CSF Flow Anatomy Flashcards
What does the carotid canal receive?
the internal carotid artery from the neck
Where does the carotid canal lie?
in the pyramid-shaped PETROUS part of the temporal bone
edges of carotid canal are smooth and rounded
What is the difference between ‘lacerated’ and ‘incised’?
lacerated = torn (with blunt force)
incised = cut (with sharp object)
Why might damage to the wall of the internal carotid artery cause Horner’s syndrome?
damage to the internal carotid (sympathetic) plexus
Why might an internal carotid aneurysm cause lateral rectus palsy?
the abducens nerve (CN VI) travels forward in the cavernous sinus with the internal carotid artery (ICA)
(other nerves travel in the lateral wall)
Abducens is therefore more vulnerable to compression due to ICA aneurysm
this can cause denervation of the ipsilateral lateral rectus muscle
=> diplopia, worse on far LATERAL GAZE of affected eye
What proportion of blood flow does the middle cerebral artery receive?
receives 80% of the internal carotid blood flow
What do the branches of the middle cerebral artery supply?
UPPER DIVISION
supplies frontal and parietal lobes
LOWER DIVISION
passes downwards to supply the temporal lobe
Why might a middle cerebral artery stroke cause paralysis of the contralateral face and upper limb but spare the lower limb?
the leg (lower limb) area of the primary motor cortex gives rise to the corticospinal cord
this is located MEDIALLY (paracentral lobule)
supplied by the anterior cerebral artery (ACA)
Areas for ‘arm’ (upper limb) and ‘face’ (cranial nerve/bulbar fibres) are supplied by the middle cerebral artery (MCA)
Area originates in the cerebral convexity
What is the most likely cause of a stroke affecting the middle cerebral artery territory?
most strokes are ischaemic (85%) rather than haemorrhagic (15%)
Ischaemic strokes are most commonly caused by an EMBOLUS coming from the heart or atherosclerotic neck vessels
What is the mechanism of AF increasing stroke risk?
AF = atrial fibrillation
mural thrombus originates from the left atrial appendage (blood sits and can easily form clots)
common cause of stroke
especially since 10% over the age of 75 have AF
What is the mechanism for clot formation in coronary circulation?
myocardial infarcts are also occlusive in origin
but most commonly caused by IN SITU THROMBUS
e.g. due to unstable atherosclerotic plaque then vascular occlusion from the displaced thrombus and downstream MI
What is the smaller branch off of the internal carotid that passes superiorly and medially into the longitudinal fissure?
anterior cerebral artery
receives 20% of the blood from the internal carotid blood flow
Where is the ACA in relation to the optic chiasm?
ACA = anterior cerebral artery
ACA sits above the optic chiasm
What is the point where the 2 anterior cerebral arteries connect called?
the anterior communicating artery
part of the circle of Willis
What is the branch of the ACA that winds around the corpus callosum?
pericallosal branch of the ACA
[other branches fan out over the medial surface of the hemisphere]
What does the ACA supply?
medial surface of the brain as far back as the deep parieto-occipital sulcus
What supplies the brain posterior to the deep parieto-occipital sulcus?
posterior cerebral artery (PCA)
supplies the posterior region = medial surface of the occipital lobe
Why might an anterior cerebral artery (ACA) stroke cause paralysis of the contralateral lower limb but spare the upper limb and face?
ACA supplies medial part of the motor strip (paracentral lobule)
this contains the primary motor cortex (controls contralateral lower extremity)
upper limb and facial areas of motor cortex supplied by MCA
Would you expect an anterior cerebral artery (ACA) stroke to cause a contralateral visual field defect (hemianopia)?
No
ACA does not supply the visual cortex or the central visual pathways from the lateral geniculate nucleus to the calcarine sulcus
Why might a MCA stoke cause a visual field defect?
can interrupt the optic radiations as they sweep down through the temporal and up through the parietal lobes to reach the visual cortex
Would an embolus travelling up the internal carotid artery be more likely to enter the ACA or MCA?
~80% of the ICA blood flows laterally into the MCA (bigger diameter)
other 20% flow passes anteriorly and medially to enter the ACA
therefore more likely for embolus to enter the MCA
Where do the paired vertebral arteries arise from?
subclavian arteries in the neck
Where do the vertebral arteries pass in the cervical vertebrae?
pass through the upper 6 cervical transverse foramina
not C7
What is the course of the vertebral arteries?
ascend the cervical vertebra roughly parallel to the carotid vessels
enter skull via the foramen magnum
they then unite to form the basilar artery
Why might a violent flexion-extension neck injury to the neck lead to a brain stem stroke?
by tearing/damaging the vertebral artery lining as it passes through the foramina transversaria to ascend the neck
may trigger local vessel thrombosis -> occlusion -? could cut off posterior circulation supply
What is the medical term for a tear in an arterial wall?
arterial dissection
DISS-SECTION not DIE-SEKSHUN
In what age group is arterial dissection more common?
more likely to occur in a younger patient <50
in those who lack the usual risk factors for stroke
however, arterial dissections are generally unusual
When may vertebral arteries be absent?
if brain stem transection is high
Where does the basilar artery ascend?
in the basilar groove of the pons
In life, where does the basilar artery lie?
rests on the sloping base of the skull (known at the CLIVUS)
Where is the terminal bifurcation of the basilar artery?
at the upper border of the pons
here it divides into the 2x posterior cerebral arteries (PCA)
Where do the posterior cerebral arteries travel?
wind around the midbrain
supply the inferior surfaces of the cerebral hemispheres
also supply the medial surface of the occipital lobe
Where do the posterior communicating arteries pass?
between the internal carotid artery and posterior cerebral artery on each side
(these vessels will form part of the circle of Willis)
Why is thrombosis of the entire basilar artery likely to be fatal?
loss of brain stem blood supply
will interfere with the vital cardiorespiratory centres
What visual field defect would you expect with occlusion of the distal basilar artery?
If a saddle embolus/thrombus lodges in the crotch of the distal basilar artery where it bifurcates
this can cut off the entire blood supply to both posterior cerebral vessels
=> ischaemia of both occipital lobes
=> total occipital infarction
=> cortical blindness
however, if one/both posterior communicating arteries is large, then it may compensate by receiving retrograde flow from the ICA
(perfusion of occipital cortex)
Where does the superior cerebellar artery (SCA) reside?
superior cerebellar artery (SCA) sits immediately below the posterior cerebral artery (PCA)
Which cranial nerve emerges between the SCA and PCA?
oculomotor nerve (CN III)
SCA = superior cerebellar artery
PCA = posterior cerebral artery
Where doe the anterior inferior cerebellar artery (AICA) arise from?
usually from the lower part of the basilar artery
Where does the posterior inferior cerebellar artery (PICA) usually arise from?
from the vertebral artery on each side
What clinical features would you expect following a large cerebellar haemorrhage?
usually cerebellar features e.g. ataxia on ipsilateral side
also, space-occupying effect of haematoma (plus oedema + swelling) would cause increased ICP
likely to be exacerbated by compression of the 4th ventricle, leading to outflow obstruction and acute hydrocephalus
If a haemorrhage affected the left cerebellar hemisphere, on which side of the body would you expect to see clinical signs?
Ipsilateral (left)
cerebellar hemispheres are “uncrossed”
Why might left cerebellar hemisphere haemorrhage be an emergency?
posterior fossa mass
=> swelling/oedema
=> secondary acute hydrocephalus
=> may lead to tonsilar herniation
=> respiratory depression/death
emergency surgical evacuation of the haematoma may therefore be required
Where does the blood supply to the brain stem arise?
from the posterior circulation
What kind of vessels are the cerebellar arteries?
long circumferential arteries
SCA
AICA
PICA
Why might occlusion of the pontine paramedian arteries cause paralysis of all 4 limbs from the neck down (quadriplegia)?
destruction of the corticospinal (pyramidal) tracts
these descend in the basal/anterior pons
corticobulbar innervation to the lower cranial nerves (e.g. tongue, lateral rectus) would probably also be affected
Why kind of eye movements may be spared/affected in an infarction of the pontine paramedian arteries? What is this clinical picture known as?
VERTICAL EYE MOVEMENTS
- vertical gaze centre in in the midbrain
- its corticobulbar innervation has already reached its target about the level of the lesion (basal pons)
LATERAL EYE MOVEMENTS
- likely to be affected
- lateral gaze centre and nucleus of the abducens (CN VI) nerve is in the pons
Clinical picture: Locked-in syndrome
What supplies the deep grey area of the brain?
central perforating vessels
Where do the lenticulo-striate arteries arise from?
middle cerebral artery
What are the boundaries between the 3 arterial territories known as?
arterial watersheds
What is the ‘triple watershed area’ of the brain?
most vulnerable region for where ACA, MCA and PCA converge in the parieto-occipital region
posterior to the lateral ventricles
Why might cardiac arrest lead to a stroke at the boundary zone between the territories of 2 major cerebral arteries (watershed infarct)?
profound arterial hypotension and relative low pressure/perfusion during resuscitation
core vascular territories might just be adequately perfused, but the peripheries of vascular territories and regions between 2 supplies will receive the least blood at the lowest pressure
=> critical ischaemia and infarction
=> “watershed” areas
Why might someone suffer a watershed infarct after being stung by a wasp?
anaphylactic shock
Which vascular territories would be affected by a complete occlusion of the left internal carotid artery?
Middle cerebral artery (MCA) and anterior cerebral artery (ACA) territories arise from the ICA
ICA occlusion -> total anterior circulation syndrome (TACS)
this carries a >60% risk of death
What are the dural venous sinuses?
valveless spaces between 2 layers of dura lined by endothelium
occur at the free and attached margins of the dural folds
Where is the superior saggital sinus?
in the attached margin of the FALX CEREBRI
major sinus: receives blood from the superior surfaces of the hemispheres
Where is the inferior saggital sinus located?
in the free margin of the FALX
drains into the straight sinus (between falx and tentorium cerebelli)
straight sinus receives blood from deep cerebral veins
Where are the transverse sinuses?
in the attached margins of the tentorium cerebelli
Where does the superior saggital sinus drain?
into the RIGHT TRANSVERSE SINUS
Where does the straight sinus drain?
into the LEFT TRANSVERSE SINUS
Where do the transverse sinuses drain?
they become the S-shaped sigmoid sinuses
which then discharge into the bulb of the internal jugular vein on each side
Why might thrombosis of the superior saggital sinus cause extensive haemorrhage over the surface of both cerebral hemispheres?
failure to drain superficial cerebral veins
leads to over-filling, confession and increasing back-pressure in the superficial cerebral venous bed
may ultimately lead to rupture with extensive superficial venous haemorrhage over both cerebral hemispheres
What are the main risk factors for venous sinus thrombosis?
- dehydration
- malignancy
- infection
- pregnancy
- OCP
- clotting abnormalities (e.g. factor V Leiden etc)
Why might meningioma be very difficult to remove surgically if it is close to/surrounding a major venous sinus?
it is liable to bleed
like a stuck pig
What is a meningioma?
a benign tumour that arises from the meninges
Where are the arachnoid granulations located?
on either side the longitudinal fissure
will correspond to an arachnoid villous projecting into the superior saggital sinus
Where is the CSF absorbed into the ventricular system?
at the point at which arachnoid villi project into the superior saggital sinus
Where is CSF produced?
in highly vascular choroid plexuses
What are the main structural horns of the lateral ventricle?
- frontal
- temporal
- occipital
Where is the cerebral aqueduct?
between the third and fourth ventricles
Where in the interventricular foramina?
[Foramina of Monro]
located between the lateral and third ventricles
How many openings for CSF are there in the fourth ventricle?
3 openings:
1 medial: FORAMEN OF MAGENDIE
2 lateral: FORMANINA OF LUSHKA
4th ventricle is diamond shaped btw
What would happen if the foramina in the 4th ventricle were occluded?
this would block outflow from the ventricular system
CSF would be continually produced by choroid plexuses with no means of escape
=> dilation of entire ventricular system
=> raised ICP
=> papilloedema (optic disc swelling)
=> acute hydrocephalus (sharp increase in ICP)
=> tonsillar herniation and death
can be managed neurosurgically: insertion of external ventricular drain or ventriculoperitoneal (VP) shunt
