Anatomy clinical correlates: Anterior blood supply to the brain

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Introduction0:00–0:37

Your brain is awake and working hard all day every day, even when you're sleeping. So it makes sense that it needs a lot of oxygen and energy, which is why it is well supplied from several major arteries.
The circulation of the brain can ultimately be divided into the anterior and posterior circulation. And the understanding of their anatomy can help us understand the clinical consequences and management of various issues that can arise.
So, let's delve into the anterior circulation of the brain. Remember that the anterior circulation supplies the anterior portion of the brain and comes from the internal carotid arteries which divide into the anterior and middle cerebral arteries.

Circle of Willis0:37–1:40

The anterior circulation then connects to the posterior circulation through the posterior communicating arteries. The posterior circulation comes from the vertebral arteries which combine to form the basilar artery.
Together. The connection between the anterior and posterior circulation form the circle of Willis, which is an anastomotic network of arteries at the base of the brain, which ensure adequate blood flow to the brain.
Even in cases where part of this circulation becomes occluded. However, there are still instances where obstruction of these arteries and their branches disrupts blood flow to the brain causing a stroke which can lead to irreversible neuronal damage.

Stroke1:40–3:35

Now, a stroke can be classified as either ischemic or hemorrhagic ischemic strokes are much more common and they happen because of an acute blockage of one of the blood vessels supplying the brain ischemic strokes can be thrombotic embolic or hypoxic.
A thrombotic stroke occurs when there's a blood clot in the artery formed directly at the site of infarction, which typically occurs because of a ruptured atherosclerotic plaque.
An embolic stroke. On the other hand is where an embolism from another part of the body travels to the site of infarction to cause obstruction.
For example, with atrial fibrillation, a blood clot can form in the heart where it then travels through the circulation to eventually obstruct brain vessels.
Then there are hypoxic strokes where there is not a direct blockage of a vessel but systemic hypoperfusion or hypoxemia of the brain.
This can cause inadequate oxygenation of the brain, especially in watershed areas which are supplied by the terminal branches of two large vessels and are therefore more prone to hypoperfusion injuries, hemorrhagic strokes.
On the other hand, occur when there is a bleed within the brain tissue called an intracerebral or intraparenchymal bleed or a bleed in the subarachnoid space called a subarachnoid hemorrhage.
This happens most often as the result of chronic untreated hypertension and associated hypertensive vasculopathy. Other causes include amyloid angiopathy, a ruptured vascular aneurysm and vascular malformations.

Middle cerebral artery lesions3:35–7:58

Now, we can typically identify which artery of the anterior circulation is affected during a stroke based on clinical symptoms.
Let's start with the middle cerebral artery or MCA for short, which supplies most of the lateral cerebral cortex of the frontal parietal and temporal lobes, the insular cortex as well as a large portion of the basal ganglia and internal capsule depending on which portion of the MCA is occluded.
Clinical presentation can vary. Now, the MCA supplies a large portion of the primary motor and sensory cortex, which can be found along the precentral and postcentral gyrus respectively.
Looking at the motor homunculus, we can see that the MCA supplies the area for the face trunk and upper extremity. Therefore, lesions of the motor cortex cause weakness or paralysis of the contralateral side of the face and arm and eventual upper motor neuron signs such as hyperreflexia, looking at the sensory homunculus.
We can see that the MCA also supplies the face and upper extremity area because of this lesions of the sensory cortex produce loss of sensations in the contralateral side of the face and arm.
The MCA also supplies the posterior part of the inferior frontal gyrus. This area is in the dominant cerebral hemisphere which is the left hemisphere for right-handed people and right hemisphere for left-handed people and is where Broca's area is located.
Lesions of this area cause Broca's aphasia where individuals have difficulties planning and executing movements necessary for the production of speech while their comprehension of speech is not affected.
Furthermore, the MCA also supplies the posterior part of the superior temporal gyrus in the dominant hemisphere. This is where Wernickes area is located when injured.
It causes Wernicke's aphasia where individuals are fluent and may even speak faster than usual. But their comprehension and repetition of spoken and written language is impaired and their speech appears meaningless.
So in a righthanded individual presenting with Broca's or Wernickes aphasia, we would suspect a lesion to the left MC an MCA artery stroke can also affect other regions of the brain such as the frontal lobe which contains the frontal eye field in the middle frontal gyrus.
This is the center for voluntary control of eye movements and conjugate gaze to the contralateral side. So when injured, it causes both eyes to deviate towards the ipsilateral side as if they are looking at the lesion.
The frontal lobe also contains the prefrontal cortex and its injuries lead to frontal lobe syndrome where individuals experience personality changes such as problems with planning, initiative and judgment.
While the individual also exhibits socially unacceptable behavior. Next, lesions of the angular gyrus of the parietal lobe on the dominant cerebral hemisphere lead to Gerstman Syndrome, which presents with four key features left right disorientation, finger agnosia, acalculia and agraphia involvement of the parietal cortex on the non dominant cerebral hemisphere.
On the other hand, would cause hemispatial Neglect syndrome. This means that individuals have agnosia or inability to process sensory information of the contralateral side of the body and space around it.
Finally, the MCA supplies the subcortical area of the temporo parietal lobe which contains the optic radiation where depending on which hemisphere is affected.
A lesion here would result in contralateral homonymous quadrantanopia. Let's take a short break and see if you can remember the most common clinical syndromes associated with a middle cerebral artery stroke.

Quiz7:58–8:13

Ok. Now, let's switch gears and discuss the anterior cerebral artery or AC A for short, which supplies the medial aspect of the frontal and parietal lobes as well as anterior portions of the basal ganglia and internal capsule.

Anterior cerebral artery lesions8:13–9:40

The AC A supplies specific portions of the primary motor and somatosensory cortex. In particular the anterior and posterior paracentral lobule.
When we look at the motor and sensory houn coli, we can see that the AC A mainly supplies the areas for the lower extremities and genitalia.
Therefore, occlusion of the AC A leads to paresis or motor loss of the contralateral leg with eventual upper motor neuron signs such as lower limb hyperreflexia and positive babinski sign.
While lesions of the sensory cortex lead to sensory loss of the contralateral leg to make matters worse. The left and right.
AC A can sometimes get occluded at the same time causing bilateral damage to both hemispheres. Motor control to the urinary sphincters is also located in the anterior paracentral lobule.
So, bilateral lesions can result in urinary incontinence. Now, as we recall, the brain is contained within the rigid bony cranium which provides great protection but doesn't allow the intracranial space to expand.

Subfalcine herniation9:40–11:14

Because of this, any mass occupying lesion such as a hemorrhagic stroke or a tumor increases the intracranial pressure and pushes the brain tissue of the frontal parietal or temporal lobe away from the lesion in a setting like this, the cingulate gyrus of the affected cerebral hemisphere is forced under the falx.
Cerebri which separates the hemispheres leading to a subfalcine or cingulate herniation. As herniation progresses, the cingulate gyrus pulls the ipsilateral anterior cerebral artery together with it under the falx where it can be compressed or occluded.
This leads to infarction in the regions of the cerebral cortex supplied by the AC A and its characteristic clinical features which are hemiplegia and hemisensory loss of the contralateral leg.
Subfalcine herniation can also obstruct the foramen of Monro which allows passage of cerebrospinal fluid from the lateral ventricles to the third ventricle causing obstructive hydrocephalus or a buildup of cerebrospinal fluid in the brain.
Now, let's discuss a particular type of stroke known as a lacunar stroke which involves small blood vessels arising from the MCA and AC A called the lenticulostriate arteries.

Lacunar stroke11:14–13:12

The lenticulostriate arteries arising from the MCA are sometimes referred to as the lateral lenticulostriate arteries and the ones arising from the AC are sometimes referred to as the medial lenticulostriate arteries.
They penetrate the brain and supply deep subcortical structures like the striatum composed of the caudate and lentiform nucleus and the internal capsule.
These arteries are susceptible to injuries, secondary to uncontrolled hypertension. The high BP damages their endothelial cells and leads to the development of hyaline atherosclerosis, thickening the arterial wall which reduces their lumen size and blood flow.
Setting the stage for a lacunar stroke lesions that develop from a lacunar stroke, commonly affect the posterior limb of the internal capsule and damage the descending corticospinal and corticobulbar tracts.
This causes hemiplegia of the contralateral side of the body with eventual signs of upper motor neuron lesion such as hyperreflexia, hyperspasticity and positive babinski sign.
The thing that differentiates lacunar stroke from other types of strokes is the absence of cortical signs such as aphasia, visual field defects, hemineglect syndrome and others.
This is why lacunar strokes are sometimes referred to as pure motor strokes. Let's take one more break and see if you can recall the areas of the brain supplied by the anterior cerebral artery.

Quiz13:12–13:26

Now let's move on to the concept of watershed zones of the brain. A watershed zone is a cortical or subcortical area supplied by two major arteries.

Watershed zones13:26–15:38

For example, between the MCA and AC A or between the MCA and PCA watershed zones, get their blood supply from the terminal branches of both adjacent arteries.
So if there's systemic hypoperfusion or hypoxemia, these terminal branches cannot provide enough blood supply to this area.
This is why watershed zones are especially prone to infarction. Some of the most common causes of hypoperfusion include cardiovascular surgery and cardiac arrest which cause bilateral lesions or major arterial stenosis such as the internal carotid artery stenosis, which often causes unilateral lesions going back to the motor homunculus.
We can see that the cortical area for the proximal part of the upper and lower extremities is a watershed zone between the AC A and MCA.
Because of this infarction of the watershed zones between the AC A and MCA causes proximal muscle weakness of both upper and lower extremities, which is sometimes referred to as the man in a Barrel syndrome.
On the other hand, infarction of the watershed zones between the MCA and PC damages areas around the visual cortex that are in charge of processing visual information.
This presents as a higher order visual dysfunction such as prosopagnosia or the inability to recognize people's faces, visual agnosia or loss of the ability to recognize objects, alexia, or the loss of the ability to comprehend written language and achromatopsia, which is a form of colorblindness.

Epidural hematoma15:38–17:30

Finally, let's look at the different types of brain bleeds that can occur depending on where the bleed occurs in relation to the dura arachnoid and pia mater.
Brain bleeds can be divided into epidural subdural and subarachnoid hemorrhages. So let's start with the epidural hematomas which typically develop after injury of the middle meningeal artery following a traumatic event and possible skull fracture.
The most common site of fracture is the pterion, which is the thinnest area of the skull. It can be found in the lateral region of the skull where the frontal parietal temporal and sphenoid bones come together, sharp bone fragments can lacerate distal branches of the middle meningeal artery coursing in the area of the pterion after it enters the foramen spinosum to act as the major blood supply for the cranial dura mater.
Once ruptured the artery, then bleeds causing a fast growing hematoma to accumulate in the space between the outer or periosteal dura layer and the inner skull on a head.
Ct, the hematoma presents as a hyperdense biconvex or classical lens shape blood collection that does not cross the suture lines of the skull.
To remember that epidural hematomas are outside of the dura. Think that when you go outside, you need to wear sunglasses with lenses in them to protect yourself from the sun.
Individuals can sometimes present with a lucid interval where they experience an interval of no symptoms. But these hematomas can progress quickly in a matter of hours and be fatal.
Next up, there are subdural hematomas which can either be acute or chronic, acute injuries can develop following a traumatic event or a blow to any part of the skull.

Subdural hematoma17:30–18:48

This blow displaces the brain damaging the bridging veins found in the subdural space that drain the superior cerebral veins into the superior sagittal sinus.
Individuals that are at higher risk of developing subdural hematomas are elderly individuals and alcoholics. These people are prone to brain atrophy which shrinks the brain down, creating more space and putting more stress on longer bridging veins within the subdural space.
So even minor traumas can cause subdural hematomas. In Children, subdural hematomas are also a sign of shaken baby syndrome, which is an indicator of child abuse once damaged.
These veins bleed below the dura in the subdural space between the dura and Arachnoid. On a head ct, there is a crescent shaped hematoma that crosses the suture lines making it distinguishable from an epidural hematoma.
Finally, there are subarachnoid hemorrhages. These can occur after trauma, aneurysmal rupture or arteriovenous malformation with a ruptured aneurysm.

Subarachnoid hemorrhage18:48–19:44

Individuals may experience what's called a thunderclap headache. Typically described as the worst headache of their life.
The bleeding occurs between the arachnoid and pia mater in the subarachnoid space that is normally filled with the cerebrospinal fluid on a head ct, a subarachnoid hemorrhage can be seen as hyperdense areas of blood collected in the subarachnoid space which can be found in the major brain fissures and sulci along the falx, cerebri tentorium, cerebelli and falx cerebelli or the cisterns of the brain.
Ok. Final quiz.
Can you name all the arteries of the anterior circulation? All right, as a quick recap.

Quiz19:44–19:58

Review19:58–21:48

The anterior circulation supplies the anterior part of the brain. The MCA supplies the lateral side of the brain.
A stroke in this area usually leads to the contralateral hemiparesis and sensory loss of the face and arm. A stroke in the dominant hemisphere can also lead to Broca's or Wernickes, aphasia or Gerstman Syndrome.
While in the nondominant side, it leads to hemineglect syndrome. The AC supplies the medial side of the brain and when occluded, that causes contralateral hemiparesis and hemisensory loss of the leg.
When occlusion is bilateral, it also leads to urinary incontinence, subfalcine herniation is when the cingulate gyrus is forced under the false cerebri to the contralateral side of the brain.
And if severe enough, the AC can become compressed and occluded. A lacunar stroke involves the lenticulostriate arteries and leads to the hemiparesis of the contralateral side of the body and can manifest as pure motor stroke.
Lesions of the watershed zones between AC A and MCA lead to the proximal muscle weakness of the arms and legs. While lesions of those between MCA and PCA produce higher order visual dysfunctions.
Intracranial bleeds can also be categorized as epidural subdural and subarachnoid hemorrhages depending on their location.