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I want to make three points about third nerve syndromes.

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First, every medical student and resident, fellow,

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and attending should know that PCOM aneurysms

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can generate a third nerve palsy.

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And that's the classic aneurysm to do so.

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Second, intranuclear ophthalmoplegia.

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Let's redraw the third nerve nucleus,

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which sits right in front of the aqueduct of Sylvius.

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There's also an accessory third nerve

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nucleus that's a little medial.

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And then the nerve kind of bows outward a little bit

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and exits the interpeduncular cistern, medial to the

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substantia nigra, which is right over here.

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And you can actually see a little bit of the third

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nerve, these little gray dots right there.

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Now, in intranuclear ophthalmoplegia,

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you'll have involvement of the nucleus itself and the

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medial longitudinal fasciculus, perhaps as an MS plaque.

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So an MS plaque might be located right over here.

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It might clip one or both of the nuclei, and you would

1:02

get an INO, an intranuclear ophthalmoplegia.

1:04

What's that look like?

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We're going to make a couple of eyes.

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Here's an eyeball.

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Here's another eyeball.

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And then we're going to have the patient look to their left.

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And when they look to their left,

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the lateral rectus pulls this eyeball over.

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But because there's a third nerve palsy,

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and I actually should make the palsy over here.

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So let's make the lesion right there, not on this side.

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I'm going to erase it

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so it's going to be on the right side.

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And this eyeball is lagging because the

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third nerve is weak.

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Therefore, the medial rectus can't pull the eyeball over,

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but the lateral rectus does because it's functioning.

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And so, this eye has an adduction problem.

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And we say this patient has a right INO.

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The third point I want to make is related to Weber's Syndrome.

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So let me draw again the third nerve.

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Here's the third nerve nucleus.

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Here's the third nerve,

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bows out a little bit, then comes in.

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And in Weber's Syndrome,

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let's go to my favorite color, yellow.

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We're going to have an area along the medial peduncle

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that clips the third nerve as it courses

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towards its apparent exit.

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So right there, it's going to get clipped and the patient

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is going to have an ipsilateral third nerve palsy with a fixed pupil.

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What else are they going to have?

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Well, let's look over here.

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Let's draw it again.

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And we relatively spare the red nucleus in Weber's Syndrome,

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but we do involve the exiting,

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as you can see over here,

2:43

the exiting third nerve.

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So ipsilateral third nerve palsy.

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You may also have a supranuclear

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7th nerve ipsilateral palsy.

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But because the corticospinal tract is going

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to decussate as internal arcuate fibers,

2:57

it's going to be a spastic hemiparesis.

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That's contralateral.

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So contralateral motor plus ipsilateral seventh,

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plus ipsilateral third is going to

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give you a Weber's syndrome.

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And every medical student absolutely has to learn this.

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What produces it?

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Occlusion of the posterior cerebral artery,

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the PCOM or posterior choroidal branches.

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There's a variation of this called Benedikt's syndrome,

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where it comes back a little further

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and it may get the red nucleus.

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It may also get the cross-descending

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superior cerebellar peduncle.

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But the bottom line, more advanced,

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not so basic-basic,

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is that you have an ipsilateral oculomotor

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nerve with a contralateral cerebellar syndrome.

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That's known as Benedikt's syndrome.

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As we finish out,

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let's give some creds to the Weber family.

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The father, Hermann David Weber,

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described this Weber's Vascular Syndrome.

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The son, Frederick Parkes Weber,

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described Klippel-Trénaunay Weber, Parkes Weber syndrome,

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Osler-Weber-Rendu.

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A prolific family.

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That's all from me.

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Pormeranz out.