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I want to give you some tricks to isolate the

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fourth nerve and the fourth nerve nucleus.

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If you find the aqueduct of Sylvius

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in the inferior midbrain,

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you go forward 7 mm and over 5 mm

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along the inner edge of this dark line,

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the posterior longitudinal fasciculus.

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You are going to run into the trochlear nucleus on

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the left and then there's a paired

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one on the opposite side.

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Now, the trochlear nerve is going to come back and

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arc around the periaqueductal gray

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and it's going to decussate,

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not going to show the decussation because I want

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you to see the decussated trochlear nerves right there.

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So, another trick to finding the nerve

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is finding the inferior colliculus.

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There's the inferior colliculus.

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There is the frenulum of the superior

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medullary velum right there.

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And I'm going to scroll a little bit now and take

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my lines away and do some scrolling for you.

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There's the frenulum of the superomedullary

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velum and right there is the

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inferior colliculus.

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And there is the trochlear nerve.

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You're seeing some wispy spidery structures here,

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but the one that is most round right

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there is the trochlear nerve itself.

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Now, another trick is to find the third nerve.

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So let's cross-reference the third nerve in

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the interpeduncular cistern right there.

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There's your third nerve on the patient's right.

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There's the third nerve on the left.

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Now, let's keep going forward.

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There's the third nerve. Keep following it.

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Keep following.

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We're not able to see the fourth nerve.

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Keep going, keep going.

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The nerve is now entering the oculomotor sulcus.

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The nerve has now entered the cavernous sinus.

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There's the carotid artery.

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And what's the first thing below it?

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This little gray structure right there.

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I'm going to put a dot on top of it.

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That is the fourth nerve right there.

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What types of pathology affect the fourth nerve?

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Well, the fourth nerve supplies the superior

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oblique muscle of the eyeball.

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So,

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one of the syndromes that affects the superior

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oblique muscle is Brown's syndrome,

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which is a contraction or scar of the pulley of

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the superior oblique muscle.

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That can actually compress and encase the fourth nerve

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and contribute to cranial nerve encroachment

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on that nerve, and what's known as Brown's syndrome

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or the fibrotic syndrome of the superior oblique pulley.

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Let's move on, shall we?