Stroke - The occluded brain

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

Stroke is a common term in today's society. Most of you problem know - or know of - someone who has suffered a stroke.  Essentially, a stroke is a same for the brain as a heart attack is for the heart.  In fact - they share a common medical name: "infarction."  An infarct is an area of dead cells caused by a blocked blood supply.  Without blood, the cells have no oxygen, no nutrients, and cannot get rid of wastes.  The blockage is usually due to a narrowing of blood vessels due to scarring, plaques or other deposits.  In the heart, the blockage grows until it blocks the blood vessel.  Blood clots can form at these sites, and may break off and stop at other narrow places "downstream" causing a sudden infarct.  Quite frequently stroke is caused by these "wandering" blood clots getting lodged in the tiny, narrow vessels of the brain.  Unless the blood supply, and in particular, the oxygen supply, to heart or brain can be replaced or restored, the cells will die. 

The picture at left is an excellent illustration - one of the best I've seen to demonstrate a "cerebral infarct" or stroke.  The picture is from "The Internet Stroke Center" at http://www.strokecenter.org/education/ais_pathogenesis/13_evol_cereb_athero.htm and I direct readers there for more clinical information.

What you should notice is that an infarct (black) can occur anywhere in the brain - and the dead neurons are downstream from the blockage of the artery.  The edges are less affected, since some blood can be delivered via arteries and capillaries (the smallest blood vessels) to the sides.  It is difficult to cut off the blood supply to too large an area of brain, because the brain has a highly redundant network of blood vessels (below, left - from The Encyclopedia of Science) at the bottom of the brain.  However, as you run out the length of the arteries, you reach brain areas that are served by only one blood vessel - typically these are on the upper surface of the brain, and provide some of the characteristics of a stroke - i.e. loss of motor ability.

Onset of a stroke is typically sudden - a clot breaks loose from somewhere else in the body, travels to brain, and blocks a narrow artery.  The brain area "downstream" becomes "ischemic" - meaning it does not have sufficient blood and oxugen flow to keep the neurons functioning.  Cells can function for a time without oxygen by breaking glucose down into lactic acid - but without blood flow the glucose depletes, the lactic acid builds up, and then cells begin to die.  As neurons die, they release neurotransmitters in such large quantities that they can further damage the surrounding neurons.  Unless blood flow and oxygen are restored, an area of brain will die, and that function will be lost.

Brains can recover - somewhat - there is enough redundancy that patients can be trained to recover some function.  There is some penetration of oxygen from unaffected areas, and a high oxygen environment is helpful. If the clot can be broken up fast enough, the neurons are damaged, but not dead, and can recover some function.  In addition, the brain is remarkably plastic, and functions can switch to other areas of the brain (for example, language and speech have been known to switch from left to right hemisphere!).

The severity and effects of a stroke depend on the location of the infarct.  In the Occipital Lobe, an infarct will result in loss of a visual area or even blindness.  In the Temporal Lobe, hearing, memory or language capability may suffer.  In the Parietal Lobe and the motor areas of the Frontal Lobe, sensory inputs and and motor outputs are the result.  Note that the latter are most common because (A) they are most noticeable effects, (B) those brain areas are further from any "collateral" blood vessels that could bypass a blockage, (C) they are survivable. Infarcts in Prefrontal Areas are less commonly noted, because in some areas, there are few obvious effects, except for eye movement and decision making.  Also, large infarcts in these areas tend not to be survivable.

More rare would be infarcts of deeper structures, but these tend to have the more interesting effects for a writer:  Infarcts of the thalamus and hypothalamus are serious - they affect large areas of the feedback control of the body. Infarcts of the visual and sensory pathways lead to an interesting form of amnesia termed "neglect."  In neglect, the patient does not acknowledge that a sensory field exists, even though the body reacts normally to those sensations.  In visual neglect, an experimenter can show a picture to one portion of the visual field - the patient is not allowed to move their eyes to "scan" the picture.  The patient denies that there is anything in the visual field, but also denies that they are *missing* any portion of the visual field.  Yet the rest of the brain knows that image is present - the eyes will track the image, an embarrassing image will cause a blush, etc.  The same is true for sensory inputs from the rest of the body.  Such effects arise from infarcts at either the inputs through thalamus, or from infarcts in the parietal association areas. 

A final note before moving on to the next topic is that some strokes may not be noticeable at all - the area affected may be small, it may be an area with less defined function (i.e. the nondominant hemisoehere of the brain), or they may be so short acting that any damage is temporary.  We will cover "Transient Ischmic Attacks" in the next blog.