0:01

I'd like to show this as a companion case

0:04

to our search for cochlear nerves.

0:07

So let's look at this patient.

0:09

As you can see,

0:10

there is dilatation of the subarachnoid space

0:13

anterior to the brain stem because this patient

0:16

has an abnormality known as a pontine

0:20

tegmental cap dysplasia.

0:22

And part of it is that the pons

0:25

is quite small in size,

0:27

but does allow us to see the cranial nerves

0:29

quite nicely. Here, for example,

0:32

one can see the six cranial nerves coursing

0:35

anteriorly and crossing dorello's canal

0:38

to get to the cavernous sinus.

0:40

We also see nerves that are going to the

0:44

internal auditory canal. However,

0:47

we see that there is a difference in the

0:50

appearance of these nerves on the right side

0:54

compared to the left side and we'll show that in

0:57

just a moment. If we go up further anteriorly,

1:01

you can start to see some of the

1:03

third cranial nerves as well.

1:06

So I want to point out, in this example, the

1:09

cochlea. So if we look at the cochlea,

1:12

we have a nice basal turn bilaterally,

1:15

we have a nice middle turn bilaterally and we also

1:20

have a nice apical turn and we have a nice

1:22

development of the cochlear mediolis bilaterally.

1:26

However,

1:27

let's look at the internal auditory canal.

1:30

When we look at the internal auditory canal on the

1:33

right side and compare it to the internal

1:36

auditory canal on the left side,

1:38

we see that there is internal

1:40

auditory canal hypoplasia.

1:42

It's quite small in size. And when that happens,

1:46

we have to be very concerned that there is absence

1:49

of the cranial nerves going through

1:51

the internal auditory canal.

1:53

So although the cochlea is well developed,

1:56

we have a problem with the internal auditory canal

2:00

on the left side compared with the right.

2:03

If we look at this in coronal plane,

2:06

we can compare the size of the internal auditory

2:10

canal on the right side with the size of the

2:15

internal auditory canal on the left side and

2:18

clearly identify that there's a problem in small

2:22

size on the left compared to the

2:24

more normal size on the right.

2:26

Now, most of the time the width of the internal

2:28

auditory canal is going to be wider at the opening

2:31

than it is in the portion of the fundus

2:34

of the internal auditory canal.

2:36

So in this case, it's probably a little

2:39

bit small on the right side as well,

2:42

but certainly dramatically small on the left side.

2:45

Now, trying to find the cochlear nerve in this very,

2:50

very tiny internal auditory canal

2:53

is going to be problematic.

2:55

But we have a high suspicion that there is

2:57

likely cochlear nerve absence on the left

3:00

because of the small size of the internal auditory canal.

3:03

So let's see what we can do on the sagittal plane.

3:06

So I'm going to magnify this for you all

3:09

and see what we can find.

3:11

So we are seeing the cranial nerves here, and this

3:16

is the very tiny internal auditory

3:19

canal on the left side, and

3:24

here's the...

3:25

you know, turns of the cochlea.

3:27

And this is portion of the vestibular system

3:30

here and the cochlea. But frankly,

3:35

we can't really identify the different nerves in

3:39

the internal auditory canal because of

3:41

its small size. And we would imply,

3:44

particularly if the patient has congenital

3:46

sensorineural hearing loss,

3:47

that there is cochlear nerve absence on the left

3:51

side. As we go to the contralateral side,

3:55

here we have a little better view of

3:57

the internal auditory canal.

4:00

And we have something here that

4:01

we're going to say, well,

4:03

we see some nerves superiorly in

4:05

the internal auditory canal,

4:07

likely being superior vestibular

4:09

nerve and the facial nerve,

4:10

particularly if the patient has normal facial

4:12

nerve function. But inferiorly, we're missing nerves.

4:16

So this is an example of bilateral cochlear

4:20

nerve hypoplasia or absence,

4:23

even though there has been normal development as seen

4:28

of the cochlea and its modiolus and the

4:33

basal, middle, and apical turns of the cochlea.

4:36

So it goes to show that there is sometimes a

4:40

separation of the bony labyrinth from the

4:44

development of the cranial nerve.