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This is a 16-year-old child

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who was in a car accident, hit their head,

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and a head CT was performed to look for signs of

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intracranial traumatic injury.

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Fortunately,

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there were no signs of traumatic injury.

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No fractures, no bleeding or anything like that.

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But one thing that caught people's attention was

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this cystic appearing area here does not

0:22

look like an acute traumatic etiology,

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but it definitely was puzzling,

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and it resulted in a brain MRI being performed.

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An MRI of the brain was performed and shows a

0:37

normal development of the cerebellar vermis.

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We see the superior medullary velum,

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the inferior medullary velum,

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that little angle where they come together

0:46

is called the fastigium,

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and there's a normal fastigial angle.

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Overall,

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the cerebellar vermis looks on this mid-sagittal

0:55

image, like the video game character Pac-Man,

0:58

directed towards the brainstem.

1:00

So the cerebellar vermis is normal,

1:03

but there is prominence of CSF posterior

1:07

to the cerebellar vermis.

1:09

It's pushing slightly to the right,

1:12

the falx cerebelli.

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This is demonstrating hyperintense

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signal on T2-weighted imaging.

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It suppresses on FLAIR imaging.

1:25

It's hypointense on T1-weighted imaging,

1:29

and it demonstrates facilitated diffusion.

1:33

All of those other features help us know

1:36

that this is just an arachnoid cyst.

1:39

It's an arachnoid cyst behind

1:40

the cerebellar vermis.

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So, it's known as a retrovermian arachnoid cyst.

1:45

This is normal.

1:46

This is not a Dandy-Walker spectrum malformation.

1:49

This does not require imaging follow-up.

1:51

This does not require neurosurgical consultation.

1:54

This does not require consternation

1:55

on the part of the clinical team.

1:57

This does not require anxiety on the part of the patient.

2:00

This is a normal finding.

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Now, notice this retrovermian arachnoid cyst results

2:06

in slight elevation of the torcula.

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So, the position of the torcula alone actually is

2:14

not a feature to diagnose

2:18

or exclude a Dandy-Walker spectrum malformation.

2:21

This patient does not have a Dandy

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Walker spectrum malformation.

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This patient does not have any

2:26

pathologic abnormality.

2:28

They have a retrovermian arachnoid cyst,

2:30

which is a normal variant.