for all stroke types.
Treatment and preventive measures depend on the type of stroke and the cause (if known). Stroke prevention is essential in patients with known stroke risk factors or history of strokes.
Nerve cells don’t regenerate. There is no treatment that would be able to promote regeneration of the nerve cells.
Irrelevantly of the stroke cause and type, prognosis depends on the brain area damaged and the volume of the brain tissue lost.
Aggressive prevention of recurrent strokes is a key factor in prevention of future disability and long-term prognosis.