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This is an MRI of the brain

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in a nine-month old child with developmental delay.

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And on this sagittal image,

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there's a few things that we can notice.

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First of all, we do not see any corpus callosum.

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But secondly,

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unlike the prior case of agenesis

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of the corpus callosum,

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we're seeing a very high positioning of the

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superior aspect of the third ventricle.

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We're seeing cerebrospinal fluid

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all the way up in the interhemispheric fissure,

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which normally you don't see.

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If we look at this on the axial image,

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we can see there's a cystic structure associated

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with the superior aspect of the third ventricle.

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It's actually displacing some

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of the vessels here,

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the distal branches of the anterior cerebral artery,

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and we can go all the way to the top.

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We do not see an intervening corpus callosum.

0:46

Similar to the other case of agenesis

0:48

of the corpus callosum.

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We have a parallel configuration of the

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body of both lateral ventricles.

0:53

We have enlargement of the atria and occipital

0:56

horns of both lateral ventricles,

0:57

known as colpocephaly.

0:59

On a coronal image,

1:00

we see the vertically oriented third ventricle

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with direct continuity with the

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interhemispheric fissure.

1:06

We see an atypical configuration between the

1:09

third ventricle and the lateral ventricles,

1:11

sometimes referred to as the Texas longhorn sign.

1:14

We see here on this case,

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because the patient is young

1:18

and the white matter is not fully myelinated,

1:21

we actually,

1:21

along the medial margin of the body

1:23

of the parallelateral ventricles,

1:25

see an area of dark white matter.

1:29

This here is very subtle, early myelination,

1:34

resulting in a T2 hypointense appearance

1:36

within the probes bundles.

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The probes bundles are anterior-posteriorly

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directed fibers that are the fibers that would

1:43

have crossed and become commissural fibers

1:46

crossing the corpus callosum,

1:48

but they stay here in this location.

1:50

So, these are the probes bundles.

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This is a vertically oriented third ventricle

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with an interhemispheric cyst.

1:57

Agenesis of the corpus callosum is often

1:59

associated with an interhemispheric cyst.

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It's unclear if it's truly an arachnoid cyst or

2:06

some people have referred to this as cystic

2:09

meningeal dysplasia related to the

2:11

abnormal development.

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Either way,

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it forms as a cyst and occasionally can enlarge

2:18

and may require shunting or fenestration.