Download Neuroscience 2a – Blood Supply to CNS

Neuroscience 2a – Blood Supply to the CNS
Anil Chopra
1. Draw a simple diagram of the blood supply to the CNS, including:
a. Circle of Willis
b. Cerebral arteries
c. Main branches of the vertebrobasilar tree
2. Draw a map of the perfusion fields for the three main cerebral arteries and
explain briefly the neurological deficits that might arise following their
disruption.
3. Outline the pattern of venous drainage of the brain.
4. Define the following terms:
a. Cerebral ischaemia
b. Cerebral infarction
c. Cerebral thrombosis
d. Cerebral embolism
e. Cerebral haemorrhage
5. Explain what is meant by the terms “stroke” and “transient ischaemic attack”
and list the main risk factors for these conditions.
6. Contrast the effects of a cerebrovascular accident in the cerebral cortex with
one in the brainstem (specific deficit vs. unconsciousness, paralysis or
combined effects).
7. Explain the possible consequences of a subdural or epidural haemorrhage.
Anatomy of Blood Supply to the Brain
- Wet weight only 1.2 kg.
- 10-20% cardiac output (highly metabolically active)
- 20% of O2 consumption
- 66% of liver glucose
Therefore it is very vulnerable if blood supply is cut off.
Two sources of blood to brain:
 The right and left Internal
Carotid Arteries –
 The common carotid artery
comes of the aortic arch on
the left, and bracheocephalic
on the right, at the level of the
Adam’s apple it bifurcates
into the external and internal
carotid. The external carotid
supplies the face, the internal
the brain
 Transverse the skull in the
carotid canal and cavernous
sinus before piercing the dura
and entering the middle
cranial fossa, just lateral to the
optic chiasm
 Prior to bifurcation the ICA supplies branches of the pituitary (hypophyseal
arteries), the eye (ophthalmic artry), parts of the basal ganglia (globus
pallidus) and the limbic system (anterior choroidal artery).
 Divide and supply blood to the anterior and middle parts of the cerebral
hemispheres (anterior and middle cerebral arteries)
 The right and left Vertebral Arteries –
 Come of the subclavian artery on both sides
 Ascend to the brainstem foramina in the transverse processes of the upper
cervical vertebrae.
 Enter the skull through the foramen magnum
 Unite at the level of the lower part of the pons to supply blood to the brain
stem via the basilar artery and the posterior parts of the cerebral hemisphere
by dividing into the two posterior cerebral arteries at the superior border of
the pons
 Before forming the basilar artery each vertebral artery has a number of
branches including the posterior spinal artery and anterior spinal artery
which supply the upper cervical cord and the posterior inferior cerebellar
artery (PICA) which supplies the medulla and cerebellum
 This whole network makes up the posterior circulation
Circle of Willis:
 In addition the posterior and anterior circulations anastomose at the base of the
brain in the circle of Willis
 This anastomose circuit allows for compensatory flow if part is impaired via the
posterior and anterior communicating arteries
 It is not patent in many people, and in many is not bi enough to compensate for
impaired flow from a main artery such as the MCA
Venous Drainage from the
Brain
» Venous drainage of the
brainstem and cerebellum is
directly into the dural venous
sinuses
» Drainage of the hemispheres
is via internal and external
cerebral veins
» The external veins drain into
the superior sagittal sinus →
transverse sinus → lateral
sinus → internal jugular vein
» The internal veins drain to
the deep structures of each
hemisphere to the great vein
of Galen → straight sinus →
internal jugular
» Venous sinuses are the main
source of drainage which are
formed from folds in the dura mater
» Drainage to the jugular vein is through the jugular foramen
Stroke
 Also known as Cerebrovascular accident (CVA)
 Definition of Stroke: A rapidly developing focal disturbance of brain function of
presumed having a vascular origin and of more than 24 hours duration
 Infarction (blockage) (85%) or haemorrhage (burst blood vessel) (15%)
 Definition of Transient ischaemic attack: rapidly developing focal disturbance of
brain function of presumed vascular origin that resolves completely within 24
hours.
Infarction: Degenerative changes which occur in tissue following occlusion of an
artery. Visible on a CT scan,
Cerebral Ischaemia: Lack of sufficient BLOOD SUPPLY to nervous tissue resulting
in permanent damage if blood flow is not restored quickly. Cerebral hypoxia refers
to deprivation of oxygen supply to brain tissue and total deprivation of oxygen to the
brain is called cerebral anoxia.
Thrombosis: formation of a blood clot (thrombus).
Embolism: when a small vessel is plugged by debris from a larger vessel. Can not
only be thrombus but also air. E.g. thrombi from the heart; atherosclerotic debis from
the internal carotid.
Stroke Epidemiology
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3rd commonest cause of death
100,000 deaths in UK per annum
50% of survivors are permanently disabled
70% show an obvious neurological deficit
Risk factors
o Age
o Hypertension
o Cardiac disease
o Smoking
o Diabetes mellitus
o Others
Anterior cerebral artery (Disinhibited frontal lobe)
• Paralysis of contralateral leg > arm, face
• Disturbance of intellect and judgment (abulia)
• Loss of appropriate social behavior
Middle cerebral artery
• “Classic stroke”
• Contralateral hemiplegia: this is when paralysis occurs on the opposite side of
the body to which the lesion is. The arm is affected more than the legs.
• Hemisensory deficits (no sensation on one side of the body)
• Hemianopia (knock out half visual field)
• Aphasia –inability to speak (Left sided lesion)
Posterior cerebral artery
• Visual deficits
– homonymous hemianopia (blindness of half the visual field)
– visual agnosia (Loss of the ability to interpret sensory stimuli, such as
sounds or images)
• Receptive aphasia – can speak by cannot understand what is being said due to
defects in Wernicke’s area.
Lacunar Infarcts
• Lacune is a small cavity
• They appear in deep structures as a result of small vessel occlusion.
• Deficit is dependent on anatomical location
• Normally occur as a result of hypertension.
• Can be clinically silent
Hemorrhagic Strokes
• Extradural – build up of blood, usually from arteries, between the dura mater
and the skull. Usually caused by trauma and has immediate effects due to the
dramatic increase in raised intra-cranial pressure (must be treated immediately
to avoid death).
• Subdural – build up of blood between the dura and arachnoid membrane, normally
from veins. It is also caused by trauma but has delayed effects as the drainage
through veins is slow, the increase in intracranial pressure is also slow.
• Subarachnoid – bleeding between arachnoid membrane and pia mater.
Normally caused by spontaneous ruptured aneurysms – typical symptom is the
thunderclap headache “worst headache ever”.
• Intracerebral – spontaneous hyper-tensive occlusions of arteries or veins
within the brain itself. Very severe as raise in intra-cranial pressure crushes
brain tissue directly.