0:00

This is an MRI of the brain in a two-year

0:03

old child with developmental delay.

0:05

So, as we're looking,

0:07

the cerebral hemispheres look fairly normal.

0:09

We have good myelination that is appropriate

0:12

for a two-year-old child.

0:14

The ventricular system is normal.

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We're not seeing any asymmetry.

0:18

We're not seeing any other suspicious lesions.

0:22

Now, if we get to the posterior fossa,

0:25

there's something that stands out,

0:27

and it can be tricky to catch if you're not paying

0:31

attention because it's a midline finding,

0:34

and it's, in particular, the absence of something.

0:37

And I'm not seeing a cerebellar vermis right here.

0:40

I'm seeing the right cerebellar hemisphere.

0:42

I'm seeing the left cerebellar hemisphere.

0:44

And in between,

0:45

I'm not really seeing a cerebellar vermis.

0:48

Now, if I go superiorly,

0:49

maybe I see a small vermis.

0:54

If I look on this sagittal image,

0:56

all this is the cerebellum.

0:59

Cerebellar hemispheres.

1:00

This is the small cerebellar vermis.

1:03

We can see the fourth ventricle.

1:06

So there's a hypoplastic cerebellar vermis,

1:10

but there's no cystic dilation

1:13

of the posterior fossa.

1:14

So, it's a hypoplastic cerebellar vermis

1:17

that could easily be overlooked.

1:20

But what else do we see?

1:23

If we look,

1:24

the superior cerebellar peduncles are parallel,

1:28

bilaterally.

1:29

They are parallel.

1:30

Ad additionally, in this appearance,

1:34

they're thickened right here.

1:37

And this appearance here has been likened

1:41

to that of a molar tooth,

1:42

where here is the visible portion of the tooth,

1:45

and these are the roots,

1:46

that thickened roots that come down

1:49

and end up being somewhat parallel.

1:53

So, the classic molar tooth abnormality

1:56

is an entity called Joubert syndrome,

1:59

where there is a molar tooth abnormality in the

2:02

setting of a hypoplastic cerebellar vermis.

2:05

The molar tooth abnormality and the parallel

2:09

thickened superior cerebellar peduncles is

2:12

related to incomplete decussation of the fibers of

2:18

the superior cerebellar peduncle in the midbrain.

2:22

some diffusion tensor fiber tracking studies

2:26

have shown that these patients sometimes also

2:29

may not have a parameter decussation.

2:32

So, what that means is their right side of the

2:36

brain could be predominantly involved in

2:39

motor for the right side of the body,

2:41

which is different than the typical configuration.

2:45

So this is a patient with a vermian hypoplasia,

2:49

a molar tooth abnormality.

2:50

And collectively,

2:52

this is known as Joubert syndrome.