0:00

So, I want to give you some tips

0:01

for isolating the third nerve.

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One tip is that you got to have high image

0:07

quality or high spatial resolution.

0:08

So thin section imaging is vital to

0:11

identifying any cranial nerve.

0:13

And I like to see something

0:14

on the order of 1.5 less.

0:17

You can then go hit the reconstruction button and

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cross-reference in the coronal or in the sagittal

0:21

projection we have reconstructed in the coronal

0:25

projection, our Sagittal is a separate flare.

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Now, one clue for finding the third nerve is just

0:31

find the interpeduncular cistern. Simple.

0:33

Once you find that cistern,

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you work your way out and you look for

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a gray signal intensity structure,

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and you follow it and you make sure

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that it exhibits no flow void,

0:43

because otherwise it's a vessel and no

0:46

slow flow phenomenon. In other words,

0:48

it shouldn't be bright white and it shouldn't

0:50

be black. It should be gray,

0:52

and it should course in the right distribution,

0:54

which this one does.

0:55

It goes into the oculomotor sulcus,

0:57

then into the cavernous sinus,

0:59

and then into the superior orbital fissure.

1:02

So that's pretty easy.

1:03

Another tip off defining it is to locate

1:07

the posterior cerebral artery. Now,

1:09

we said in our first vignette that masses around the

1:12

posterior cerebral artery or the superior cerebellar

1:14

artery can compress the third nerve.

1:17

But the classic compression of the third nerve

1:19

as an aneurysm comes from the PCOM,

1:21

which you all know is a more common aneurysm

1:24

than any of these two other arteries.

1:26

So PCOM aneurysms will be in close opposition to the

1:30

third nerve and may produce a third nerve palsy.

1:32

But once you find the PCA,

1:34

you'll know that the third nerve

1:35

is going to be right nearby,

1:37

and it is now in the coronal projection.

1:41

All you got to do is find and cross-reference the

1:45

third nerve with the third nerve

1:47

in the axial projection.

1:48

So you have to be able to locate it in the

1:50

axial projection, which is easier.

1:52

Then I like to follow the third nerve back toward

1:55

the midbrain. Another way to locate the third nerve,

1:58

if you're spot localizing it

1:59

in the coronal projection,

2:01

is to go down and to the side of the mammillary body.

2:04

So there are the two mammillary bodies down and to

2:07

the side. There's your third nerve on the left.

2:09

There is your third nerve on the right.

2:11

Now let's come forward. Now,

2:14

that looks like initially a vessel.

2:17

There's a vessel right next to it, but it's not.

2:19

You continue to follow it.

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It's gray. It exhibits no flow phenomenon,

2:24

no flow void. We keep following it, following it,

2:27

following it. And what are we next to?

2:29

We are next to the uncus,

2:31

so Uncal Herniation will produce a third nerve palsy

2:36

right there we are in the oculomotor sulcus now,

2:39

as we move forward.

2:40

And now we've entered the upper portion

2:42

of the cavernous sinus.

2:44

I'm just showing you the nerve on the patient's

2:46

left as we continue to move forward

2:50

in the Sagittal projection.

2:51

We've already shown you the third nerve,

2:53

which is probably the easiest projection to see.

2:55

Thin-section imaging shows you the

2:57

nerve in the interpeduncular cistern.

2:59

One last caveat.

3:01

What does the third nerve innervate?

3:04

It innervates all the extraocular

3:06

muscles except for two.

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The lateral rectus, which is innervated by the 6th,

3:13

and the superior oblique,

3:15

which is innervated by the fourth.

3:18

And finally,

3:18

the additional supply of the third nerve is the

3:20

levator of the upper eyelid as well as the annular

3:23

fibers of the ciliary muscle and the sphincter

3:26

muscles of the pupil parasympathetic innervation.

3:29

That concludes our discussion of the third nerve.

3:32

We'll talk about third nerve syndromes when we

3:34

get into cranial nerve pathology.

3:36

Pomeranz out.