Blood supply to the CNS Flashcards
Summarise the metabolic demands of the brain
Brain makes up 2% of body weight but uses:
10-20% of cardiac output
20% of body O2 consumption
66% of liver glucose
The brain is therefore very vulnerable if its blood supply is impaired
What can left carotid artery stenosis lead to
Stroke
Summarise the two sources of blood supply to the brain
Vertebral arteries (posteriorly) Internal carotid arteries (anteriorly)
Where does the common carotid artery split
At the level of the laryngeal prominence (Adam’s apple)
Splits into external and internal carotid arteries
External supplies the vessels of the face and the surface soft tissue
The internal carotid- unbranched- emerges through the base of the skull- to supply the anterior part of the circulation. in the anterior cranial cavity.
What is the key difference between the external and internal carotid arteries
External- unbranched
Internal- branched
Describe the passage of the vertebral arteries into the skull
First branch of the subclavian artery
The vertebral arteries pass through the transverse foramina of the cervical vertebrae and through the foramen magnum into the brain to join the rest of the circulation
Summarise the carotid arteries
Carotid arteries: Left common carotid formed from aortic arch and right common carotid from the right subclavian; branch at the carotid sinus to form the internal and external carotids - the internal carotids then ascend to supply the brain
How does the internal carotid artery enter the skull
Through the carotid canal
What is the circle of willis
Anastamoses of the ICA and vertebral arteries- gives off the cerebellar arteries
The loop formed between the basilar artery and the internal carotid vessels (via the anterior and posterior communicating arteries) is known as the circle of Willis.
What is important to remember about the vertebral arteries
Often asymmetric in size
Describe the two branches of the internal carotid arteries
The internal carotid arteries send off two branches (the anterior and posterior communicating arteries) before becoming the middle cerebral artery. This artery has an extensive territory, supplying most of the surface of the brain and some of the basal ganglia.
Describe the anterior cerebral arteries
The anterior cerebral arteries travel forward on either side of the longitudinal fissure to supply the medial surface of each cerebral hemisphere.
Describe the basilar artery
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The vertebral arteries join at the inferior border of the pons to form the single basilar artery. Branches of the vertebral arteries and the basilar artery supply the medulla, pons and cerebellum.
Describe the posterior cerebral arteries
The posterior cerebral arteries supply the occipital and temporal lobes. Most of their input is derived from the basilar artery, with a contribution from the carotid vessels via the posterior communicating arteries.
Describe carotid endarterectomy
Some people can have their common carotid artery completely occluded by stenosis- but blood flow will be compensated by the other carotid artery (circle of willis)
However, we still need to treat the occlusion- pipe clean the carotid arteries
Which of the following arteries of the circle of Willis is unpaired?
Anterior communicating artery
Where does the cerebral veins drain
Into the sinuses- the cranial cavity is not about large cerebral veins.
Ultimately, where do the venous sinuses drain
Into the back of the head- which then then drains into the internal jugular veins- in side of neck next to carotid arteries
Summarise the venous drainage of the brain
Cerebral veins
Venous sinuses
Dura mater
Internal jugular vein
State the names of the sinuses that are at the top and bottom of the falx cerebri
Superior and inferior saggital sinuses
What is the name given to the place where all the sinuses meet
The confluence of sinuses
What connects the inferior sagittal sinus to the confluence of sinuses
The straight sinus
Which sinus ascends to join the confluence of sinuses
the occipital sinus
Which two sinuses run along the temporal bone
The superior and inferior petrosal sinuses
- Which main sinus drains into the internal jugular vein through the jugular foramen?
Sigmoid sinus
- Which sinus connects the confluence of sinuses to the sigmoid and superior petrosal sinuses?
transverse sinus
- Which sinus runs along the sphenoid and parietal bones?
Sphenoparietal sinus
- Which sinuses run on either side of the pituitary stalk?
Anterior and posterior inter cavernous sinus
Summarise venous drainage of the brain
Venous drainage of the brain begins internally as networks of small venous channels lead to larger cerebral veins, cerebellar veins, and veins draining the brainstem, which eventually empty into dural venous sinuses. The dural venous sinuses are endothelial-lined spaces between the outer periosteal and the inner meningeal layers of the dura mater, and eventually lead to the internal jugular veins.
What else drains into the dural sinuses
Also emptying into the dural venous sinuses are diploic veins, which run between the internal and external tables of compact bone in the roof of the cranial cavity, and emissary veins, which pass from outside the cranial cavity to the dural venous sinuses
The emissary veins are important clinically because they can be a conduit through which infections can enter the cranial cavity because they have no valves.
Summarise the small branches of the basilar artery
The basilar artery travels in a rostral direction along the anterior aspect of the pons (Fig. 8.39). Its branches in a caudal to rostral direction include the anterior inferior cerebellar arteries, several small pontine arteries, and the superior cerebellar arteries. The basilar artery ends as a bifurcation, giving rise to two posterior cerebral arteries.
What are the branches of the internal carotid artery
Entering the cranial cavity each internal carotid artery gives off the ophthalmic artery, the posterior communicating artery, the middle cerebral artery, and the anterior cerebral artery
Describe the anastomotic interconnection of the cerebral arteries
The cerebral arterial circle (of Willis) is formed at the base of the brain by the interconnecting vertebrobasilar and internal carotid systems of vessels (Fig. 8.38). This anastomotic interconnection is accomplished by:
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an anterior communicating artery connecting the left and right anterior cerebral arteries to each other, and
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two posterior communicating arteries, one on each side, connecting the internal carotid artery with the posterior cerebral artery
Which vein drains the dural venous sinuses
The internal jugular vein
Define stroke
Cerebrovascular accident (CVA) Definition: rapidly developing focal disturbance of brain function of presumed vascular origin and of >24 hours duration Results in impaired neurological function
What re the two types of stroke
Infarction (85%) or haemorrhage (15%)
Blockage or bleed- haemorrhage- likely trauma related
Define transient ischaemic attack (mini-stroke)
rapidly developing focal disturbance of brain function of presumed vascular origin that resolves completely within 24 hours
What are the two causes of ischaemic stroke (infarction)
Ischemic strokes are further divided into those caused by thrombotic or embolic phenomena. The latter is by far the commonest type of stroke and is often caused by emboli that originate from atherosclerotic plaques in the carotid arteries that migrate into and block smaller intracranial vessels.
Describe some rare causes of ischaemic strokes
Thromboembolic infarcts are predominantly secondary to atheroma or cardiogenic emboli, although very rarely infarcts may be seen secondary to embolic material such as fat (fractured long bones) or malignancy.
What is meant by infarction
Degenerative changes which occur in tissue following occlusion of an artery
Essentially, dead tissue that has lost its blood supply- no oxygen or nutrients
Define cerebral ischaemia
Lack of sufficient blood supply to nervous tissue resulting in permanent damage if blood flow is not restored quickly
hypoxia/anoxia- this is the oxygen component
But not just about oxygenation- reduced supply of nutrients and glucose too- global insufficient blood supply
describe the significance of TIAs
They are warnings
Elderly patients may come in with a. bit of a turn- dizziness, confusion
may be indicative of a big risk of stroke
temporary blockage to vessel- clot (most likely) or bit of atherosclerotic broken off- but quickly resolved and cleared- so no neurological deficit- but big risk of full-blown stroke- needs to be carefully managed.
Define thrombus
formation of a blood clot (thrombus)
If blood is static in the heart- it can begin to form thrombi- and if turbulent flow- can be thrown to the head- thrombosis
Define embolism
plugging of small vessel by material carried from larger vessel e.g. thrombi from the heart or atherosclerotic debris from the internal carotid
can break away to vessels in the brain and cause stroke
can get fat and air emboli
How will atherosclerosis show on the post-mortem
Yellow colour in the vessel wall- atheroma- fatty build up at vessel wall (most likely to occur at bifurcations)- completely occluded vessel
What should you look for on a post-mortem or imaging when looking for signs of pathology
Look for any asymmetries
Changes in colour- lose differentiation of colour from white to grey mater- sign of infarction or dead tissue
If it takes out a lot of the lateral part of the brain (on a coronal section)- likely to be middle cerebral artery
Summarise haemorrhagic strokes
Haemorrhagic infarcts are mostly seen in the setting of hypertension, although other causes include underlying malignancy or vascular malformation, trauma, vasculitis, recreational drugs and iatrogenic causes. Hypertensive infarcts occur most frequently in the following sites: • The putamen and the internal capsule • Central white matter • Thalamus • Cerebellar hemisphere • Pons.
Describe the nature of TIAs and what patients present with
They are thromboembolic in nature and should be recognized and managed promptly because they are an indication that a full stroke may be imminent. Carotid territory transient ischaemic attacks present with:
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Transient monocular blindness (amaurosis fugax)
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Transient sensory or motor symptoms of the face, arm or leg
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Transient aphasia.
Patients often describe amaurosis fugax as ‘a black curtain descending over one eye’.
Summarise vertebrobasilar TIAs
Vertebrobasilar transient ischaemic attacks present with a combination of: • Dysarthria • Vertigo and unsteadiness • Diplopia • Circumoral paraesthesiae • Sensory or motor symptoms affecting the limbs singly or in combination • Cranial nerve palsies.
Summarise the epidemiology of strokes
3rd commonest cause of death
100,000 deaths in UK per annum
50% of survivors are permanently disabled
70% show an obvious neurological deficit
List the risk factors for stroke
Largely lifestyle:
Age
Hypertension- particularly for haemorrhagic stroke- especially if you have aneurysms or inherent weakness in vessel wall- can burst
Cardiac disease- static blood in valves of heart- may start throwing of thrombi
Smoking
Diabetes mellitus- microvascular complications
Describe the perfusion field of the anterior cerebral artery
medial, anterior and superior regions of the cerebrum - supplies the lower limb and frontal lobe (decision making and social interaction)
e anterior cerebral artery actually supplies quite far back to the parieto-occipital fissure.
Describe the perfusion field of the middle cerebral artery
lateral and anteroinferior regions of the cerebrum - supplies deep structures as cortex fibres will transverse these region
The middle cerebral artery extends laterally and emerges through the lateral fissure between the frontal and temporal lobes – it supplies the front 2/3 of the lateral part of the hemisphere
The middle cerebral artery also supplies some of the sub-cortical grey matter towards the centre of the brain.
Describe the perfusion field of the posterior cerebral artery
posteroinferior regions of the cerebrum - largely occipital lobe with inferior temporal lobe
The posterior cerebral artery supplies the medial and lateral parts of the posterior part of the hemisphere
Summarise CNS tissue vascularisation
CNS Tissue Vascularisation: arteries enter tissue as branches of surface pial vessels, which then branch to penetrate into the parenchyma - forming capillaries that drain into venules
Relate the importance of the motor and sensory homunculus to the blood supply
Arranges as an upside-down man
Therefore, middle cerebral will supply the torso, head and abdomen
Whereas the anterior cerebral will supply the lower limbs
Essentially, the leg is more medial to the arm
This relates to the differences in clinical presentations seen between anterior and middle cerebral strokes
Summarise the presentation of a stroke where the anterior cerebral artery is affected
Paralysis (hemiparesis) of contralateral leg > arm, face
Disturbance of intellect, executive function and judgement (abulia)
Loss of appropriate social behaviour- hyper sexual, over-aggressive- due to frontal lobe deficit
Summarise the presentation of a stroke where the middle cerebral artery is affected
This is a CLASSIC STROKE
Contralateral hemiplegia in the ARM more than the leg
Contralateral hemisensory deficits- sensory cortex in post-central gyrus
Hemianopia- blindness over half the field of vision- exposes a design flaw in the brain- eyes at front- processing at back- vision integral thoughout brain
Aphasia (can’t speak) – left-sided lesion of the middle cerebral artery will result in aphasia because the language centres are more on the left side than the right
More likely to be complete paralysis (hemiplegia) as although superficially the motor cortex may be spared- the sub-cortical motor fibres going down to the body may be affected
What is meant by an expressive aphasia
Can understand by can’t formulate words
Broca’s > Wernicke’s
Summarise the presentation of a stroke where the posterior cerebral artery is affected
The posterior cerebral artery supplies the occipital lobe, which is where the primary visual cortex is located
This causes visual defects such as homonymous hemianopia and visual agnosia (unable to recognise what you are seeing)
prosopagnosia- the inability to recognise the face of someone you have known
Highlights the role of occipital lobe in not only processing the image but understanding and interpreting it too
Summarise lacune infarcts
Lacune is a small cavity in the brain- seen on post-mortem
Appear in deep structures as a result of small vessel occlusio body has compensatory mechanisms- will clear clot and dead tissue- leaving cavity- thus the association with hypertension
Deficit is dependent on anatomical location- most likely to occur in basal ganglia
Hypertension
Often no clinical presentations
Summarise the four types of haemorrhagic stroke
Extradural - trauma, immediate effects
Subdural - trauma, delayed effects
Subarachnoid - ruptured aneurysms
Intracerebral – spontaneous hyper-tensive
Describe an extradural haemorrhage
Trauma of middle cerebral artery- weak point in side of head (temple)- where four bones meet
Bleed raises ICP- which pushes the dura mater away from the inside of the skull- can shut down on brainstem as it is pushed down by the raised ICP
Neurosurgical emergency- drill a flap into the bone- remove the bleed- re-build the vessel- need to get. rid of bleed quickly or the patient will die
no space-blood creates the space- why ICP raises
Describe a subdural haemorrhage
Bridging veins between dura and arachnoid space
Trauma- ruptures these veins- low pressure-so delayed effects- difficult to pick up- subdural space where blood can accumulate without any noticeable effects initially
History of loss of consciousness- hit head- back to normal- die 24 hours later
In U,K- if patient presents with history of head injury and loss of consciousness- kept in overnight for observation
Describe subarachnoid haemorrhages
All vessels at base of brain are in subarachnoid space- if burst- subarachnoid bleed
Most likely due to inherent weaknesses in the vessel walls (Berry’s aneurysms) most people live without knowing them- often picked up accidentally on a scan
Choice to intervene dependent of clinician
If intervening- go through vessels in the groin- to insert a platinum coil to prevent rupturing
Describe intracerebral haemorrghages
Bleed within brain tissue itself
Goes back to hypertension
usually due to spontaneous hypertensive rupture of small vessels
Summarise sub-arachnoid haemorrhages
Subarachnoid haemorrhage is relatively uncommon and typically occurs in people between the ages of 35 and 65 years. Rupture of a cerebral berry aneurysm is the commonest cause (70%), with arteriovenous malformations accounting for 15% of cases. Berry aneurysms result from a defect in the media and elastica of the cerebral arteries, causing the media to bulge outward covered only by the adventitia. Genetic factors are important, particularly polycystic kidney disease and coarctation of the aorta.
describe the presentation of a sub-arachnoid haemorrhage
The severity of symptoms is related to the severity of the bleed with:
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Severe headache ‘as if hit on the head with a sledge-hammer’
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Nausea and vomiting.
The signs of subarachnoid haemorrhage are:
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Neck stiffness, positive Kernig’s sign (pain on passively extending the knee when the hip is flexed to 90º) – both are signs of meningeal irritation, which develop 6 hours after the bleed
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Focal neurological signs (particularly III nerve palsy in posterior communicating artery aneurysms)
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Drowsiness, depressed consciousness
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Retinal haemorrhages.
What is common with a sub-arachnoid haemorrhage
This may show frank blood or xanthochromia (indicative of previous blood in the cerebrospinal fluid).
Vessels in contact with CSF
What is meant by a blown lobe
The whole lobe is covered with blood in the sub-dural space- subdural haemorrhage
Describe the appearance of a sub-arachnoid bleed on the post-mortem
Everything is obscured by blood in the sub-arachnoid space
Difficult to treat as you need to get into the vessels
Describe brainstem (vertebrobasillar strokes)
cranial nerve palsies, ataxia, spinal tract symptoms, depressed consciousness Difficulty breathing. Difficulty speaking. Problems with chewing and swallowing. Partial or complete hearing loss. Blurred vision. Weakness of the limbs. Paralysis. Numbness or loss of sensation.
How can the full extent of neurological injury be evaluated
The full extent of neurological injury can be evaluated on subsequent magnetic resonance imaging (MRI) of the brain, which has better soft tissue resolution compared to CT. MRI is also useful for identifying strokes that may be too small to detect on a CT scan. MRI scans are produced by using complicated algorithms that create a series of images, also known as sequences. Various sequences can be obtained to assess different anatomical and physiological properties of the brain. A stroke, whether acute or chronic, will appear as a bright region on a sequence that is sensitive to fluid (T2 weighted)
Describe the additional diagnostic workup of stroke
Additional diagnostic workup of stroke includes hematological and biochemical blood tests to identify causes such as hypoglycemia or underlying clotting disorders. A toxicology screen may be useful to identify substance intoxication, which can mimic stroke.
What are the most likely causes of stroke in younger patients
In younger patients underlying clotting disorders, use of oral contraceptives, and illicit substance abuse (such as cocaine) are additional causes.
What should be done if a stroke is suspected
Following initial clinical history taking and neurological examination, all patients with suspected stroke should undergo urgent brain imaging with computed tomography (CT). This is to identify hemorrhagic strokes for which thrombolytic therapy is contraindicated and to exclude an alternative diagnosis such as malignancy. In ischemic stroke, early CT imaging may appear normal or can show a relatively darker area of low density that corresponds to the region of abnormal brain perfusion. Due to subsequent brain edema and swelling, the affected brain also loses its normal sulcal pattern (Fig. 8.40A). If thrombolysis is performed, a 24-hour follow-up CT scan is routinely carried out to evaluate for complications such as intracranial hemorrhage.
What is meant by the patient becoming ‘coned’
When a patient is “coned”, their brainstem falls through the foramen magnum and the become braindead.
