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Mechanisms of Brain Herniations

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I'd like to thank the medical illustrators,

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Paul and Lisa,

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here at MRI online for some of these diagrams that are going

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to nicely demonstrate the findings of brain herniation.

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On this initial scan,

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we see an intraparenchymal hematoma,

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which is in the right frontal lobe.

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This is a demonstration where we have

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the falx in the coronal plane.

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And what we see is a portion of the tissue,

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in this case the cingulum.

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The cingulate gyrus

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herniating across midline under the falx.

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This would be, by definition,

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subfalcian herniation or transfalcian herniation,

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where brain tissue, as you see,

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has herniated across the midline.

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So this is above and beyond just midline shift.

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You actually have tissue herniating across the midline.

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In this location.

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We're just above the Sylvian Fissure with

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this intraparenchymal hematoma.

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And so you see that this mass has

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impact on the medial temporal lobe.

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What this illustration is demonstrating is that

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the medial temporal lobe, this being the uncus,

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including the amygdala,

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has shifted over and caused narrowing of the adjacent

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sulcus compared to the contralateral side.

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So by this we would say that there is

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uncal herniation or uncal deviation.

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However,

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because of the size of this intraparenchymal hematoma,

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we note that brain tissue is herniating

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downward across the tentorium.

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This therefore fulfills the downward

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criterion

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for transtentorial uncal herniation.

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So it is crossing the tentorium and going downward.

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So these are all the different qualities and quantities

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and degrees of herniation midline shift leading to,

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in this case, some falsean herniation,

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transfalcian herniation and uncal deviation

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with transtentorial downward herniation.

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This is with this intraparenchymal hematoma.

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This is another nice diagram of the various

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types of herniation. In this situation,

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we have a herniation that I haven't mentioned before,

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and that is the transcranial herniation.

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This is an example where brain tissue is herniating outward

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from the calvarium where there has been an open fracture

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or wide fracture. So this would be our transcranial.

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Herniation very much like a meningoencephalocele,

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if you will.

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For congenital basis we see this large epidural hematoma.

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How do we know it's epidural?

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It's lenticular

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and the dura is displaced inward and we have an associated

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fracture and therefore likely injury

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to the middle meningeal artery.

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This is leading to transtentorial herniation with a facement

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of the adjacent cisterns from uncal

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herniating over and downward.

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And the same collection is leading to our subfalcian

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herniation with a portion of the cingulum herniating

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across midline from left to right.

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In this diagram we see the same epidural hematoma and this

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is demonstrating not just the transtentorial herniation but

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a herniation that was present also in the previous

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diagrams and that is tonsillar herniation.

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By tonsillar herniation we refer to the degree of descent of

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the tonsil through the foramen magnum

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greater than four to 5 mm.

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Now,

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if there is asymmetry from right to left with the side

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showing the hemorrhage having greater downward herniation

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than the other we will describe it

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even if it's less than 5 mm.

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But in this situation what we see is the tonsil herniating

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downward and impacting the lower medulla.

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Usually this occurs with posterior fossa hemorrhages.

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This is a diagram that is a little bit artificial in

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claiming that this epidural hematoma by virtue of its size

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had enough pressure that it led to tonsillar

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herniation through the foramen magnum.

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That would be unusual.

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Here we have a more typical example of tonsillar herniation.

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We have a collection of blood which,

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as you can see is deep to the dura and therefore must be a

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subdural hematoma and it's compressing the cerebellum

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from right to left as well as downward.

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So this is a more typical scenario where we have tonsillar

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herniation downward through the foramen magnum

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and right to left shift of the medulla.

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In this situation, as you can see,

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it may sometimes put pressure on the posterior inferior

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cerebellar artery and potentially lead to its narrowing

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and occlusion which could create an infarction.

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This is a more typical scenario for tonsillar herniation

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than that in which we see a supratentorial collection in

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this. Case the epidural hematoma displacing tissue downward.

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This is an old slide,

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but I do want to emphasize two things

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with regard to this slide.

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We see that the patient has

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abnormality in the midbrain from an acceleration

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deceleration injury and that midbrain has banged up against

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the 1000 and oral edge leading to the midbrain injury.

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However,

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we see findings suggestive of

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an infarction in the left posterior cerebral artery.

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Left and right posterior cerebral artery infarctions are a

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risk factor when one has uncal or transtentorial

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herniation of temporal lobe tissue.

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The posterior cerebral artery may be compressed by the

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downward herniating or medially herniating temporal lobe and

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that can lead to an occlusion of either the right or the

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left or portions of those blood vessels

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and lead to infarction. Again,

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these are all secondary effects of the trauma,

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not the primary injury.

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This is secondary to the herniation that can occur.

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This is another example of a patient who had trauma

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and had secondary infarction. As you can see,

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the patient has left greater than right areas of infarction

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involving the parietal lobe. In this case,

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the abnormality was secondary to involvement of the

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posterior cerebral arteries superior branches

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involving the parietal lobe. However,

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we can see sometimes anterior cerebral artery

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infarctions from the subfalcian herniation

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subfalcian herniation. Remember,

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we have that diagram of the brain in coronal imaging

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with subfalcine herniation of brain tissue.

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This subfalcine herniation can lead to compression of

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anterior cerebral artery and therefore lead to

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an anterior cerebral artery infarction.

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So,

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usually posterior cerebral artery is due

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to uncal or transtentorial herniation.

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Anterior cerebral artery is usually due to subfalcine

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herniation and posterior inferior cerebellar artery

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infarction is usually due to the downward

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tonsillar herniation.

Report

Description

Faculty

David M Yousem, MD, MBA

Professor of Radiology, Vice Chairman and Associate Dean

Johns Hopkins University

Tags

Vascular Imaging

Vascular

Trauma

Temporal bone

Spine

Skull Base

Non-infectious Inflammatory

Neuroradiology

MRI

Head and Neck

Emergency

Brain

Bone & Soft Tissues

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