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This is an eleven-year-old child with a history

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of myelomeningocele of the lumbosacral spine

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at birth that was repaired.

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So, therefore, there's a known

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Chiari type II malformation,

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and we can see some of the posterior fossa

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abnormalities here that we'll

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go into in more detail.

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But while we're talking predominantly about

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posterior fossa malformations,

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we really need to also discuss the supratentorial

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abnormalities that go along with

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the Chiari Type II malformation.

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We can see that there's posterior

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corpus callosal dysgenesis.

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The rostrum and genu are slightly thinned,

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but the body of the corpus callosum,

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as we go posteriorly, thins out.

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And we can barely see the

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isthmus and splenium of the corpus callosum.

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We have a large massa intermedia.

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The massa intermedia,

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or interthalamic adhesion,

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is typically large in Chiari Type II malformations.

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If we look at this axial image,

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we can see this prior occipital predominant

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white matter volume loss.

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We also can see right here,

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there's a ventriculostomy catheter,

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because here's a ventriculostomy catheter in

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the left lateral ventricle and another

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ventriculostomy catheter

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in the right lateral ventricle,

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because patients with the Chiari type II malformation

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very, very often will need a shunt for hydrocephalus.

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Now, there's some additional features we can see.

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If we go off to the side into

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a parasagittal image,

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we can see a lot of these sulci are fairly

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narrowly spaced and shallow,

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but this is not true polymicrogyria.

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Polymicrogyria represents a malformation

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of postmigrational organization.

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This represents an entity called stenogyria.

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S-T-E-N-O-G-Y-R-I-A.

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Stenogyria.

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That's something that's commonly seen

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in a Chiari type II malformation.

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There's also an element of ulegyria,

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U-L-E-G-Y-R-I-A,

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which is gyri that are narrowly spaced,

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not due to a malformation

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of cortical development,

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but due to a paucity of white matter

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subjacent to these gyri.

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So, we see the corpus callosal dysgenesis,

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posterior predominant white matter volume loss,

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a prominent massa intermedia.

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Well, what else do we see?

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There's somewhat of a horizontally configured

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appearance of the tectal plate, in particular,

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protrusion of the inferior colliculi.

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This is sometimes referred to as tectal beaking,

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which can be a variable severity.

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And then, we see some of the more classic findings.

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We see the fourth ventricle,

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is small and vertically oriented.

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It's very small.

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On this axial image,

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we can see the foramen magnum is very large.

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There's a large foramen magnum.

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And while in a Chiari type I malformation,

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we talk about cerebellar tonsillar ectopia.

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If you look closely,

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this is the cerebellar vermis.

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The cerebellar vermis extends the

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foramen magnum too.

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So, this is not cerebellar tonsillar ectopia.

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This is just cerebellar ectopia.

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So a Chiari type II malformation has cerebellar ectopia

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through an enlarged foramen magnum.

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We can see that the cerebellar tonsils are

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almost wrapped around the brainstem here.

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We can see the cerebellar vermis

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extending this low.

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So, this is a patient with a Chiari Type II malformation

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with many of the very characteristic findings.