Brain Blood Supply and CVA

Brain Blood Supply and CVA

• A stroke happens when blood flow to the brain is interrupted.
• Interruption of blood flow can be due to a blockage from occlusive stroke (85% of cases) or bleeding from haemorrhagic stroke (15% of cases)

Blood Supply

• The common carotid artery (originating from the thorax) divides into internal and external carotid arteries
• The internal carotid artery goes into the skull through the temporal bone before branching into the anterior cerebral artery, middle cerebral artery, and posterior communicating artery
• The vertebral artery goes up the neck through cervical vertebrae
• The vertebral arteries go into the skull through the foramen magnum and fuse into the basilar artery, which leads to cerebellar arteries and posterior cerebral arteries
• The Circle of Willis is an arterial circle around the pituitary gland that allows blood to be distributed to the brain
• Blockage of some blood supply may be compensated as a result of the circle of willis

Venous Drainage

• Venous sinuses are located in dura mater
• Sinuses converge at the back of the skull to the sigmoid sinus on each side
• Sigmoid sinus moves to the internal jugular vein

Blood-Brain Barrier

• Most brain capillaries are "tightly sealed", preventing many substances from crossing freely from the blood into the brain's intercellular fluid
• Blood gases, water, alcohol, and anaesthetics move freely through the barrier
• Many antibiotics and drugs do not pass through
• Glucose and amino acids need proteins for their carriage
• The blood-brain barrier is less effective in newborns
• Brain inflammation reduces the effectiveness of the barrier because capillaries become "leaky"

Intracranial Pressure

• Within the skull, the brain occupies 80%, blood occupies 12%, and CSF occupies 8%
• The total volume within the skull is 1600ml
• The skull is a rigid box
• Fluid is equalized initially but the pressure will rise if the volume of the contents increases
• Intracranial Pressure (ICP) is normally 5-13 mmHg
• Intracranial pressure has limited capacity

Brains and Blood

• The brain weighs roughly 1.5 kg, about 2% of body weight
• The brain requires continuous supply of oxygen (20% of body's amount) and glucose (70% of blood glucose)
• The brain also removes H+ produced by metabolism
• The brain has no anaerobic ATP production capacity
• The brain does not store fuel like glucose
• Deprival of oxygen/glucose for a few minutes leads to irreversible damage

Cerebral Blood Flow

• The brain needs a steady blood flow of 1000mL per minute, about 20% of cardiac output
• Arterioles in the brain automatically dilate to increase brain flow if systematic blood pressue falls, and rise of blood pressue causes constriction
• Local increase in CO2 in a particular region causes vasodilation, increasing flow to carry away the CO2

Defining Stroke

• Stroke, also labelled Cerebrovascular Accident (CVA), is a sudden onset of a focal (or sometimes global) neurological impairment that lasts more than 24 hours (or leads to death) and is presumed to be of vascular origin.
• CVA is the rapidly developing loss of brain functions due to a disturbance in blood vessels supplying blood to the brain, caused by thrombosis, embolism, or hemorrhage
• Infarction is the term for tissues that have died due to failed blood supply
• Liquefactive necrosis is the process the brain undergoes when part of it dies
• When blood supply is spontaneously restored before tissue death occurs, and recovery is complete, the event is a transient ischaemic attack (TIA) (aka "mini stroke")
• Signs and symptoms of stroke or TIA depend on which areas of the CNS have been deprived of blood

Risk Factors for Stroke

• Increasing age
• Hypertension
• Heart disease
• Atherosclerosis
• High blood cholesterol
• Hypercoagulability
• Diabetes mellitus
• Smoking
• Oral contraceptives
• Pregnancy
• Cocaine use

Transient Ischaemic Attack (TIA)

• Blood flow occlusion temporarily reduces blood flow to part of the brain (like ischaemic stroke) for less than 24 hours
• Occurs in an eye, causing fleeting blindness on one side
• The occlusion clears itself before brain tissue has died
• Damage may be shown on MRI after TIA
• Is an increased risk of future stroke

Occlusive Stroke

• Thrombosis is obstruction of vessels from blood clot forming locally, arising from atherosclerosis, usually diabetes or HTN
• Embolism is obstruction from a blood clot from elsewhere, particularly the heart (atrial fibrillation, heart valve disease)
• Obstruction/failure of arterial blood flow is caused by venous thrombosis or shock

Blood Clots in the Brain

• Clots can form in large or small blood vessels
• In large vessels, clots can form in the carotid arteries, vertebral arteries, or the Circle of Willis
• In small vessels, clots can form in the arteries within the brain itself
• Most common sites of thrombotic occlusion are branch points of arteries
• Emboli commonly originate from the heart, especially in atrial fibrillation

Location and Impact of Clots

• The affected area depends on the distribution of the artery blocked
• Arterial anastomoses (direct arterial connections, as in the Circle of Willis), facilitate blood flow to some areas even when the main artery is blocked
• The cortex (outer layer) can undergo the process due to arterial anastomes, whereas the internal structures (ie basal nuclei) do not due to a lack of arterial anastomes

Impacts of Ischaemic Tissue

• ATP Depletion
• Membrane ion transport systems stop functioning
• Depolarization of neurone
• Influx of calcium
• Release of neurotransmitters, including glutamate, activates N-methyl-D-aspartate and other excitatory receptors on other neurones
• Further depolarisation of cell
• Further calcium influx

Ischaemic Penumbra

• Within the ischaemic area, there are the core ischaemic zone and the ischaemic penumbra
• The penumbra defines ischaemic yet still viable cerebral tissue
• In the core zone, severe ischemia can result in necrosis in neurones and glial cells
• Penumbra has a rim of mild to moderately ischaemic tissue
• Brain cells within the penumbra may remain viable for several hours.
• the penumbral zone is supplied with blood by collateral arteries anastomosing with branches of the occluded vessel.
• The extent of the penumbra varies directly with the number and patency of collateral arteries
• Cells will die if blood does not resume in hours due to insufficient circulation
• Pharmocologic interventions are most effective in the penumbra

Necrosis

• Severe ischaemia causes neurones and glial cells die, leading to liquefactive necrosis
• Cells breakdown by their own enzymes, causing tissue to be replaced w/ cerebralspinal fluid
• Astrocytes proliferate at the margin of the cavity to line it, and this process takes months during large infarcts

Types of Oedema

• Cytotoxic oedema is swelling of all cellular elements of affected area, occurring minutes to hours after, caused by hypoxia that leads to failure of ATP-dependent transport
• Vasogenic oedema is the increase of interstitial fluid volume, occurring hours to days after, because of increased permeability of small vessels because of inflammatory response from cell damage

More on Oedema

• Cerebral oedema peaks about 2-5 days after stroke, followed by stabilization and lessening
• Recovery of function post-stroke involves resorption of Oedema
• Oedema increase life-threatening intercranial pressure

Herniation

• Occurs whenever pressure rises inside the skull and displaces brain tissues
• Can occur through the opening at base of skull (occipital foramen)
• Can occur in compartments like the one separated by the tentorium
• Rapidly leads to death because of compression of blood vessels as those in the respiratory centre

Stroke in Evolution

• Thrombosis can take minutes, hours, or days to evolve
• Stroke is actively progressing as a result of increasing occlusion and ischaemia
• Larger blood vessels (carotid, middle cerebral, or basilar arteries) can take longer to become occluded than a smaller vessel and there may be warning signs, like transient ischemic attacks

Haemorrhagic Stroke

• 10-15% of strokes, with higher mortality than ischaemic strokes
• May occur within brain or surrounding menigeal spaces
• Cannot distinguish between ischaemic and haemorrhagic stroke based on history
• More likely when patient is generally more ill
• Presentation includes raised intracranial pressure symptoms via nausea, vomiting, and headache

Haemorrhagic Stroke, continued

• Seizures are more common compared to ischaemic strokes
• Symptoms of meningeal irritation may result from blood in the ventircles
• Haemorrhage is more acute than infarct from same sized
• When hematoma is resorbed, tissue generally retains much function
• Less severe long term disabilities compared to infarct

Haemorrhage

• Results in tissue injury
• Compression of expanding haematoma to distory and injure surrounding tissue, increasing pressure and reducing blood flow
• Increase intercranial pressure with possibility of herniation
• Blood emptys into ventricular system or the pial system from hematoma
• Blood in the ventricles is noxious and painful when in the subarachnoid space

Basal Ganglia Area and Haemorrhage

• Haemorrhages in area of basal ganglia is contributed by hypertension
• Hypertension damages and weakens the penetrating arteries
• Mass effect in midline shift
• Secondary oedema can lead to herniation

Subarachnoid Haemorrhage

• A "berry" aneurysm bursts, and blood erupts into the subarachnoid space
• This causes excruciating pain, followed by neurologic problems
• Vasospasm occur, producing additional cerebral damage
• High chance of death or rebleeding
• Subarachnoid haemorrhage from ruptured aneurysm more of irritant producing vasospasm than mass lesion

Aneurysm

• Defect in arterial wall is present from birth and considered "congenital"
• Aneurysms take years to develop and rupture in middle age, commonly in the Circle of Willis

Presentations of Acute Stroke

• Onset of focal neurological symptoms and signs, and patient was well beforehand

Altered Consciousness

• Stupor or coma
• Confusion or agitation
• Memory loss
• Seizures
• Delirium
• Usually intense or unusually severe

Headache

• Associated with decreased level of consiousness or neurological deficit
• With unusual/severe neck or facial pain
• Incoherent speech
• Difficulty understanding speech

Other Presentations

• Facial weakness or asymmetry
• Incoordination, weakness, paralysis, or sensory loss (usually one half of the body and particularly the hand)
• Ataxia, via poor balance, clumsiness, or difficulty walking
• Visual loss, monocular or binocular with partial loss of field
• Intensive vertigo, double vision, unilateral hearing loss, nausea, vomiting, photophobia

Common Stroke Patterns

• Left Hemisphere Strokes result in aphasia, rightsided issues (hemiparesis and sensory loss), right visual field defect, and poor right conjugate gaze, dysarthria and difficulty with reading and calculating
• Right Hemisphere Strokes result in a neglect of left visual field, extinction of left sided stimuli, left hemiparesis and sensory loss, left visual field defect, poor left conjugate gaze, dysarthria and difficulty with spatial orientation
• Brain Stem, Cerebellum Strokes result in loss in all 4 limbs, limb or gait ataxia, dysarthria, dysconjugate gaze, nystagmus and amnesia
• Subcortical Hemisphere Stroke results in weakness of face and limbs

Head Injury

• Classified as closed or open
• In closed head injuries, brain trauma usually occurs from blunt trauma and presents as diffuse axonal injury or focal brain injury, while the skull is still intact
• In open head injuries, penetrating trauma occurs, the dura mater in broken, causing focal brain injury and skull penetration from eg bone fragments or a foreign body

Head Injury, Continued

• Damage occurs from primary, secondary, and tertiary injury
• Primary injury occurs from acceleration/deceleration, direct impact and damage
• Can affect neurons, glia, and blood vessels
• Seconary injury factors include cerebral oedema, haematoma, increased intercranial pressure and infection
• Tertiary injury is from apnoea, hypotension, and respiratory and cardiovascular effects from primary and secondary injury

Intercranial Pressure

• Increase in content like haematoma, and inflammation causes increase in pressure
• Damage to blood vessels
• Arterial bleeding can rapidly bring on injury hours
• Venous bleeding can be delayed for days/weeks
• Signs when injury occurs
  • Arterial Bleeding
    • High risk of rebleed
  • Venous Bleeding
    • Common to occur during infanthood or at elderly stage

Effects of Increasing Intracranial Pressure

• Decreating levels of consiousness by hindering pressure on RAS in Brainsteam or cerebral vortex
• Stretching wall of blood vessels can result in headache
• Pressure of vomitting can act on the emetic center in the medulla
• Increasing pulse pressure with intercranial pressure
• Cushing's reflex can systemically vasoconstrict
• Increase of CSF pressure, causes optic disc to swell

General Indicators of intercranial pressure

• The brain has compliance to a point
• Some increase of CSF will be accommodated, but not too much, otherwise intracranial pressure will rise

Increase the Intercranial pressure

• Compensation of increase in systolic, pulse, and diastolic pressure
• Resperation increases and lessens with time

Progression of injury

• Level of consiousness can vary by extent of injury, medical intervention, fever and environment.
• Deterioration will increase from the fundamental reflexes of the brainstem

Common Factors Related to Brain injuries

• Respiratory rate changes
• Important to assess severity of injury from Glasgow Coma Scale -Standard Stimuli -Score 3-15, 3 is the worst, 15 is the best

Key Clinical Management

• May require sedation, like coma patients
• Oxygen, steroids, and diuretics are the standard approach, as well as ventilation

More on Amnesia

• Loss of memory around time of trauma
• Agitation, disorientation, and decreased cognitive ability and attention are major signs
• Duration is indicative to the level of injury
• Can be rated as "very mild injury" after 5 minutes, moderate to 24 minutes, or may be greater than 7 days