0:01

Let's continue with the embryology of the inner

0:04

ear structures. And as I said earlier,

0:07

this occurs very early in gestation.

0:10

So when you talk about the spiralization of the

0:13

cochlea into two and a half to

0:15

two and three quarters turns,

0:17

that is occurring in the 6th to 8th week

0:19

of gestation. At the same time,

0:21

the vestibular aqueduct is elongating from the

0:25

vestibule into the posterior portion

0:28

of the temporal bone.

0:30

So one can have abnormal development of this

0:34

spiralization in what is referred

0:37

to as the Mondini malformation

0:39

which is the arrest of spiralization

0:41

at one and a half turns.

0:43

You can have enlargement of the vestibular aqueduct

0:46

which, as I mentioned,

0:47

is associated with the most common cause of

0:51

congenital sensorineural hearing loss.

0:53

When we combine the enlargement of the vestibular

0:56

aqueduct with the abnormal spiralization

0:58

of the cochlea,

1:00

we're starting to talk about an entity

1:01

known as incomplete partition

1:05

type two. And I will refer to that in just a moment.

1:09

By the 10th week of gestation,

1:12

the bony labyrinth around all of these cochlear

1:16

and vestibular structures should be complete.

1:18

The last thing to cover, if you will,

1:21

is the superior semicircular canal.

1:24

So when we talk about superior semicircular

1:26

canal dehiscence,

1:27

that's really at the endpoint of the development

1:30

of the inner ear structures.

1:32

And the dehiscence would be something that occurs at

1:35

a congenital problem at the 8th

1:37

to 10th week of gestation.

1:41

This is an anatomic drawing of some of the

1:44

congenital abnormalities that we might see in

1:47

patients who have problems in the inner ear

1:50

and congenital sensorineural hearing loss.

1:52

So we've talked about cochlear aplasia, and I showed

1:55

an MRI example where you don't see

1:57

the cochlea developed at all.

1:59

And then we've seen other examples where

2:01

we have cochlear hypoplasia,

2:03

where you see maybe a basal turn but nothing else.

2:06

You don't have any of the middle and apical turns.

2:09

Another of the abnormalities is what is known as the

2:11

common cavity. I refer to that as the cochlear deformity,

2:14

but it is also known in the current vernacular

2:18

as the incomplete partition type one.

2:22

And you can see that it may be severe or mild

2:26

in this incomplete partition. If you have the absence of the

2:30

complete spiralization of the cochlea, and you have the

2:34

enlargement of the vestibular aqueduct,

2:36

we are at the incomplete partition type two.

2:39

And this is what we would normally expect,

2:42

to have the full spiralization of the cochlea,

2:45

as well as the development of the semicircular

2:47

canals in the vestibule.

2:48

So this is what it looks like diagrammatically.

2:51

Now, I promised you that I would give you a table.

2:55

And this is taken from Neurographics, December 2019,

3:00

article by Traylor et al., and I have

3:02

permission to present this.

3:04

And this lists some of the genetic mutations that are associated

3:08

with different syndromes that affect the inner ear.

3:13

And as you can see, many of these syndromes include

3:16

things such as cochlear hypoplasia, cochlear aplasia,

3:19

common cavity, semicircular canal-

3:23

vestibular globular abnormality,

3:25

or the incomplete partitions type two or type one.

3:30

And here is incomplete partition type three

3:31

associated with the gene.

3:34

And then from there, you have the syndrome that is named after it.

3:38

So as I said, there are many, many syndromes

3:41

that are associated with inner ear anomalies.

3:43

This is sort of a simplified version of the various

3:45

syndromes that are associated and the CT and MR findings.

3:50

I want to spend a little bit of time to review

3:52

this with you. So we can have

3:54

complete absence of development of the labyrinth,

3:56

what we called the Michel's aplasia

3:59

where you see nothing. You don't see the cochlea,

4:01

you don't see the vestibule, and you won't see any of the cranial nerves.

4:02

You can have a rudimentary otocyst

4:05

which leads to atresia of the

4:06

internal auditory canal.

4:09

We demonstrated one case where we had a very

4:11

small internal auditory canal, and that is often

4:13

associated with absence of the cochlear nerve

4:14

or the nerves of the 8th cranial nerve,

4:17

the cochlear vestibular nerves.

4:30

hypoplasia where you may see either absence of

4:32

the cochlea or just a rudimentary cochlea.

4:35

And quite often, again,

4:37

these may have either small

4:39

or absent cochlear nerves.

4:41

We're now going to start looking at some examples of

4:44

the common cavity where you have a single cavity

4:47

associated with the cochlea and the vestibule,

4:50

and they may or may not have cranial nerves.

4:52

Usually they do. Cochlear hypoplasia.

4:56

We talked about it with regard to aplasia,

4:59

where you have separation of

5:00

the cochlea and vestibule,

5:01

but you have abnormal turns of the cochlea, and they

5:05

may or may not have a hypoplastic cochlear nerve.

5:10

The incomplete partitions are kind of

5:12

associated with this common cavity,

5:14

in which case you have a malformed vestibule

5:17

and abnormal cochlea forming a figure-eight

5:21

configuration of this cochlear and vestibular

5:23

cavity. And they again may have hypoplastic nerves,

5:27

but a normal vestibular aqueduct.

5:29

The thing that's associated with the enlarged

5:31

vestibular aqueduct, as I mentioned,

5:33

is the incomplete partition type two,

5:35

in which case you have abnormal spiralization of the

5:38

cochlea and the dilated vestibule and

5:40

the enlarged vestibular aqueduct.

5:43

Incomplete partition type three is very uncommon.

5:46

It's associated with absent modiolus and what's called

5:49

a corkscrew cochlea that looks like a dilated

5:53

corkscrew in the inner ear structure with a

5:56

bulbous internal auditory canal. If we just

6:00

look at the cochlear development

6:02

in these different syndromes,

6:04

we have the normal development of

6:06

the cochlea with its apical,

6:09

middle and basal turns and the normal modiolus.

6:12

When we talk about incomplete partition type one,

6:15

it's often associated with this expanded cochlea

6:19

that may fuse into the vestibule.

6:24

The type two is incomplete spiralization.

6:28

So you see it's partly developed,

6:30

but you don't have the full two and a half,

6:33

two and three quarters turns.

6:35

And then we have the type three

6:36

which is our corkscrew modiolus,

6:40

our corkscrew cochlea.

6:42

So let's look at some of the examples.

6:45

This is an example of incomplete partition type one.

6:48

Let's go back in our slides to see what

6:50

we should be looking for.

6:52

So,

6:53

incomplete partition one, absent modiolus,

6:56

malformed vestibule,

6:58

creating a figure eight configuration with

7:01

often hypoplastic cochlear nerve.

7:04

And this is our type one

7:06

where we don't really have a good

7:07

development of the modiolus.

7:09

So here we have the cochlea, and we don't have a good

7:14

development of the modiolus of the cochlea.

7:17

And what's this?

7:18

This little bulbous area right here is

7:20

actually a portion of the vestibule.

7:22

So this is kind of what some people refer to

7:24

as a figure eight or this common cavity,

7:28

this cystic cochlear vestibule, combination of the

7:32

two with malformed cochlea, as well as malformed

7:36

vestibule in incomplete partition type one.

7:39

In this example,

7:41

you may see a dilated

7:45

posterior or superior vestibular semicircular canal.

7:51

So in this case,

7:52

you see it's kind of a fat one right there.

7:56

One other thing to make sure we look at is the

7:58

internal auditory canal. So in this example,

8:01

in B and D,

8:05

we have a very narrow internal auditory canal.

8:09

That's going to imply that there's unlikely to be a

8:12

normal cochlear nerve going to this cystic

8:17

cochlear vestibular structure.

8:19

Here it is, another example on MRI and another

8:24

CT example. So incomplete partition type one,

8:28

in which we have sort of this common cavity between

8:32

the cochlea and the vestibule on the CT scan.

8:36

So we're going to say this is probably

8:38

the development of the cochlea.

8:39

This is probably the development of the vestibule.

8:41

No modiolus, no turns of the cochlea.

8:45

And it has what some people refer to as a figure

8:50

eight configuration. The same is true here.

8:54

This is the left side with an MRI scan.

8:58

So on this MRI scan, on the left side,

9:00

we have the cochlea, and we have the vestibule,

9:03

and we have this common cavity with no development

9:05

of the spiral lamina in this incomplete partition

9:09

type one, cystic cochlear vestibular malformation.