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Saturday, July 16, 2011

TIA - the "temporary stroke" [Full link to blog for email clients.][FT:C44]

This is part two of a four-part series on the interrelated subjects of stroke, TIA, seizures and epilepsy.

In the previous blog, I spoke of strokes - cerebral infarctions which are analogous to the myocardial infarction of the heart (heart attack). Just as the heart has symptoms of angina from temporary interruption of blood flow, so does the brain have TIAs - transient ischemic attacks. TIAs are essentially miniature strokes,without the long-lasting obvious effects of a stroke.

TIAs can result from three factors - (A) low blood-pressure-induced lack of blood flow to the brain, (B) spasm of the blood vessels interrupting blood flow to the brain, (C) breakaway clots that temporarily block blood vessels in the brain.  The key feature of all is that they are transient.  TYpe A requires two factors - a malformation of blood vessels in the brain that cause the blood flow to be decreased under normal conditions, and an event that further reduces blood flow.  The former can be caused by previous blockage, arterial plaques (i.e. atherosclerosis), scar tissue, or malformed vessels.  The hypotensive or low whole-body blood flow event can be caused by rapid posture change (usually standing up), using the bathroom (especially if constipated), or medications to lower blood pressure.  One example of TIA is thus a person suddenly standing up, getting dizzy, feeling pain, and losing consciousness.

An important issue about blood pressure is that there are two pressure states - the highest pressure is encountered as the heart is actively pushing blood into the highly elastic arteries at the peak of the heart beat.  As the heart rests and refills, the pressure in the blood vessels is largely a function of the elasticity of the vessels and smooth muscles lining the vessels which can constrict the diameter and regulate the blood pressure.  Thus the high pressure is the result of the heart's ability to pump, and the low pressure is a result of the rest of the circulatory system regulating the pressure via blood vessel size. This minimum pressure is import for being able to force blood through the smallest vessels (capillaries) that provide the actual exchange of oxygen, carbon dioxide, glucose and metabolic wastes at the cells themselves.    If there is a partial blockage anywhere, or an area that restricts the blood flow for only a short space, the area downstream will have lower blood pressure and less "perfusion" of blood through the capillaries. 

Thus if the whole body blood pressure drops - such as when a person on heart and blood pressure medication stands up suddenly - the pressure downstream from a partial blockage may be too low to perfuse a particular brain area.  The neurons are temporarily deprived of oxygen - i.e. transient ischemia - and for a very brief time the patient has symptoms of a stroke - albeit a small one.

A similar temporary occlusions can result from spasm of the smooth muscle fibers that regulate the size of blood vessels.  Spasm can be due to drugs or medications, stress, environmental conditions or seizure-like activity (see the next blog!).  A vasospasm is also likely to occur if there is an unusual shape or configuration or even damage to blood vessels in the brain.  The source can be congenital or due to head injury or even prior strokes.

The final cause is from mobile clots or debris in the blood.  The technical term is "emboli" and typical sources include clots due to a damaged heart valve, traumatic injury with extensive bleeding (and subsequent clotting) or peripheral artery disease in which clots form in the legs of patients who have been immobile for too long.  Emboli will travel through the blood until they are either broken apart or reach a blood vessel narrower than the clot.  If the clot reaches the brain, but once lodged in a vessel it breaks down further, a TIA results.  If the clot cannot be dislodged or more clots build up, a stroke results.

TIAs resulting from blood pressure drops or vasospasm may occur anywhere in the brain, but will most likely involve the same area each time if there is a malformed or partially occluded blood vessel that is sensitive to low perfusion.  TIAs due to emboli will most likely be different each time since the eventual destination of the clots will vary with each instance.

TIA symptoms vary - essentially they are miniature strokes.  Thus the same symptoms of strokes can be applied to TIAs with the exception that they are transient hence the effects may last only seconds to minutes with a complete return to normal function.  Some typical TIA symptoms include migraine-like headache that lasts less than 10 minutes, sudden paralysis or weakness that disappears within just a few minutes.  Sudden blindness, deafness, aphasia (inability to speak) or incoherent speech that returns to normal before the patient can even seek assistance. 

Rarely does unconsciousness result from a TIA, but if it does it may indicate that a more serious condition is developing.  Repeated strokes or TIAs may result in scar tissue formation that leads to seizures or even epilepsy.  We will begin our coverage of that topic in the next blog.

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