0:01

Let's finish our discussion of congenital anomalies

0:04

of the inner ear with a discussion about

0:06

superior semicircular canal dehiscence.

0:09

This is appropriate because the covering of the

0:12

superior semicircular canal is one of the last

0:15

evolutionary and embryological steps in the

0:19

development of inner ear structure.

0:21

When one has dehiscence of the superior semicircular canal,

0:26

you have effectively a third window.

0:28

We have our round window, we have our oval window,

0:30

but now we've created a third window where there may

0:34

be transmission of pulsations from the cerebrospinal

0:38

fluid to the inner ear structures and vice versa.

0:42

What one has, clinically, in patients who have superior

0:46

semicircular canal dehiscence is something

0:48

called Tullio's phenomenon,

0:49

which is dizziness associated with loud

0:52

noises or the Valsalva manoeuvre.

0:56

The patients may also experience Oscillopsia,

0:58

which is the apparent

1:00

phenomenon of motion,

1:02

even though an object may be stationary.

1:04

So they perceive it as showing motion.

1:07

And normally, as you know,

1:09

when we have our closed system,

1:12

when something in the oval window moves,

1:14

particularly the footplate of the stapes, from sound,

1:17

the round window will move in this closed system.

1:20

But when you have this third window, that is,

1:23

this third opportunity for the transmission of vibratory

1:27

motion in the endolymph and perilymph

1:30

that can be dismissed through the

1:34

dehiscent superior semicircular canal,

1:36

you then have that sense of motion that

1:40

is abnormal with the various sounds.

1:43

Patients may have some element of hearing loss and

1:46

this dehiscence in the superior semicircular canal is defective in,

1:51

bilaterally, in 17% of patients.

1:54

For this, we usually do what I mentioned previously

1:57

was called spiral reconstructions of axial CT data.

2:02

So that way we put the superior semicircular canal in

2:06

a perpendicular plane to the angle of reconstruction.

2:10

So here in this example of the reconstructed image,

2:15

we have the vestibule,

2:16

we have the superior semicircular canal.

2:18

And as you can see,

2:19

there is no bony covering of the

2:21

superior semicircular canal,

2:23

such that it is communicating the subarachnoid

2:26

space of the temporal bone.

2:28

And if we do this in a slice perpendicular to the

2:32

plane of the superior semicircular canal,

2:35

what you see is it cut in cross-section with absence

2:39

of bony covering of the superior semicircular canal.

2:45

So, as I mentioned,

2:46

the lateral semicircular canal and the superior

2:49

semicircular canal are the last to develop,

2:52

and they may have hypoplasia.

2:54

There are certain syndromes that are associated

2:57

with semicircular canal hypoplasia,

3:00

and that includes Pendred syndrome,

3:02

which is associated with enlargement

3:03

of the vestibular aqueduct.

3:05

And with Pendred syndrome, you may also show a

3:09

multinodular goitre and have hypo or hyperthyroidism.

3:14

This may also be associated with incomplete

3:17

partition phenomenon type 2.