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Blood Supply to the
Brain
The
consequences of stroke depend upon which areas of the brain
are affected by ischemia, hemorrhage, hematoma or aneurysm.
Therefore, it is important to understand the organization of
the cerebral blood supply system.
Two arterial
systems supply the brain with blood: the internal carotid
and basilar. The internal
carotids and basilar arteries are connected via the circle
of Willis, which allows blood to pass from one
system to another in the event of blockage.
The Internal Carotid
Arteries
There
are two internal carotid arteries. They ascend from the
aortic arch toward the brain along each side of the neck.
They pass behind the ear into the temporal
lobe
and enter the subarachnoid space. They then go to the circle
of Willis where each bifurcates to form two main branches:
the anterior cerebral
artery and the middle cerebral
artery.
The course of the internal carotid arteries and their
branches is tortuous. Because of their many twists and turns
these arteries are all subject to blockages caused by
cholesterol build-up. For this reason, the internal
carotids, and the middle and anterior cerebral arteries are
more vulnerable to ischemic rather than hemorrhagic stroke.
According to FitzGerald, 1996, the contralateral internal
carotid may use the circle of Willis/anterior communicating
artery to supply blood to both pairs of anterior and middle
cerebral arteries. Complete interruption of blood flow in
the anterior cerebral artery is not very likely because the
opposite anterior cerebral can use the anterior
communicating artery to make up for the deficiency. Even the
facial artery (external carotid) can shunt blood to the
anterior and middle cerebral arteries via the ophthalmic
artery.
The Anterior Cerebral
Arteries
The anterior cerebral arteries supply blood to the medial
cortex, including the medial aspects of the motor and
sensory strips. Due to the upside-down representation of the
homunculus in both of these area, a
blockage in an anterior cerebral artery could cause
paralysis or sensory deficits which affect the opposite side
of the body from the hips on down. For example, a patient
who has suffered such a blockage might have a paralyzed leg
or be incontinent of bowel and/or bladder. Since the parts
of the motor and sensory strips connected to the arm receive
some blood from the anterior cerebral artery, the arm may
also be somewhat affected by this kind of blockage. Finally,
apraxia
of gait may also occur if an anterior
cerebral artery fails to supply an adequate amount of blood
to the medial part of the motor strip.
The anterior cerebral arteries also supply blood to the
anterior aspects of the frontal
lobes.
As these areas are involved in higher level cognition such as
reasoning and judgment, a condition called cerebral
dementia may result from anterior cerebral
artery blockages. Confused
language, or a language indicative of
cognitive impairment, may also occur.
The
Middle Cerebral Arteries
These large arteries have tree-like branches that bring
blood to most of the lateral cortex of each cerebral
hemisphere. This means that the middle cerebral arteries
supply blood to the cortical areas involved in speech,
language and swallowing. The left middle cerebral artery
provides Broca's
area, Wernicke's
area, Heschl's
gyrus,
and the angular
gyrus with blood. Also, the "head" and "neck" areas of the motor and sensory
strips in both hemispheres receive their
blood supply from the middle cerebral arteries. Damage to
these cortical areas on either side of the brain can impair
motor speech and swallowing functions.
In addition, the middle cerebral arteries provide most of
the blood supply to the corpus
striatum. The striatas, which are arterial
branches of the middle cerebral arteries, are known as the arteries of
stroke as they are the main source of blood
for the internal
capsule. A rupture of the lenticulo-striate
artery results in bleeding usually in the region of the
internal capsule (Steadman, 1997). When one of these
arteries is damaged, the bottleneck of fibers within the internal capsule , including the pyramidal tract, can be
affected, causing many disabilities. The striatas have
relatively thin walls and pressure within them is high. For
this reason, they are more vulnerable to hemorrhages than to
blockages.
Because the middle cerebral arteries supply blood to so many
areas of the brain, blockages in this part of the
circulatory system can cause numerous types of
disorders.
If the left
middle cerebral artery is blocked, aphasia and apraxia may occur.
Blockages of the right middle cerebral artery can cause
left
side neglect, which is an impairment in the
ability to recognize and respond to stimuli on the left side
of the body, prosopagnosia or the inability to
recognize faces, and various cognitive
problems, including agnosia. All of these are part of
the right hemisphere syndrome.
Blockages of either middle cerebral artery can cause
contralateral hemiplegia or hemiparesis as well as contralateral
hyposthesia. Paralysis and sensory deficits will
affect the head and neck most. Because the "hand" and "arm"
areas are superior to the "head" and "neck" areas of the pre
and postcentral gyri, they receive some blood supply from
the anterior cerebral arteries. For this reason, they may
not suffer as much after a middle cerebral artery
blockage.
Damage to the internal capsule caused by blockage or
hemorrhage of the striata (branch of middle cerebral) can
cause upper motor neuron dysarthria and dysphagia.
In addition, damage to either middle cerebral artery can
cause hemianopsia or the loss of sight in
one half of the visual field. This blindness affects the
contralateral aspect of both visual fields. For example, a
blockage in the right middle cerebral artery will cause left
hemianopsia or blindness in the left visual field of
both eyes.
The Vertebro-Basilar
System
Both
of the vertebral arteries
ascend through the spinal column and
enter the brain through the magnum foramen. Once in the
brain, they continue to ascend, traveling beside the brain
stem. At the lower border of the pons the two vertebral
arteries join together to form the basilar
artery or vertebro-basilar
artery.
The vertebral
arteries and the basilar are straight arteries and therefore
not as subject to blockages due to the build up of
cholesterol as are the internal carotids.
The posterior
inferior cerebellar not only supply the cerebellum but take
blood to the lateral medulla. Anterior and posterior spinal
arteries supply the ventral and dorsal medulla, respectively
(FitzGerald 1996). The three arteries are branches of the
vertebral.
The side of the
pons and the cerebellum receive blood from the anterior
inferior cerebellar artery and the superior cerebellar
artery. These arteries are branches of the basilar. The
anterior inferior cerebellar artery also has a branch, the
labyrinthine artery, that supplies the inner ear. The
basilar also gives off about twelve pontine arteries that
supply the medial pons (FitzGerald, 1996).
At the superior
border of the pons, the basilar artery divides to form the
two posterior cerebral
arteries.
Before the basilar
artery divides, several other arteries arise from it. These
include the anterior, inferior, and posterior cerebellar
arteries as well as pontine
branches. So, the cerebellum and pons are
mainly supplied by branches of the basilar.
The
Posterior Cerebral Arteries
At the superior
border of the pons, the basilar artery divides to form the
two posterior cerebral
arteries. These arteries supply blood to the
part of the brain that lies in the posterior fossa of the
skull, including the medial aspects of the occipital
lobes,
the inferior portions of the temporal
lobes,
the brainstem, and the cerebellum. They also deliver blood
to the thalamus and some other subcortical structures. A
vascular lesion may result in Thalamic Aphasia or Thalamic
Syndrome where the posterior thalamic nucleus is
disconnected from the sensory cortex. This rare syndrome
causes complete contralateral sensory loss alternating with
bouts of severe pain (FitzGerald, 1997).
As the vertebral and basilar are somewhat wide arteries and
their course is relatively straight, they are not as subject
to blockages caused by the build-up of cholesterol as the
internal carotids. This means that, overall, cerebrovascular
accidents effect the basilar system less frequently than the
internal carotid system. However, the basilar and posterior
cerebral arteries may hemorrhage due to shearing injuries
caused by accidents or pressure due to edema of the
brain.
Since they provide blood to the occipital lobes, damage to
these arteries can cause a variety of visual problems,
including cortical
blindness and alexia, which is an inability to read and visual
agnosia, or the inability to recognize
stimuli presented visually. Because these arteries supply
blood to the cerebellum and to the brain stem, blockages or
hemorrhaging there can cause either ataxic (cerebellar) or flaccid
(lower motor neuron) dysarthria..
It is important to note that the effects of ischemic stroke
are almost always contralateral and therefore unilateral. On the other hand, hemorrhagic
strokes may damage both hemispheres, depending upon the size
and location of the resulting hematoma.
One exception to this pattern is alternating
hemiplegia. This is a paralysis that can be
caused by ischemic stroke and affects different structures
on each side of the body. It results from a brain
stem
lesion that damages cranial nerve nuclei and one side of the
pyramidal tract in the same area. Lesions of cranial nerve
nuclei will cause a paralysis of the structures served by
that nerve on the same side of the body as the injury.
Because the pyramidal tract provides only contralateral
innervation to the spinal nerves, damage to these fibers
will cause a paralysis of different structures on the other
side of the body. For example, a lesion that affects the
right nucleus of the trigeminal nerve and the right half of
the pyramidal tract will cause paralysis of the right side
of the jaw and of the skeletal muscles on the left side of
the body.
The Circle of
Willis
The Circle
of Willis or the Circulus
Arteriosus is the main arterial anastomatic
trunk of the brain. According to Bhatnagar and Andy (1995), anastomosis occurs when vessels bring
blood to one spot and then redistribute it. The two internal
carotid arteries and the basilar arteries feed into the
Circle of Willis. The blood is then redistributed by the
anterior, middle and posterior cerebral arteries. Thus, the
ends of the internal carotids and the basilar artery enter
the Circle while the three cerebral arteries arise and exit
from it. (Neither the internal carotids nor the basilar
arteries can be considered "cerebral" arteries since they do
not begin within the brain).
In the Circle of
Willis the two internal carotids are joined together by the
anterior
communicating artery while the posterior
communicating artery links the internal carotid system
with the basilar artery. These connections make
collateral
circulation, which Love and Webb (1992, p. 40)
define as "the flow of blood through an alternate route,"
possible. This is a safety mechanism, allowing brain areas
to continue receiving adequate blood supply even when there
is a blockage somewhere in an arterial system. When all
arteries are functioning normally, their blood supplies will
not mix where they meet in the Circle because the pressure
of their streams will be equal.
As long as the
Circle of Willis can maintain blood pressure at fifty
percent of its normal level, no infarction or death of tissue will
occur in an area where a blockage exists. If collateral
circulation is good, sometimes a blockage will have no
permanent effects. Sometimes, an adjustment time is required
before collateral circulation can reach a level that
supports normal functioning; the communicating arteries will
enlarge as blood flow through them increases. In such cases,
a transient ischemic
attack may occur, meaning that parts of the
brain are temporarily deprived of oxygen.
Collateral
circulation is not always sufficient to prevent a stroke.
Some people lack one or more of the communicating arteries
in the Circle of Willis. This means that, if a blockage
develops, blood cannot be redistributed from another
arterial system and stroke will occur. FitzGerald (1996),
writes that 25% of people get blood to their posterior
cerebral arteries from the internal carotids, rather than
from the vertebro-basilar system. Also, even when all the
communicating arteries exist, the influence of several extraneural
factors including abnormal blood pressure,
cerebrovascular resistance, and arteriosclerosis may impede
collateral circulation. Low blood
pressure, which can result from surgical
shock, may prevent blood from traveling through the smaller
diameter arteries. Pressure or hypertension can cause stenosis of the
arteries by pushing plaque up against their walls. Cerebrovascular
resistance makes it more difficult for blood to
flow from one area to another. It can be caused by arterial
spasm, a high level of tri-glycerides in the blood which
increases its viscosity, or by elevated levels of cerebral
spinal fluid. Arteriosclerosis or the formation of
"plaque" consisting of muscle cells and fats on arterial
walls also makes it more difficult for blood to flow through
communicating arteries.
Watershed
Areas
Parts
of the brain located at the ends of the distribution areas
of the vascular systems are called the watershed
areas.
According to Bhatnager and Andy (1995, p.345), a watershed
area is a "tertiary brain area located peripheral to primary
distribution areas for anterior, posterior, and middle
cerebral arteries; it derives blood via the branches of all
three cortical arteries." Because they are at the extreme
ends of arterial distribution they are particularly
vulnerable to ischemia and infarction in those who have
circulation problems. Lesions
in the watershed areas may cause transcortical sensory
aphasia.
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