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Middle Fossa Arachnoid Cyst

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This is an MRI scan

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in a seven-year-old child with headache.

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And at the level of the body

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of the lateral ventricles,

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there's normal symmetry between both cerebral

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hemispheres, and everything looks okay.

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If we go inferiorly,

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we see this prominent area of bright

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signal on T2-weighted imaging.

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If I go anteriorly on the coronal STIR,

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we also see the anterior aspect of the left

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middle cranial fossa is filled with fluid

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and is also actually,

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if we look at the transverse dimension of the

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anterior aspect of the left

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middle cranial fossa,

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it's slightly larger than the anterior aspect

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of the right middle cranial fossa.

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Enlargement of an area

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is typically a sign of a chronic process.

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If we look at FLAIR imaging,

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we can see that the area that was bright

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on T2-weighted imaging is dark.

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This confirms that we have pure fluid signal.

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On T1-weighted imaging,

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we can see the posterior margin

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of this fluid collection.

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There's brain parenchyma with gray matter.

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This is important because if there was a prior

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stroke involving the anterior aspect

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of the left temporal lobe,

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then we would not have a margin of

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gray matter at the interface.

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The gray matter would have been injured,

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and you would go straight to white matter.

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So this is a sign that this is not

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an acquired injury to the brain.

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Diffusion-weighted imaging shows that this area

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has no restricted diffusion.

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At this point,

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we can be fairly confident that this finding in

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the anterior aspect of the left middle

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cranial fossa is an arachnoid cyst.

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It is bright on T2-weighted imaging,

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it shows suppression on FLAIR imaging,

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it shows absence of diffusion restriction,

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and it does not...

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we do not see any signs of injury

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to the subjacent brain.

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In contrast,

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if we saw bright signal

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on diffusion-weighted imaging,

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that would be suggestive of

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this being an epidermoid.

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Diffusion is always important to look

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at on a suspected arachnoid cyst

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because an epidermoid cyst can look like

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an arachnoid cyst on regular imaging,

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it can be bright on T2-weighted imaging,

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and it can actually suppress on FLAIR imaging.

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While there is no signs of injury

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to the left temporal lobe,

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the space-occupying aspect of this arachnoid

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cyst does result in slight hypoplasia

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of the left temporal pole.

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It also results in slight bony expansion of

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the anterior aspect of the cranial fossa.

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We can see on the right,

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a normal greater sphenoid wing.

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We can see the contour along the anterior aspect

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of the middle cranial fossa.

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On the left side,

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this is bowed anteriorly,

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in contrast to the margins of the right greater

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sphenoid wing.

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The left looks smaller.

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That's because there's been bony remodeling from

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chronic pressure from this arachnoid cyst.

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This is chronic low pressure, not high pressure.

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This is important to notice because these findings,

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if they can confirm an arachnoid cyst,

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indicate this is an incidental finding.

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Arachnoid cysts very rarely cause symptoms

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and very rarely require intervention.

Report

Description

Faculty

Asim F Choudhri, MD

Chief, Pediatric Neuroradiology

Le Bonheur Children's Hospital

Tags

Pediatrics

Neuroradiology

MRI

Congenital

Brain

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